SHOW EXPLAIN: merge to 10.0-base.

.bzrignore

@@ -1136,6 +1136,7 @@ plugin/handler_socket/perl-Net-HandlerSocket/Makefile.PL
 libmysqld/libmysqld_exports_file.cc
 libmysqld/gcalc_slicescan.cc
 libmysqld/gcalc_tools.cc
+libmysqld/my_apc.cc
 sql/share/errmsg.sys
 sql/share/mysql
 install_manifest.txt

CMakeLists.txt

@@ -285,6 +285,7 @@ IF(WITH_UNIT_TESTS)
  ADD_SUBDIRECTORY(unittest/strings)
  ADD_SUBDIRECTORY(unittest/examples)
  ADD_SUBDIRECTORY(unittest/mysys)
+ ADD_SUBDIRECTORY(unittest/sql)
 ENDIF()
 
 IF(NOT WITHOUT_SERVER)

client/mysqltest.cc

@@ -84,6 +84,8 @@ static my_bool non_blocking_api_enabled= 0;
 #define QUERY_SEND_FLAG  1
 #define QUERY_REAP_FLAG  2
 
+#define QUERY_PRINT_ORIGINAL_FLAG 4
+
 #ifndef HAVE_SETENV
 static int setenv(const char *name, const char *value, int overwrite);
 #endif
@@ -345,6 +347,7 @@ enum enum_commands {
   Q_SEND,		    Q_REAP,
   Q_DIRTY_CLOSE,	    Q_REPLACE, Q_REPLACE_COLUMN,
   Q_PING,		    Q_EVAL, 
+  Q_EVALP,
   Q_EVAL_RESULT,
   Q_ENABLE_QUERY_LOG, Q_DISABLE_QUERY_LOG,
   Q_ENABLE_RESULT_LOG, Q_DISABLE_RESULT_LOG,
@@ -410,6 +413,7 @@ const char *command_names[]=
   "replace_column",
   "ping",
   "eval",
+  "evalp",
   "eval_result",
   /* Enable/disable that the _query_ is logged to result file */
   "enable_query_log",
@@ -8316,7 +8320,8 @@ void run_query(struct st_connection *cn, struct st_command *command, int flags)
   /*
     Evaluate query if this is an eval command
   */
-  if (command->type == Q_EVAL || command->type == Q_SEND_EVAL)
+  if (command->type == Q_EVAL || command->type == Q_SEND_EVAL || 
+      command->type == Q_EVALP)
   {
     init_dynamic_string(&eval_query, "", command->query_len+256, 1024);
     do_eval(&eval_query, command->query, command->end, FALSE);
@@ -8348,11 +8353,21 @@ void run_query(struct st_connection *cn, struct st_command *command, int flags)
   */
   if (!disable_query_log && (flags & QUERY_SEND_FLAG))
   {
-    replace_dynstr_append_mem(ds, query, query_len);
+    char *print_query= query;
+    int print_len= query_len;
+    if (flags & QUERY_PRINT_ORIGINAL_FLAG)
+    {
+      print_query= command->query;
+      print_len= command->end - command->query;
+    }
+    replace_dynstr_append_mem(ds, print_query, print_len);
     dynstr_append_mem(ds, delimiter, delimiter_length);
     dynstr_append_mem(ds, "\n", 1);
   }
   
+  /* We're done with this flag */
+  flags &= ~QUERY_PRINT_ORIGINAL_FLAG;
+
   /*
     Write the command to the result file before we execute the query
     This is needed to be able to analyse the log if something goes
@@ -9212,6 +9227,7 @@ int main(int argc, char **argv)
       case Q_EVAL_RESULT:
         die("'eval_result' command  is deprecated");
       case Q_EVAL:
+      case Q_EVALP:
       case Q_QUERY_VERTICAL:
       case Q_QUERY_HORIZONTAL:
 	if (command->query == command->query_buf)
@@ -9239,6 +9255,9 @@ int main(int argc, char **argv)
           flags= QUERY_REAP_FLAG;
         }
 
+        if (command->type == Q_EVALP)
+          flags |= QUERY_PRINT_ORIGINAL_FLAG;
+
         /* Check for special property for this query */
         display_result_vertically|= (command->type == Q_QUERY_VERTICAL);
 

libmysqld/CMakeLists.txt

@@ -96,6 +96,7 @@ SET(SQL_EMBEDDED_SOURCES emb_qcache.cc libmysqld.c lib_sql.cc
            ../sql/create_options.cc ../sql/rpl_utility.cc
            ../sql/rpl_reporting.cc
            ../sql/sql_expression_cache.cc
+           ../sql/my_apc.cc ../sql/my_apc.h
            ${GEN_SOURCES}
            ${MYSYS_LIBWRAP_SOURCE}
 )

mysql-test/r/show_explain_ps.result

@@ -0,0 +1,27 @@
+drop table if exists t0, t1;
+select * from performance_schema.setup_instruments where name like '%show_explain%';
+NAME	ENABLED	TIMED
+wait/synch/cond/sql/show_explain	YES	YES
+# We've got no instances
+select * from performance_schema.cond_instances where name like '%show_explain%';
+NAME	OBJECT_INSTANCE_BEGIN
+# Check out if our cond is hit.
+create table t0 (a int);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+set @show_explain_probe_select_id=1;
+set debug_dbug='d,show_explain_probe_join_exec_start';
+select count(*) from t0 where a < 100000;
+show explain for $thr2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t0	ALL	NULL	NULL	NULL	NULL	10	Using where
+Warnings:
+Note	1003	select count(*) from t0 where a < 100000
+count(*)
+10
+set debug_dbug='';
+select event_name
+from performance_schema.events_waits_history_long 
+where event_name='wait/synch/cond/sql/show_explain';
+event_name
+wait/synch/cond/sql/show_explain
+drop table t0;

mysql-test/t/show_explain_ps.test

@@ -0,0 +1,51 @@
+#
+# Test how SHOW EXPLAIN is represented in performance schema
+#
+--source include/have_debug.inc
+--source include/have_perfschema.inc
+# Like all other perfschema tests, we don't work on embedded server:
+--source include/not_embedded.inc
+
+--disable_warnings
+drop table if exists t0, t1;
+--enable_warnings
+
+select * from performance_schema.setup_instruments where name like '%show_explain%';
+
+--echo # We've got no instances
+select * from performance_schema.cond_instances where name like '%show_explain%';
+
+--echo # Check out if our cond is hit.
+
+create table t0 (a int);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+
+let $thr1=`select connection_id()`;
+connect (con1, localhost, root,,);
+connection con1;
+let $thr2=`select connection_id()`;
+connection default;
+
+let $wait_condition= select State='show_explain_trap' from information_schema.processlist where id=$thr2;
+
+#
+# Test SHOW EXPLAIN for simple queries
+#
+connection con1;
+set @show_explain_probe_select_id=1;
+set debug_dbug='d,show_explain_probe_join_exec_start';
+send select count(*) from t0 where a < 100000;
+
+connection default;
+--source include/wait_condition.inc
+evalp show explain for $thr2;
+connection con1;
+reap;
+
+set debug_dbug='';
+
+select event_name
+from performance_schema.events_waits_history_long 
+where event_name='wait/synch/cond/sql/show_explain';
+
+drop table t0;

sql/CMakeLists.txt

@@ -86,6 +86,7 @@ SET (SQL_SOURCE
                gcalc_slicescan.cc gcalc_tools.cc
 			   threadpool_common.cc 
 			   ../sql-common/mysql_async.c
+               my_apc.cc my_apc.h
                ${GEN_SOURCES}
                ${MYSYS_LIBWRAP_SOURCE}
 			   )

sql/filesort.cc

@@ -581,7 +581,6 @@ static ha_rows find_all_keys(Sort_param *param, SQL_SELECT *select,
   my_off_t record;
   TABLE *sort_form;
   THD *thd= current_thd;
-  volatile killed_state *killed= &thd->killed;
   handler *file;
   MY_BITMAP *save_read_set, *save_write_set, *save_vcol_set;
   
@@ -603,6 +602,11 @@ static ha_rows find_all_keys(Sort_param *param, SQL_SELECT *select,
   if (flag)
     ref_pos= &file->ref[0];
   next_pos=ref_pos;
+
+  DBUG_EXECUTE_IF("show_explain_in_find_all_keys", 
+                  dbug_serve_apcs(thd, 1);
+                 );
+
   if (!quick_select)
   {
     next_pos=(uchar*) 0;			/* Find records in sequence */
@@ -665,7 +669,7 @@ static ha_rows find_all_keys(Sort_param *param, SQL_SELECT *select,
 	break;
     }
 
-    if (*killed)
+    if (thd->check_killed())
     {
       DBUG_PRINT("info",("Sort killed by user"));
       if (!quick_select)
@@ -1459,18 +1463,13 @@ int merge_buffers(Sort_param *param, IO_CACHE *from_file,
   void *first_cmp_arg;
   element_count dupl_count= 0;
   uchar *src;
-  killed_state not_killable;
   uchar *unique_buff= param->unique_buff;
-  volatile killed_state *killed= &current_thd->killed;
+  const bool killable= !param->not_killable;
+  THD* const thd=current_thd;
   DBUG_ENTER("merge_buffers");
 
-  status_var_increment(current_thd->status_var.filesort_merge_passes);
+  status_var_increment(thd->status_var.filesort_merge_passes);
-  current_thd->query_plan_fsort_passes++;
+  thd->query_plan_fsort_passes++;
-  if (param->not_killable)
-  {
-    killed= &not_killable;
-    not_killable= NOT_KILLED;
-  }
 
   error=0;
   rec_length= param->rec_length;
@@ -1547,7 +1546,7 @@ int merge_buffers(Sort_param *param, IO_CACHE *from_file,
 
   while (queue.elements > 1)
   {
-    if (*killed)
+    if (killable && thd->check_killed())
     {
       error= 1; goto err;                        /* purecov: inspected */
     }

sql/handler.h

@@ -600,6 +600,7 @@ enum enum_schema_tables
   SCH_COLUMN_PRIVILEGES,
   SCH_ENGINES,
   SCH_EVENTS,
+  SCH_EXPLAIN,
   SCH_FILES,
   SCH_GLOBAL_STATUS,
   SCH_GLOBAL_VARIABLES,

sql/item_func.cc

@@ -4331,7 +4331,7 @@ longlong Item_func_sleep::val_int()
 
 #define extra_size sizeof(double)
 
-static user_var_entry *get_variable(HASH *hash, LEX_STRING &name,
+user_var_entry *get_variable(HASH *hash, LEX_STRING &name,
 				    bool create_if_not_exists)
 {
   user_var_entry *entry;

sql/item_subselect.cc

@@ -1919,7 +1919,7 @@ bool Item_allany_subselect::is_maxmin_applicable(JOIN *join)
     WHERE condition.
   */
   return (abort_on_null || (upper_item && upper_item->is_top_level_item())) &&
-      !join->select_lex->master_unit()->uncacheable && !func->eqne_op();
+      !(join->select_lex->master_unit()->uncacheable & ~UNCACHEABLE_EXPLAIN) && !func->eqne_op();
 }
 
 

sql/my_apc.cc

@@ -0,0 +1,270 @@
+/*
+   Copyright (c) 2011 - 2012, Monty Program Ab
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; version 2 of the License.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */
+
+
+#ifndef MY_APC_STANDALONE
+
+#include "sql_priv.h"
+#include "sql_class.h"
+
+#endif
+
+/* For standalone testing of APC system, see unittest/sql/my_apc-t.cc */
+
+/* 
+  Initialize the target. 
+   
+  @note 
+  Initialization must be done prior to enabling/disabling the target, or making
+  any call requests to it.
+  Initial state after initialization is 'disabled'.
+*/
+void Apc_target::init(mysql_mutex_t *target_mutex)
+{
+  DBUG_ASSERT(!enabled);
+  LOCK_thd_data_ptr= target_mutex;
+#ifndef DBUG_OFF
+  n_calls_processed= 0;
+#endif
+}
+
+
+/* 
+  Destroy the target. The target must be disabled when this call is made.
+*/
+void Apc_target::destroy()
+{
+  DBUG_ASSERT(!enabled);
+}
+
+
+/* 
+  Enter ther state where the target is available for serving APC requests
+*/
+void Apc_target::enable()
+{
+  /* Ok to do without getting/releasing the mutex: */
+  enabled++;
+}
+
+
+/* 
+  Make the target unavailable for serving APC requests. 
+  
+  @note
+    This call will serve all requests that were already enqueued
+*/
+
+void Apc_target::disable()
+{
+  bool process= FALSE;
+  mysql_mutex_lock(LOCK_thd_data_ptr);
+  if (!(--enabled))
+    process= TRUE;
+  mysql_mutex_unlock(LOCK_thd_data_ptr);
+  if (process)
+    process_apc_requests();
+}
+
+
+/* [internal] Put request qe into the request list */
+
+void Apc_target::enqueue_request(Call_request *qe)
+{
+  mysql_mutex_assert_owner(LOCK_thd_data_ptr);
+  if (apc_calls)
+  {
+    Call_request *after= apc_calls->prev;
+    qe->next= apc_calls;
+    apc_calls->prev= qe;
+     
+    qe->prev= after;
+    after->next= qe;
+  }
+  else
+  {
+    apc_calls= qe;
+    qe->next= qe->prev= qe;
+  }
+}
+
+
+/* 
+  [internal] Remove request qe from the request queue. 
+  
+  The request is not necessarily first in the queue.
+*/
+
+void Apc_target::dequeue_request(Call_request *qe)
+{
+  mysql_mutex_assert_owner(LOCK_thd_data_ptr);
+  if (apc_calls == qe)
+  {
+    if ((apc_calls= apc_calls->next) == qe)
+    {
+      apc_calls= NULL;
+    }
+  }
+
+  qe->prev->next= qe->next;
+  qe->next->prev= qe->prev;
+}
+
+#ifdef HAVE_PSI_INTERFACE
+
+/* One key for all conds */
+PSI_cond_key key_show_explain_request_COND;
+
+static PSI_cond_info show_explain_psi_conds[]=
+{
+  { &key_show_explain_request_COND, "show_explain", 0 /* not using PSI_FLAG_GLOBAL*/ }
+};
+
+void init_show_explain_psi_keys(void)
+{
+  if (PSI_server == NULL)
+    return;
+
+  PSI_server->register_cond("sql", show_explain_psi_conds, 
+                            array_elements(show_explain_psi_conds));
+}
+#endif
+
+
+/*
+  Make an APC (Async Procedure Call) to another thread. 
+ 
+  @detail
+  Make an APC call: schedule it for execution and wait until the target
+  thread has executed it. 
+
+  - The caller is responsible for making sure he's not posting request
+    to the thread he's calling this function from.
+
+  - The caller must have locked target_mutex. The function will release it.
+
+  @retval FALSE - Ok, the call has been made
+  @retval TRUE  - Call wasnt made (either the target is in disabled state or
+                    timeout occured)
+*/
+
+bool Apc_target::make_apc_call(THD *caller_thd, Apc_call *call, 
+                               int timeout_sec, bool *timed_out)
+{
+  bool res= TRUE;
+  *timed_out= FALSE;
+
+  if (enabled)
+  {
+    /* Create and post the request */
+    Call_request apc_request;
+    apc_request.call= call;
+    apc_request.processed= FALSE;
+    mysql_cond_init(key_show_explain_request_COND, &apc_request.COND_request,
+                    NULL);
+    enqueue_request(&apc_request);
+    apc_request.what="enqueued by make_apc_call";
+ 
+    struct timespec abstime;
+    const int timeout= timeout_sec;
+    set_timespec(abstime, timeout);
+
+    int wait_res= 0;
+    const char *old_msg;
+    old_msg= caller_thd->enter_cond(&apc_request.COND_request, 
+                                    LOCK_thd_data_ptr, "show_explain");
+    /* todo: how about processing other errors here? */
+    while (!apc_request.processed && (wait_res != ETIMEDOUT))
+    {
+      /* We own LOCK_thd_data_ptr */
+      wait_res= mysql_cond_timedwait(&apc_request.COND_request,
+                                     LOCK_thd_data_ptr, &abstime);
+                                      // &apc_request.LOCK_request, &abstime);
+      if (caller_thd->killed)
+        break;
+    }
+
+    if (!apc_request.processed)
+    {
+      /* 
+        The wait has timed out, or this thread was KILLed.
+        Remove the request from the queue (ok to do because we own
+        LOCK_thd_data_ptr)
+      */
+      apc_request.processed= TRUE;
+      dequeue_request(&apc_request);
+      *timed_out= TRUE;
+      res= TRUE;
+    }
+    else
+    {
+      /* Request was successfully executed and dequeued by the target thread */
+      res= FALSE;
+    }
+    /* 
+      exit_cond() will call mysql_mutex_unlock(LOCK_thd_data_ptr) for us:
+    */
+    caller_thd->exit_cond(old_msg);
+
+    /* Destroy all APC request data */
+    mysql_cond_destroy(&apc_request.COND_request);
+  }
+  else
+  {
+    mysql_mutex_unlock(LOCK_thd_data_ptr);
+  }
+  return res;
+}
+
+
+/*
+  Process all APC requests.
+  This should be called periodically by the APC target thread.
+*/
+
+void Apc_target::process_apc_requests()
+{
+  while (1)
+  {
+    Call_request *request;
+ 
+    mysql_mutex_lock(LOCK_thd_data_ptr);
+    if (!(request= get_first_in_queue()))
+    {
+      /* No requests in the queue */
+      mysql_mutex_unlock(LOCK_thd_data_ptr);
+      break;
+    }
+
+    /* 
+      Remove the request from the queue (we're holding queue lock so we can be 
+      sure that request owner won't try to remove it)
+    */
+    request->what="dequeued by process_apc_requests";
+    dequeue_request(request);
+    request->processed= TRUE;
+
+    request->call->call_in_target_thread();
+    request->what="func called by process_apc_requests";
+
+#ifndef DBUG_OFF
+    n_calls_processed++;
+#endif
+    mysql_cond_signal(&request->COND_request);
+    mysql_mutex_unlock(LOCK_thd_data_ptr);
+  }
+}
+

sql/my_apc.h

@@ -0,0 +1,138 @@
+#ifndef INCLUDES_MY_APC_H
+#define INCLUDES_MY_APC_H
+/*
+   Copyright (c) 2011 - 2012, Monty Program Ab
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; version 2 of the License.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */
+
+/*
+  Interface
+  ~~~~~~~~~
+   (
+    - This is an APC request queue
+    - We assume there is a particular owner thread which periodically calls
+      process_apc_requests() to serve the call requests.
+    - Other threads can post call requests, and block until they are exectued.
+  )
+
+  Implementation
+  ~~~~~~~~~~~~~~
+  - The target has a mutex-guarded request queue.
+
+  - After the request has been put into queue, the requestor waits for request
+    to be satisfied. The worker satisifes the request and signals the
+    requestor.
+*/
+
+class THD;
+
+/*
+  Target for asynchronous procedure calls (APCs). 
+   - A target is running in some particular thread, 
+   - One can make calls to it from other threads.
+*/
+class Apc_target
+{
+  mysql_mutex_t *LOCK_thd_data_ptr;
+public:
+  Apc_target() : enabled(0), apc_calls(NULL) {} 
+  ~Apc_target() { DBUG_ASSERT(!enabled && !apc_calls);}
+
+  void init(mysql_mutex_t *target_mutex);
+  void destroy();
+  void enable();
+  void disable();
+  
+  void process_apc_requests();
+  /* 
+    A lightweight function, intended to be used in frequent checks like this:
+
+      if (apc_target.have_requests()) apc_target.process_apc_requests()
+  */
+  inline bool have_apc_requests()
+  {
+    return test(apc_calls);
+  }
+  
+  /* Functor class for calls you can schedule */
+  class Apc_call
+  {
+  public:
+    /* This function will be called in the target thread */
+    virtual void call_in_target_thread()= 0;
+    virtual ~Apc_call() {}
+  };
+  
+  /* Make a call in the target thread (see function definition for details) */
+  bool make_apc_call(THD *caller_thd, Apc_call *call, int timeout_sec, bool *timed_out);
+
+#ifndef DBUG_OFF
+  int n_calls_processed; /* Number of calls served by this target */
+#endif
+private:
+  class Call_request;
+
+  /* 
+    Non-zero value means we're enabled. It's an int, not bool, because one can
+    call enable() N times (and then needs to call disable() N times before the 
+    target is really disabled)
+  */
+  int enabled;
+
+  /* 
+    Circular, double-linked list of all enqueued call requests. 
+    We use this structure, because we 
+     - process requests sequentially: requests are added at the end of the 
+       list and removed from the front. With circular list, we can keep one
+       pointer, and access both front an back of the list with it.
+     - a thread that has posted a request may time out (or be KILLed) and 
+       cancel the request, which means we need a fast request-removal
+       operation.
+  */
+  Call_request *apc_calls;
+ 
+  class Call_request
+  {
+  public:
+    Apc_call *call; /* Functor to be called */
+
+    /* The caller will actually wait for "processed==TRUE" */
+    bool processed;
+
+    /* Condition that will be signalled when the request has been served */
+    mysql_cond_t COND_request;
+    
+    /* Double linked-list linkage */
+    Call_request *next;
+    Call_request *prev;
+    
+    const char *what; /* (debug) state of the request */
+  };
+
+  void enqueue_request(Call_request *qe);
+  void dequeue_request(Call_request *qe);
+
+  /* return the first call request in queue, or NULL if there are none enqueued */
+  Call_request *get_first_in_queue()
+  {
+    return apc_calls;
+  }
+};
+
+#ifdef HAVE_PSI_INTERFACE
+void init_show_explain_psi_keys(void);
+#endif
+
+#endif //INCLUDES_MY_APC_H
+

sql/mysqld.cc

@@ -3349,6 +3349,7 @@ SHOW_VAR com_status_vars[]= {
   {"show_engine_status",   (char*) offsetof(STATUS_VAR, com_stat[(uint) SQLCOM_SHOW_ENGINE_STATUS]), SHOW_LONG_STATUS},
   {"show_events",          (char*) offsetof(STATUS_VAR, com_stat[(uint) SQLCOM_SHOW_EVENTS]), SHOW_LONG_STATUS},
   {"show_errors",          (char*) offsetof(STATUS_VAR, com_stat[(uint) SQLCOM_SHOW_ERRORS]), SHOW_LONG_STATUS},
+  {"show_explain",         (char*) offsetof(STATUS_VAR, com_stat[(uint) SQLCOM_SHOW_EXPLAIN]), SHOW_LONG_STATUS},
   {"show_fields",          (char*) offsetof(STATUS_VAR, com_stat[(uint) SQLCOM_SHOW_FIELDS]), SHOW_LONG_STATUS},
 #ifndef DBUG_OFF
   {"show_function_code",   (char*) offsetof(STATUS_VAR, com_stat[(uint) SQLCOM_SHOW_FUNC_CODE]), SHOW_LONG_STATUS},
@@ -3930,6 +3931,7 @@ static int init_thread_environment()
 #ifdef HAVE_EVENT_SCHEDULER
   Events::init_mutexes();
 #endif
+  init_show_explain_psi_keys();
   /* Parameter for threads created for connections */
   (void) pthread_attr_init(&connection_attrib);
   (void) pthread_attr_setdetachstate(&connection_attrib,

sql/opt_subselect.cc

@@ -1657,6 +1657,7 @@ static bool convert_subq_to_sj(JOIN *parent_join, Item_in_subselect *subq_pred)
       parent_lex->ftfunc_list->push_front(ifm);
   }
 
+  parent_lex->have_merged_subqueries= TRUE;
   DBUG_RETURN(FALSE);
 }
 
@@ -1767,6 +1768,8 @@ static bool convert_subq_to_jtbm(JOIN *parent_join,
   create_subquery_temptable_name(tbl_alias, hash_sj_engine->materialize_join->
                                               select_lex->select_number);
   jtbm->alias= tbl_alias;
+
+  parent_lex->have_merged_subqueries= TRUE;
 #if 0
   /* Inject sj_on_expr into the parent's WHERE or ON */
   if (emb_tbl_nest)

sql/protocol.h

@@ -35,6 +35,7 @@ class Protocol
 protected:
   THD	 *thd;
   String *packet;
+  /* Used by net_store_data() for charset conversions */
   String *convert;
   uint field_pos;
 #ifndef DBUG_OFF
@@ -49,6 +50,10 @@ protected:
   MYSQL_FIELD *next_mysql_field;
   MEM_ROOT *alloc;
 #endif
+  /* 
+    The following two are low-level functions that are invoked from
+    higher-level store_xxx() funcs.  The data is stored into this->packet.
+  */
   bool net_store_data(const uchar *from, size_t length,
                       CHARSET_INFO *fromcs, CHARSET_INFO *tocs);
   bool store_string_aux(const char *from, size_t length,

sql/share/errmsg-utf8.txt

@@ -4345,13 +4345,13 @@ ER_TOO_MANY_USER_CONNECTIONS 42000
 ER_SET_CONSTANTS_ONLY  
         dan "Du må kun bruge konstantudtryk med SET"
         nla "U mag alleen constante expressies gebruiken bij SET"
-        eng "You may only use constant expressions with SET"
+        eng "You may only use constant expressions in this statement"
         est "Ainult konstantsed suurused on lubatud SET klauslis"
         fre "Seules les expressions constantes sont autorisées avec SET"
-        ger "Bei SET dürfen nur konstante Ausdrücke verwendet werden"
+        ger "Bei diesem Befehl dürfen nur konstante Ausdrücke verwendet werden"
         ita "Si possono usare solo espressioni costanti con SET"
         por "Você pode usar apenas expressões constantes com SET"
-        rus "Вы можете использовать в SET только константные выражения"
+        rus "С этой командой вы можете использовать только константные выражения"
         serbian "Možete upotrebiti samo konstantan iskaz sa komandom 'SET'"
         spa "Tu solo debes usar expresiones constantes con SET"
         swe "Man kan endast använda konstantuttryck med SET"
@@ -6588,6 +6588,8 @@ ER_QUERY_EXCEEDED_ROWS_EXAMINED_LIMIT
 ER_NO_SUCH_TABLE_IN_ENGINE 42S02 
         eng "Table '%-.192s.%-.192s' doesn't exist in engine"
         swe "Det finns ingen tabell som heter '%-.192s.%-.192s' i handlern"
+ER_TARGET_NOT_EXPLAINABLE
+        eng "Target is not running an EXPLAINable command"
 ER_CONNECTION_ALREADY_EXISTS
         eng "Connection '%.*s' conflicts with existing connection '%.*s'"
 ER_MASTER_LOG_PREFIX

sql/sp_head.cc

@@ -218,6 +218,7 @@ sp_get_flags_for_command(LEX *lex)
   case SQLCOM_SHOW_CREATE_TRIGGER:
   case SQLCOM_SHOW_DATABASES:
   case SQLCOM_SHOW_ERRORS:
+  case SQLCOM_SHOW_EXPLAIN:
   case SQLCOM_SHOW_FIELDS:
   case SQLCOM_SHOW_FUNC_CODE:
   case SQLCOM_SHOW_GRANTS:

sql/sql_class.cc

@@ -1206,6 +1206,7 @@ void THD::init(void)
   /* Initialize the Debug Sync Facility. See debug_sync.cc. */
   debug_sync_init_thread(this);
 #endif /* defined(ENABLED_DEBUG_SYNC) */
+  apc_target.init(&LOCK_thd_data);
 }
 
  
@@ -1371,6 +1372,7 @@ void THD::cleanup(void)
     ull= NULL;
   }
 
+  apc_target.destroy();
   cleanup_done=1;
   DBUG_VOID_RETURN;
 }
@@ -2023,6 +2025,20 @@ CHANGED_TABLE_LIST* THD::changed_table_dup(const char *key, long key_length)
 int THD::send_explain_fields(select_result *result)
 {
   List<Item> field_list;
+  make_explain_field_list(field_list);
+  return (result->send_result_set_metadata(field_list,
+                                           Protocol::SEND_NUM_ROWS | 
+                                           Protocol::SEND_EOF));
+}
+
+
+/*
+  Populate the provided field_list with EXPLAIN output columns.
+  this->lex->describe has the EXPLAIN flags
+*/
+
+void THD::make_explain_field_list(List<Item> &field_list)
+{
   Item *item;
   CHARSET_INFO *cs= system_charset_info;
   field_list.push_back(item= new Item_return_int("id",3, MYSQL_TYPE_LONGLONG));
@@ -2061,10 +2077,9 @@ int THD::send_explain_fields(select_result *result)
   }
   item->maybe_null= 1;
   field_list.push_back(new Item_empty_string("Extra", 255, cs));
-  return (result->send_result_set_metadata(field_list,
-                                           Protocol::SEND_NUM_ROWS | Protocol::SEND_EOF));
 }
 
+
 #ifdef SIGNAL_WITH_VIO_CLOSE
 void THD::close_active_vio()
 {
@@ -2296,6 +2311,7 @@ int select_send::send_data(List<Item> &items)
   DBUG_RETURN(0);
 }
 
+
 bool select_send::send_eof()
 {
   /* 
@@ -3189,6 +3205,10 @@ void THD::end_statement()
 }
 
 
+/*
+  Start using arena specified by @set. Current arena data will be saved to
+  *backup.
+*/
 void THD::set_n_backup_active_arena(Query_arena *set, Query_arena *backup)
 {
   DBUG_ENTER("THD::set_n_backup_active_arena");
@@ -3203,6 +3223,12 @@ void THD::set_n_backup_active_arena(Query_arena *set, Query_arena *backup)
 }
 
 
+/*
+  Stop using the temporary arena, and start again using the arena that is 
+  specified in *backup.
+  The temporary arena is returned back into *set.
+*/
+
 void THD::restore_active_arena(Query_arena *set, Query_arena *backup)
 {
   DBUG_ENTER("THD::restore_active_arena");
@@ -3777,6 +3803,7 @@ void THD::restore_backup_open_tables_state(Open_tables_backup *backup)
   @retval 1 the user thread has been killed
 
   This is used to signal a storage engine if it should be killed.
+  See also THD::check_killed().
 */
 
 extern "C" int thd_killed(const MYSQL_THD thd)
@@ -3784,6 +3811,10 @@ extern "C" int thd_killed(const MYSQL_THD thd)
   if (!thd)
     thd= current_thd;
 
+  Apc_target *apc_target= (Apc_target*)&thd->apc_target;
+  if (apc_target->have_apc_requests())
+      apc_target->process_apc_requests(); 
+
   if (!(thd->killed & KILL_HARD_BIT))
     return 0;
   return thd->killed;

sql/sql_class.h

@@ -43,7 +43,7 @@
 #include "violite.h"              /* vio_is_connected */
 #include "thr_lock.h"             /* thr_lock_type, THR_LOCK_DATA,
                                      THR_LOCK_INFO */
-
+#include "my_apc.h"
 
 class Reprepare_observer;
 class Relay_log_info;
@@ -1529,6 +1529,11 @@ private:
 
 extern "C" void my_message_sql(uint error, const char *str, myf MyFlags);
 
+class THD;
+#ifndef DBUG_OFF
+void dbug_serve_apcs(THD *thd, int n_calls);
+#endif 
+
 /**
   @class THD
   For each client connection we create a separate thread with THD serving as
@@ -2198,6 +2203,16 @@ public:
   */
   killed_state volatile killed;
 
+  /* See also thd_killed() */
+  inline bool check_killed()
+  {
+    if (killed)
+      return TRUE;
+    if (apc_target.have_apc_requests())
+      apc_target.process_apc_requests(); 
+    return FALSE;
+  }
+
   /* scramble - random string sent to client on handshake */
   char	     scramble[SCRAMBLE_LENGTH+1];
 
@@ -2406,6 +2421,17 @@ public:
   /** Disconnect the associated communication endpoint. */
   void disconnect();
 
+
+  /*
+    Allows this thread to serve as a target for others to schedule Async 
+    Procedure Calls on.
+
+    It's possible to schedule any code to be executed this way, by
+    inheriting from the Apc_call object. Currently, only
+    Show_explain_request uses this.
+  */
+  Apc_target apc_target;
+
 #ifndef MYSQL_CLIENT
   enum enum_binlog_query_type {
     /* The query can be logged in row format or in statement format. */
@@ -2599,7 +2625,7 @@ public:
   void add_changed_table(const char *key, long key_length);
   CHANGED_TABLE_LIST * changed_table_dup(const char *key, long key_length);
   int send_explain_fields(select_result *result);
-
+  void make_explain_field_list(List<Item> &field_list);
   /**
     Clear the current error, if any.
     We do not clear is_fatal_error or is_fatal_sub_stmt_error since we
@@ -3240,10 +3266,42 @@ public:
 
 class JOIN;
 
-class select_result :public Sql_alloc {
+/* Pure interface for sending tabular data */
+class select_result_sink: public Sql_alloc
+{
+public:
+  /*
+    send_data returns 0 on ok, 1 on error and -1 if data was ignored, for
+    example for a duplicate row entry written to a temp table.
+  */
+  virtual int send_data(List<Item> &items)=0;
+  virtual ~select_result_sink() {};
+};
+
+
+/*
+  Interface for sending tabular data, together with some other stuff:
+
+  - Primary purpose seems to be seding typed tabular data:
+     = the DDL is sent with send_fields()
+     = the rows are sent with send_data()
+  Besides that,
+  - there seems to be an assumption that the sent data is a result of 
+    SELECT_LEX_UNIT *unit,
+  - nest_level is used by SQL parser
+*/
+
+class select_result :public select_result_sink 
+{
 protected:
   THD *thd;
+  /* 
+    All descendant classes have their send_data() skip the first 
+    unit->offset_limit_cnt rows sent.  Select_materialize
+    also uses unit->get_unit_column_types().
+  */
   SELECT_LEX_UNIT *unit;
+  /* Something used only by the parser: */
 public:
   select_result();
   virtual ~select_result() {};
@@ -3261,11 +3319,6 @@ public:
   virtual uint field_count(List<Item> &fields) const
   { return fields.elements; }
   virtual bool send_result_set_metadata(List<Item> &list, uint flags)=0;
-  /*
-    send_data returns 0 on ok, 1 on error and -1 if data was ignored, for
-    example for a duplicate row entry written to a temp table.
-  */
-  virtual int send_data(List<Item> &items)=0;
   virtual bool initialize_tables (JOIN *join=0) { return 0; }
   virtual void send_error(uint errcode,const char *err);
   virtual bool send_eof()=0;
@@ -3292,6 +3345,32 @@ public:
 };
 
 
+/*
+  This is a select_result_sink which simply writes all data into a (temporary)
+  table. Creation/deletion of the table is outside of the scope of the class
+  
+  It is aimed at capturing SHOW EXPLAIN output, so:
+  - Unlike select_result class, we don't assume that the sent data is an 
+    output of a SELECT_LEX_UNIT (and so we dont apply "LIMIT x,y" from the
+    unit)
+  - We don't try to convert the target table to MyISAM 
+*/
+
+class select_result_explain_buffer : public select_result_sink
+{
+public:
+  select_result_explain_buffer(THD *thd_arg, TABLE *table_arg) : 
+    thd(thd_arg), dst_table(table_arg) {};
+
+  THD *thd;
+  TABLE *dst_table; /* table to write into */
+
+  /* The following is called in the child thread: */
+  int send_data(List<Item> &items);
+};
+
+
+
 /*
   Base class for select_result descendands which intercept and
   transform result set rows. As the rows are not sent to the client,
@@ -3863,6 +3942,8 @@ class user_var_entry
   DTCollation collation;
 };
 
+user_var_entry *get_variable(HASH *hash, LEX_STRING &name,
+				    bool create_if_not_exists);
 
 /*
    Unique -- class for unique (removing of duplicates).

sql/sql_join_cache.cc

@@ -2236,7 +2236,7 @@ enum_nested_loop_state JOIN_CACHE::join_matching_records(bool skip_last)
   
   while (!(error= join_tab_scan->next()))   
   {
-    if (join->thd->killed)
+    if (join->thd->check_killed())
     {
       /* The user has aborted the execution of the query */
       join->thd->send_kill_message();
@@ -2506,7 +2506,7 @@ enum_nested_loop_state JOIN_CACHE::join_null_complements(bool skip_last)
 
   for ( ; cnt; cnt--)
   {
-    if (join->thd->killed)
+    if (join->thd->check_killed())
     {
       /* The user has aborted the execution of the query */
       join->thd->send_kill_message();
@@ -3356,7 +3356,7 @@ int JOIN_TAB_SCAN::next()
     update_virtual_fields(thd, table);
   while (!err && select && (skip_rc= select->skip_record(thd)) <= 0)
   {
-    if (thd->killed || skip_rc < 0) 
+    if (thd->check_killed() || skip_rc < 0) 
       return 1;
     /* 
       Move to the next record if the last retrieved record does not

sql/sql_lex.cc

@@ -1883,6 +1883,7 @@ void st_select_lex::init_query()
   nest_level= 0;
   link_next= 0;
   is_prep_leaf_list_saved= FALSE;
+  have_merged_subqueries= FALSE;
   bzero((char*) expr_cache_may_be_used, sizeof(expr_cache_may_be_used));
   m_non_agg_field_used= false;
   m_agg_func_used= false;
@@ -3474,7 +3475,7 @@ bool st_select_lex::optimize_unflattened_subqueries(bool const_only)
         if (options & SELECT_DESCRIBE)
         {
           /* Optimize the subquery in the context of EXPLAIN. */
-          sl->set_explain_type();
+          sl->set_explain_type(FALSE);
           sl->options|= SELECT_DESCRIBE;
           inner_join->select_options|= SELECT_DESCRIBE;
         }
@@ -3924,9 +3925,12 @@ void st_select_lex::update_correlated_cache()
 
 /**
   Set the EXPLAIN type for this subquery.
+  
+  @param on_the_fly  TRUE<=> We're running a SHOW EXPLAIN command, so we must 
+                     not change any variables
 */
 
-void st_select_lex::set_explain_type()
+void st_select_lex::set_explain_type(bool on_the_fly)
 {
   bool is_primary= FALSE;
   if (next_select())
@@ -3948,6 +3952,9 @@ void st_select_lex::set_explain_type()
     }
   }
 
+  if (on_the_fly && !is_primary && have_merged_subqueries)
+    is_primary= TRUE;
+
   SELECT_LEX *first= master_unit()->first_select();
   /* drop UNCACHEABLE_EXPLAIN, because it is for internal usage only */
   uint8 is_uncacheable= (uncacheable & ~UNCACHEABLE_EXPLAIN);
@@ -4000,9 +4007,14 @@ void st_select_lex::set_explain_type()
       else
       {
         type= is_uncacheable ? "UNCACHEABLE UNION": "UNION";
+        if (this == master_unit()->fake_select_lex)
+          type= "UNION RESULT";
+
       }
     }
   }
+
+  if (!on_the_fly)
     options|= SELECT_DESCRIBE;
 }
 
@@ -4150,6 +4162,116 @@ bool st_select_lex::is_merged_child_of(st_select_lex *ancestor)
 }
 
 
+int print_explain_message_line(select_result_sink *result, 
+                               SELECT_LEX *select_lex,
+                               bool on_the_fly,
+                               uint8 options,
+                               const char *message);
+
+
+int st_select_lex::print_explain(select_result_sink *output, 
+                                 uint8 explain_flags,
+                                 bool *printed_anything)
+{
+  int res;
+  if (join && join->have_query_plan == JOIN::QEP_AVAILABLE)
+  {
+    /*
+      There is a number of reasons join can be marked as degenerate, so all
+      three conditions below can happen simultaneously, or individually:
+    */
+    *printed_anything= TRUE;
+    if (!join->table_count || !join->tables_list || join->zero_result_cause)
+    {
+      /* It's a degenerate join */
+      const char *cause= join->zero_result_cause ? join-> zero_result_cause : 
+                                                   "No tables used";
+      res= join->print_explain(output, explain_flags, TRUE, FALSE, FALSE, 
+                               FALSE, cause);
+    }
+    else
+    {
+      res= join->print_explain(output, explain_flags, TRUE,
+                               join->need_tmp, // need_tmp_table
+                               !join->skip_sort_order && !join->no_order &&
+                               (join->order || join->group_list), // bool need_order
+                               join->select_distinct, // bool distinct
+                               NULL); //const char *message
+    }
+    if (res)
+      goto err;
+
+    for (SELECT_LEX_UNIT *unit= join->select_lex->first_inner_unit();
+         unit;
+         unit= unit->next_unit())
+    {
+      /* 
+        Display subqueries only if they are not parts of eliminated WHERE/ON
+        clauses.
+      */
+      if (!(unit->item && unit->item->eliminated))
+      {
+        if ((res= unit->print_explain(output, explain_flags, printed_anything)))
+          goto err;
+      }
+    }
+  }
+  else
+  {
+    const char *msg;
+    if (!join)
+      DBUG_ASSERT(0); /* Seems not to be possible */
+
+    /* Not printing anything useful, don't touch *printed_anything here */
+    if (join->have_query_plan == JOIN::QEP_NOT_PRESENT_YET)
+      msg= "Not yet optimized";
+    else
+    {
+      DBUG_ASSERT(join->have_query_plan == JOIN::QEP_DELETED);
+      msg= "Query plan already deleted";
+    }
+    res= print_explain_message_line(output, this, TRUE /* on_the_fly */,
+                                    0, msg);
+  }
+err:
+  return res;
+}
+
+
+int st_select_lex_unit::print_explain(select_result_sink *output, 
+                                      uint8 explain_flags, bool *printed_anything)
+{
+  int res= 0;
+  SELECT_LEX *first= first_select();
+  
+  if (first && !first->next_select() && !first->join)
+  {
+    /*
+      If there is only one child, 'first', and it has join==NULL, emit "not in
+      EXPLAIN state" error.
+    */
+    const char *msg="Query plan already deleted";
+    res= print_explain_message_line(output, first, TRUE /* on_the_fly */,
+                                    0, msg);
+    return 0;
+  }
+
+  for (SELECT_LEX *sl= first; sl; sl= sl->next_select())
+  {
+    if ((res= sl->print_explain(output, explain_flags, printed_anything)))
+      break;
+  }
+
+  /* Note: fake_select_lex->join may be NULL or non-NULL at this point */
+  if (fake_select_lex)
+  {
+    res= print_fake_select_lex_join(output, TRUE /* on the fly */,
+                                    fake_select_lex, explain_flags);
+  }
+  return res;
+}
+
+
 /**
   A routine used by the parser to decide whether we are specifying a full
   partitioning or if only partitions to add or to split.

sql/sql_lex.h

@@ -194,6 +194,7 @@ enum enum_sql_command {
   SQLCOM_SHOW_RELAYLOG_EVENTS, 
   SQLCOM_SHOW_USER_STATS, SQLCOM_SHOW_TABLE_STATS, SQLCOM_SHOW_INDEX_STATS,
   SQLCOM_SHOW_CLIENT_STATS,
+  SQLCOM_SHOW_EXPLAIN,
 
   /*
     When a command is added here, be sure it's also added in mysqld.cc
@@ -357,6 +358,8 @@ typedef uchar index_clause_map;
 #define INDEX_HINT_MASK_ALL (INDEX_HINT_MASK_JOIN | INDEX_HINT_MASK_GROUP | \
                              INDEX_HINT_MASK_ORDER)
 
+class select_result_sink;
+
 /* Single element of an USE/FORCE/IGNORE INDEX list specified as a SQL hint  */
 class Index_hint : public Sql_alloc
 {
@@ -717,6 +720,8 @@ public:
   friend int subselect_union_engine::exec();
 
   List<Item> *get_unit_column_types();
+  int print_explain(select_result_sink *output, uint8 explain_flags,
+                    bool *printed_anything);
 };
 
 typedef class st_select_lex_unit SELECT_LEX_UNIT;
@@ -778,6 +783,12 @@ public:
   */
   List<Item_in_subselect> sj_subselects;
   
+  /*
+    Needed to correctly generate 'PRIMARY' or 'SIMPLE' for select_type column
+    of EXPLAIN
+  */
+  bool have_merged_subqueries;
+
   List<TABLE_LIST> leaf_tables;
   List<TABLE_LIST> leaf_tables_exec;
   List<TABLE_LIST> leaf_tables_prep;
@@ -1001,7 +1012,7 @@ public:
   bool is_part_of_union() { return master_unit()->is_union(); }
   bool optimize_unflattened_subqueries(bool const_only);
   /* Set the EXPLAIN type for this subquery. */
-  void set_explain_type();
+  void set_explain_type(bool on_the_fly);
   bool handle_derived(LEX *lex, uint phases);
   void append_table_to_list(TABLE_LIST *TABLE_LIST::*link, TABLE_LIST *table);
   bool get_free_table_map(table_map *map, uint *tablenr);
@@ -1025,8 +1036,10 @@ public:
 
   bool save_leaf_tables(THD *thd);
   bool save_prep_leaf_tables(THD *thd);
-  bool is_merged_child_of(st_select_lex *ancestor);
 
+  bool is_merged_child_of(st_select_lex *ancestor);
+  int print_explain(select_result_sink *output, uint8 explain_flags, 
+                    bool *printed_anything);
   /*
     For MODE_ONLY_FULL_GROUP_BY we need to maintain two flags:
      - Non-aggregated fields are used in this select.
@@ -2348,7 +2361,7 @@ struct LEX: public Query_tables_list
   char *backup_dir;				/* For RESTORE/BACKUP */
   char* to_log;                                 /* For PURGE MASTER LOGS TO */
   char* x509_subject,*x509_issuer,*ssl_cipher;
-  String *wild;
+  String *wild; /* Wildcard in SHOW {something} LIKE 'wild'*/ 
   sql_exchange *exchange;
   select_result *result;
   Item *default_value, *on_update_value;

sql/sql_parse.cc

@@ -336,6 +336,7 @@ void init_update_queries(void)
   sql_command_flags[SQLCOM_SHOW_ENGINE_STATUS]= CF_STATUS_COMMAND;
   sql_command_flags[SQLCOM_SHOW_ENGINE_MUTEX]= CF_STATUS_COMMAND;
   sql_command_flags[SQLCOM_SHOW_ENGINE_LOGS]= CF_STATUS_COMMAND;
+  sql_command_flags[SQLCOM_SHOW_EXPLAIN]= CF_STATUS_COMMAND;
   sql_command_flags[SQLCOM_SHOW_PROCESSLIST]= CF_STATUS_COMMAND;
   sql_command_flags[SQLCOM_SHOW_GRANTS]=      CF_STATUS_COMMAND;
   sql_command_flags[SQLCOM_SHOW_CREATE_DB]=   CF_STATUS_COMMAND;
@@ -2175,6 +2176,33 @@ mysql_execute_command(THD *thd)
     execute_show_status(thd, all_tables);
     break;
   }
+  case SQLCOM_SHOW_EXPLAIN:
+  {
+    if (!thd->security_ctx->priv_user[0] &&
+        check_global_access(thd,PROCESS_ACL))
+      break;
+
+    /*
+      The select should use only one table, it's the SHOW EXPLAIN pseudo-table
+    */
+    if (lex->sroutines.records || lex->query_tables->next_global)
+    {
+      my_message(ER_SET_CONSTANTS_ONLY, ER(ER_SET_CONSTANTS_ONLY),
+		 MYF(0));
+      goto error;
+    }
+
+    Item **it= lex->value_list.head_ref();
+    if (!(*it)->basic_const_item() ||
+        (!(*it)->fixed && (*it)->fix_fields(lex->thd, it)) || 
+        (*it)->check_cols(1))
+    {
+      my_message(ER_SET_CONSTANTS_ONLY, ER(ER_SET_CONSTANTS_ONLY),
+		 MYF(0));
+      goto error;
+    }
+    /* no break; fall through */
+  }
   case SQLCOM_SHOW_DATABASES:
   case SQLCOM_SHOW_TABLES:
   case SQLCOM_SHOW_TRIGGERS:
@@ -6666,6 +6694,35 @@ void add_join_natural(TABLE_LIST *a, TABLE_LIST *b, List<String> *using_fields,
 }
 
 
+/**
+  Find a thread by id and return it, locking it LOCK_thd_data
+
+  @param id  Identifier of the thread we're looking for
+
+  @return NULL    - not found
+          pointer - thread found, and its LOCK_thd_data is locked.
+*/
+
+THD *find_thread_by_id(ulong id)
+{
+  THD *tmp;
+  mysql_mutex_lock(&LOCK_thread_count); // For unlink from list
+  I_List_iterator<THD> it(threads);
+  while ((tmp=it++))
+  {
+    if (tmp->command == COM_DAEMON)
+      continue;
+    if (tmp->thread_id == id)
+    {
+      mysql_mutex_lock(&tmp->LOCK_thd_data);    // Lock from delete
+      break;
+    }
+  }
+  mysql_mutex_unlock(&LOCK_thread_count);
+  return tmp;
+}
+
+
 /**
   kill on thread.
 
@@ -6684,20 +6741,7 @@ uint kill_one_thread(THD *thd, ulong id, killed_state kill_signal)
   DBUG_ENTER("kill_one_thread");
   DBUG_PRINT("enter", ("id: %lu  signal: %u", id, (uint) kill_signal));
 
-  mysql_mutex_lock(&LOCK_thread_count); // For unlink from list
+  if ((tmp= find_thread_by_id(id)))
-  I_List_iterator<THD> it(threads);
-  while ((tmp=it++))
-  {
-    if (tmp->command == COM_DAEMON)
-      continue;
-    if (tmp->thread_id == id)
-    {
-      mysql_mutex_lock(&tmp->LOCK_thd_data);    // Lock from delete
-      break;
-    }
-  }
-  mysql_mutex_unlock(&LOCK_thread_count);
-  if (tmp)
   {
     /*
       If we're SUPER, we can KILL anything, including system-threads.

sql/sql_prepare.cc

@@ -2134,6 +2134,7 @@ static bool check_prepared_statement(Prepared_statement *stmt)
       Note that we don't need to have cases in this list if they are
       marked with CF_STATUS_COMMAND in sql_command_flags
     */
+  case SQLCOM_SHOW_EXPLAIN:
   case SQLCOM_DROP_TABLE:
   case SQLCOM_RENAME_TABLE:
   case SQLCOM_ALTER_TABLE:

sql/sql_priv.h

@@ -331,6 +331,7 @@ enum enum_yes_no_unknown
   External variables
 */
 
+
 /* sql_yacc.cc */
 #ifndef DBUG_OFF
 extern void turn_parser_debug_on();

sql/sql_select.cc

@@ -277,6 +277,57 @@ JOIN_TAB *first_breadth_first_tab(JOIN *join, enum enum_exec_or_opt tabs_kind);
 JOIN_TAB *next_breadth_first_tab(JOIN *join, enum enum_exec_or_opt tabs_kind,
                                  JOIN_TAB *tab);
 
+#ifndef DBUG_OFF
+
+/*
+  SHOW EXPLAIN testing: wait for, and serve n_calls APC requests.
+*/
+void dbug_serve_apcs(THD *thd, int n_calls)
+{
+  const char *save_proc_info= thd->proc_info;
+  /* This is so that mysqltest knows we're ready to serve requests: */
+  thd_proc_info(thd, "show_explain_trap");
+  
+  /* Busy-wait for n_calls APC requests to arrive and be processed */
+  int n_apcs= thd->apc_target.n_calls_processed + n_calls;
+  while (thd->apc_target.n_calls_processed < n_apcs)
+  {
+    my_sleep(300);
+    if (thd->check_killed())
+      break;
+  }
+  thd_proc_info(thd, save_proc_info);
+}
+
+
+/*
+  Debugging: check if @name=value, comparing as integer
+
+  Intended usage:
+  
+  DBUG_EXECUTE_IF("show_explain_probe_2", 
+                     if (dbug_user_var_equals_int(thd, "select_id", select_id)) 
+                        dbug_serve_apcs(thd, 1);
+                 );
+
+*/
+
+bool dbug_user_var_equals_int(THD *thd, const char *name, int value)
+{
+  user_var_entry *var;
+  LEX_STRING varname= {(char*)name, strlen(name)};
+  if ((var= get_variable(&thd->user_vars, varname, FALSE)))
+  {
+    bool null_value;
+    longlong var_value= var->val_int(&null_value);
+    if (!null_value && var_value == value)
+      return TRUE;
+  }
+  return FALSE;
+}
+#endif 
+
+
 /**
   This handles SELECT with and without UNION.
 */
@@ -939,6 +990,19 @@ err:
 }
 
 
+int JOIN::optimize()
+{
+  int res= optimize_inner();
+  /*
+    If we're inside a non-correlated subquery, this function may be 
+    called for the second time after the subquery has been executed
+    and deleted. The second call will not produce a valid query plan, it will
+    short-circuit because optimized==TRUE.
+  */
+  if (!res && have_query_plan != QEP_DELETED)
+    have_query_plan= QEP_AVAILABLE;
+  return res;
+}
 /**
   global select optimisation.
 
@@ -952,7 +1016,7 @@ err:
 */
 
 int
-JOIN::optimize()
+JOIN::optimize_inner()
 {
   ulonglong select_opts_for_readinfo;
   uint no_jbuf_after;
@@ -2176,6 +2240,32 @@ JOIN::save_join_tab()
 }
 
 
+void JOIN::exec()
+{
+  /*
+    Enable SHOW EXPLAIN only if we're in the top-level query.
+  */
+  thd->apc_target.enable();
+  DBUG_EXECUTE_IF("show_explain_probe_join_exec_start", 
+                  if (dbug_user_var_equals_int(thd, 
+                                               "show_explain_probe_select_id", 
+                                               select_lex->select_number))
+                        dbug_serve_apcs(thd, 1);
+                 );
+
+  exec_inner();
+
+  DBUG_EXECUTE_IF("show_explain_probe_join_exec_end", 
+                  if (dbug_user_var_equals_int(thd, 
+                                               "show_explain_probe_select_id", 
+                                               select_lex->select_number))
+                        dbug_serve_apcs(thd, 1);
+                 );
+
+  thd->apc_target.disable();
+}
+
+
 /**
   Exec select.
 
@@ -2187,8 +2277,8 @@ JOIN::save_join_tab()
   @todo
     When can we have here thd->net.report_error not zero?
 */
-void
+
-JOIN::exec()
+void JOIN::exec_inner()
 {
   List<Item> *columns_list= &fields_list;
   int      tmp_error;
@@ -2494,6 +2584,10 @@ JOIN::exec()
       DBUG_PRINT("info",("Creating group table"));
       
       /* Free first data from old join */
+      
+      /*
+        psergey-todo: this is the place of pre-mature JOIN::free call.
+      */
       curr_join->join_free();
       if (curr_join->make_simple_join(this, curr_tmp_table))
 	DBUG_VOID_RETURN;
@@ -3766,7 +3860,7 @@ make_join_statistics(JOIN *join, List<TABLE_LIST> &tables_list,
     goto error;
 
   /* Generate an execution plan from the found optimal join order. */
-  DBUG_RETURN(join->thd->killed || get_best_combination(join));
+  DBUG_RETURN(join->thd->check_killed() || get_best_combination(join));
 
 error:
   /*
@@ -6840,7 +6934,7 @@ best_extension_by_limited_search(JOIN      *join,
   DBUG_ENTER("best_extension_by_limited_search");
 
   THD *thd= join->thd;
-  if (thd->killed)  // Abort
+  if (thd->check_killed())  // Abort
     DBUG_RETURN(TRUE);
 
   DBUG_EXECUTE("opt", print_plan(join, idx, read_time, record_count, idx,
@@ -6997,7 +7091,7 @@ find_best(JOIN *join,table_map rest_tables,uint idx,double record_count,
 {
   DBUG_ENTER("find_best");
   THD *thd= join->thd;
-  if (thd->killed)
+  if (thd->check_killed())
     DBUG_RETURN(TRUE);
   if (!rest_tables)
   {
@@ -7693,6 +7787,7 @@ get_best_combination(JOIN *join)
   join->table_access_tabs= join->join_tab;
   join->top_table_access_tabs_count= join->top_join_tab_count;
 
+
   update_depend_map(join);
   DBUG_RETURN(0);
 }
@@ -7936,6 +8031,7 @@ static bool create_ref_for_key(JOIN *join, JOIN_TAB *j,
   j->ref.null_rejecting= 0;
   j->ref.disable_cache= FALSE;
   j->ref.null_ref_part= NO_REF_PART;
+  j->ref.const_ref_part_map= 0;
   keyuse=org_keyuse;
 
   store_key **ref_key= j->ref.key_copy;
@@ -7971,6 +8067,13 @@ static bool create_ref_for_key(JOIN *join, JOIN_TAB *j,
       if (keyuse->null_rejecting) 
         j->ref.null_rejecting |= 1 << i;
       keyuse_uses_no_tables= keyuse_uses_no_tables && !keyuse->used_tables;
+      /*
+        Todo: we should remove this check for thd->lex->describe on the next
+        line. With SHOW EXPLAIN code, EXPLAIN printout code no longer depends
+        on it. However, removing the check caused change in lots of query
+        plans! Does the optimizer depend on the contents of
+        table_ref->key_copy ? If yes, do we produce incorrect EXPLAINs? 
+      */
       if (!keyuse->val->used_tables() && !thd->lex->describe)
       {					// Compare against constant
 	store_key_item tmp(thd, 
@@ -7983,6 +8086,7 @@ static bool create_ref_for_key(JOIN *join, JOIN_TAB *j,
 	if (thd->is_fatal_error)
 	  DBUG_RETURN(TRUE);
 	tmp.copy();
+        j->ref.const_ref_part_map |= key_part_map(1) << i ;
       }
       else
 	*ref_key++= get_store_key(thd,
@@ -8094,6 +8198,7 @@ JOIN::make_simple_join(JOIN *parent, TABLE *temp_table)
       !(parent->join_tab_reexec= (JOIN_TAB*) thd->alloc(sizeof(JOIN_TAB))))
     DBUG_RETURN(TRUE);                        /* purecov: inspected */
 
+  // psergey-todo: here, save the pointer for original join_tabs.
   join_tab= parent->join_tab_reexec;
   table= &parent->table_reexec[0]; parent->table_reexec[0]= temp_table;
   table_count= top_join_tab_count= 1;
@@ -10357,7 +10462,7 @@ ha_rows JOIN_TAB::get_examined_rows()
 {
   ha_rows examined_rows;
 
-  if (select && select->quick)
+  if (select && select->quick && use_quick != 2)
     examined_rows= select->quick->records;
   else if (type == JT_NEXT || type == JT_ALL ||
            type == JT_HASH || type ==JT_HASH_NEXT)
@@ -10411,9 +10516,18 @@ bool JOIN_TAB::preread_init()
       mysql_handle_single_derived(join->thd->lex,
                                     derived, DT_CREATE | DT_FILL))
       return TRUE;
+
   preread_init_done= TRUE;
   if (select && select->quick)
     select->quick->replace_handler(table->file);
+
+  DBUG_EXECUTE_IF("show_explain_probe_join_tab_preread", 
+                  if (dbug_user_var_equals_int(join->thd, 
+                                               "show_explain_probe_select_id", 
+                                               join->select_lex->select_number))
+                        dbug_serve_apcs(join->thd, 1);
+                 );
+
   return FALSE;
 }
 
@@ -10622,6 +10736,8 @@ void JOIN::cleanup(bool full)
   DBUG_ENTER("JOIN::cleanup");
   DBUG_PRINT("enter", ("full %u", (uint) full));
   
+  have_query_plan= QEP_DELETED;
+
   if (table)
   {
     JOIN_TAB *tab;
@@ -10641,10 +10757,23 @@ void JOIN::cleanup(bool full)
 
     if (full)
     {
+      JOIN_TAB *sort_tab= first_linear_tab(this, WITHOUT_CONST_TABLES);
+      if (pre_sort_join_tab)
+      {
+        if (sort_tab && sort_tab->select == pre_sort_join_tab->select)
+        {
+          pre_sort_join_tab->select= NULL;
+        }
+        else
+          clean_pre_sort_join_tab();
+      }
+
       for (tab= first_linear_tab(this, WITH_CONST_TABLES); tab; 
            tab= next_linear_tab(this, tab, WITH_BUSH_ROOTS))
+      {
 	tab->cleanup();
       }
+    }
     else
     {
       for (tab= first_linear_tab(this, WITH_CONST_TABLES); tab; 
@@ -15539,7 +15668,7 @@ create_internal_tmp_table_from_heap2(THD *thd, TABLE *table,
     DBUG_EXECUTE_IF("raise_error", write_err= HA_ERR_FOUND_DUPP_KEY ;);
     if (write_err)
       goto err;
-    if (thd->killed)
+    if (thd->check_killed())
     {
       thd->send_kill_message();
       goto err_killed;
@@ -15798,6 +15927,14 @@ do_select(JOIN *join,List<Item> *fields,TABLE *table,Procedure *procedure)
   else
   {
     DBUG_ASSERT(join->table_count);
+
+    DBUG_EXECUTE_IF("show_explain_probe_do_select", 
+                    if (dbug_user_var_equals_int(join->thd, 
+                                                 "show_explain_probe_select_id", 
+                                                 join->select_lex->select_number))
+                          dbug_serve_apcs(join->thd, 1);
+                   );
+
     if (join->outer_ref_cond && !join->outer_ref_cond->val_int())
       error= NESTED_LOOP_NO_MORE_ROWS;
     else
@@ -15928,7 +16065,7 @@ sub_select_cache(JOIN *join, JOIN_TAB *join_tab, bool end_of_records)
       rc= sub_select(join, join_tab, end_of_records);
     DBUG_RETURN(rc);
   }
-  if (join->thd->killed)
+  if (join->thd->check_killed())
   {
     /* The user has aborted the execution of the query */
     join->thd->send_kill_message();
@@ -16215,7 +16352,7 @@ evaluate_join_record(JOIN *join, JOIN_TAB *join_tab,
     DBUG_RETURN(NESTED_LOOP_ERROR);
   if (error < 0)
     DBUG_RETURN(NESTED_LOOP_NO_MORE_ROWS);
-  if (join->thd->killed)			// Aborted by user
+  if (join->thd->check_killed())			// Aborted by user
   {
     join->thd->send_kill_message();
     DBUG_RETURN(NESTED_LOOP_KILLED);            /* purecov: inspected */
@@ -16960,6 +17097,14 @@ int read_first_record_seq(JOIN_TAB *tab)
 static int
 test_if_quick_select(JOIN_TAB *tab)
 {
+  DBUG_EXECUTE_IF("show_explain_probe_test_if_quick_select", 
+                  if (dbug_user_var_equals_int(tab->join->thd, 
+                                               "show_explain_probe_select_id", 
+                                               tab->join->select_lex->select_number))
+                        dbug_serve_apcs(tab->join->thd, 1);
+                 );
+
+
   delete tab->select->quick;
   tab->select->quick=0;
   return tab->select->test_quick_select(tab->join->thd, tab->keys,
@@ -17438,7 +17583,7 @@ end_write(JOIN *join, JOIN_TAB *join_tab __attribute__((unused)),
     }
   }
 end:
-  if (join->thd->killed)
+  if (join->thd->check_killed())
   {
     join->thd->send_kill_message();
     DBUG_RETURN(NESTED_LOOP_KILLED);             /* purecov: inspected */
@@ -17521,7 +17666,7 @@ end_update(JOIN *join, JOIN_TAB *join_tab __attribute__((unused)),
   }
   join->send_records++;
 end:
-  if (join->thd->killed)
+  if (join->thd->check_killed())
   {
     join->thd->send_kill_message();
     DBUG_RETURN(NESTED_LOOP_KILLED);             /* purecov: inspected */
@@ -17571,7 +17716,7 @@ end_unique_update(JOIN *join, JOIN_TAB *join_tab __attribute__((unused)),
       DBUG_RETURN(NESTED_LOOP_ERROR);            /* purecov: inspected */
     }
   }
-  if (join->thd->killed)
+  if (join->thd->check_killed())
   {
     join->thd->send_kill_message();
     DBUG_RETURN(NESTED_LOOP_KILLED);             /* purecov: inspected */
@@ -17649,7 +17794,7 @@ end_write_group(JOIN *join, JOIN_TAB *join_tab __attribute__((unused)),
   if (join->procedure)
     join->procedure->add();
 end:
-  if (join->thd->killed)
+  if (join->thd->check_killed())
   {
     join->thd->send_kill_message();
     DBUG_RETURN(NESTED_LOOP_KILLED);             /* purecov: inspected */
@@ -18907,6 +19052,8 @@ create_sort_index(THD *thd, JOIN *join, ORDER *order,
   TABLE *table;
   SQL_SELECT *select;
   JOIN_TAB *tab;
+  int err= 0;
+  bool quick_created= FALSE;
   DBUG_ENTER("create_sort_index");
 
   if (join->table_count == join->const_tables)
@@ -18915,17 +19062,46 @@ create_sort_index(THD *thd, JOIN *join, ORDER *order,
   table=  tab->table;
   select= tab->select;
   
+  JOIN_TAB *save_pre_sort_join_tab= NULL;
+  if (join->pre_sort_join_tab)
+  {
+    /*
+      we've already been in this function, and stashed away the original access 
+      method in join->pre_sort_join_tab, restore it now.
+    */
+    
+    /* First, restore state of the handler */
+    if (join->pre_sort_index != MAX_KEY)
+    {
+      if (table->file->ha_index_or_rnd_end())
+        goto err;
+      if (join->pre_sort_idx_pushed_cond)
+      {
+        table->file->idx_cond_push(join->pre_sort_index,
+                                 join->pre_sort_idx_pushed_cond);
+      }
+    }
+    else
+    {
+      if (table->file->ha_index_or_rnd_end() || 
+          table->file->ha_rnd_init(TRUE))
+        goto err;
+    }
+
+    /* Second, restore access method parameters */
+    tab->records=           join->pre_sort_join_tab->records;
+    tab->select=            join->pre_sort_join_tab->select;
+    tab->select_cond=       join->pre_sort_join_tab->select_cond;
+    tab->type=              join->pre_sort_join_tab->type;
+    tab->read_first_record= join->pre_sort_join_tab->read_first_record; 
+
+    save_pre_sort_join_tab= join->pre_sort_join_tab;
+    join->pre_sort_join_tab= NULL;
+  }
+
   /* Currently ORDER BY ... LIMIT is not supported in subqueries. */
   DBUG_ASSERT(join->group_list || !join->is_in_subquery());
 
-  /* 
-    If we have a select->quick object that is created outside of
-    create_sort_index() and this is part of a subquery that
-    potentially can be executed multiple times then we should not
-    delete the quick object on exit from this function.
-  */
-  bool keep_quick= select && select->quick && join->join_tab_save;
-
   /*
     When there is SQL_BIG_RESULT do not sort using index for GROUP BY,
     and thus force sorting on disk unless a group min-max optimization
@@ -18977,7 +19153,7 @@ create_sort_index(THD *thd, JOIN *join, ORDER *order,
 			    get_quick_select_for_ref(thd, table, &tab->ref, 
                                                      tab->found_records))))
 	goto err;
-      DBUG_ASSERT(!keep_quick);
+      quick_created= TRUE;
     }
   }
 
@@ -18995,6 +19171,37 @@ create_sort_index(THD *thd, JOIN *join, ORDER *order,
                             &examined_rows, &found_rows);
   table->sort.found_records= filesort_retval;
   tab->records= found_rows;                     // For SQL_CALC_ROWS
+
+  if (quick_created)
+  {
+    /* This will delete the quick select. */
+    select->cleanup();
+  }
+
+  if (!join->pre_sort_join_tab)
+  {
+    if (save_pre_sort_join_tab)
+      join->pre_sort_join_tab= save_pre_sort_join_tab;
+    else if (!(join->pre_sort_join_tab= (JOIN_TAB*)thd->alloc(sizeof(JOIN_TAB))))
+      goto err;
+  }
+
+  *(join->pre_sort_join_tab)= *tab;
+  
+  if (table->file->inited == handler::INDEX)
+  {
+    // Save index #, save index condition
+    join->pre_sort_index= table->file->active_index;
+    join->pre_sort_idx_pushed_cond= table->file->pushed_idx_cond;
+    // no need to save key_read? 
+    err= table->file->ha_index_end();
+  }
+  else
+    join->pre_sort_index= MAX_KEY;
+
+  /*TODO: here, close the index scan, cancel index-only read. */
+#if 0 
+  /* MariaDB doesn't need the following: */
   if (select)
   {
     /*
@@ -19011,6 +19218,7 @@ create_sort_index(THD *thd, JOIN *join, ORDER *order,
     tablesort_result_cache= table->sort.io_cache;
     table->sort.io_cache= NULL;
 
+   // select->cleanup();				// filesort did select
     /*
       If a quick object was created outside of create_sort_index()
       that might be reused, then do not call select->cleanup() since
@@ -19033,18 +19241,60 @@ create_sort_index(THD *thd, JOIN *join, ORDER *order,
     // Restore the output resultset
     table->sort.io_cache= tablesort_result_cache;
   }
+#endif
+  tab->select=NULL;
   tab->set_select_cond(NULL, __LINE__);
-  tab->last_inner= 0;
+//  tab->last_inner= 0;
-  tab->first_unmatched= 0;
+//  tab->first_unmatched= 0;
   tab->type=JT_ALL;				// Read with normal read_record
   tab->read_first_record= join_init_read_record;
+  tab->table->file->ha_index_or_rnd_end();
+  
+  if (err)
+    goto err;
+
   tab->join->examined_rows+=examined_rows;
-  table->disable_keyread(); // Restore if we used indexes
   DBUG_RETURN(filesort_retval == HA_POS_ERROR);
 err:
   DBUG_RETURN(-1);
 }
 
+void JOIN::clean_pre_sort_join_tab()
+{
+  //TABLE *table=  pre_sort_join_tab->table;
+  /*
+   Note: we can come here for fake_select_lex object. That object will have
+   the table already deleted by st_select_lex_unit::cleanup().  
+    We rely on that fake_select_lex didn't have quick select.
+  */
+#if 0  
+  if (pre_sort_join_tab->select && pre_sort_join_tab->select->quick)
+  {
+    /*
+      We need to preserve tablesort's output resultset here, because
+      QUICK_INDEX_MERGE_SELECT::~QUICK_INDEX_MERGE_SELECT (called by
+      SQL_SELECT::cleanup()) may free it assuming it's the result of the quick
+      select operation that we no longer need. Note that all the other parts of
+      this data structure are cleaned up when
+      QUICK_INDEX_MERGE_SELECT::get_next encounters end of data, so the next
+      SQL_SELECT::cleanup() call changes sort.io_cache alone.
+    */
+    IO_CACHE *tablesort_result_cache;
+
+    tablesort_result_cache= table->sort.io_cache;
+    table->sort.io_cache= NULL;
+    pre_sort_join_tab->select->cleanup();
+    table->quick_keys.clear_all();  // as far as we cleanup select->quick
+    table->intersect_keys.clear_all();
+    table->sort.io_cache= tablesort_result_cache;
+  }
+#endif
+  //table->disable_keyread(); // Restore if we used indexes
+  if (pre_sort_join_tab->select && pre_sort_join_tab->select->quick)
+  {
+    pre_sort_join_tab->select->cleanup();
+  }
+}
 /*****************************************************************************
   Remove duplicates from tmp table
   This should be recoded to add a unique index to the table and remove
@@ -19163,7 +19413,7 @@ static int remove_dup_with_compare(THD *thd, TABLE *table, Field **first_field,
   error= file->ha_rnd_next(record);
   for (;;)
   {
-    if (thd->killed)
+    if (thd->check_killed())
     {
       thd->send_kill_message();
       error=0;
@@ -19295,7 +19545,7 @@ static int remove_dup_with_hash_index(THD *thd, TABLE *table,
   for (;;)
   {
     uchar *org_key_pos;
-    if (thd->killed)
+    if (thd->check_killed())
     {
       thd->send_kill_message();
       error=0;
@@ -21317,53 +21567,50 @@ void JOIN::clear()
   }
 }
 
-/**
-  EXPLAIN handling.
-
-  Send a description about what how the select will be done to stdout.
-*/
-
-static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
-			    bool distinct,const char *message)
-{
-  List<Item> field_list;
-  List<Item> item_list;
-  THD *thd=join->thd;
-  select_result *result=join->result;
-  Item *item_null= new Item_null();
-  CHARSET_INFO *cs= system_charset_info;
-  int quick_type;
-  DBUG_ENTER("select_describe");
-  DBUG_PRINT("info", ("Select 0x%lx, type %s, message %s",
-		      (ulong)join->select_lex, join->select_lex->type,
-		      message ? message : "NULL"));
-  /* Don't log this into the slow query log */
-  thd->server_status&= ~(SERVER_QUERY_NO_INDEX_USED | SERVER_QUERY_NO_GOOD_INDEX_USED);
-  join->unit->offset_limit_cnt= 0;
 
 /*
-    NOTE: the number/types of items pushed into item_list must be in sync with
+  Print an EXPLAIN line with all NULLs and given message in the 'Extra' column
-    EXPLAIN column types as they're "defined" in THD::send_explain_fields()
 */
-  if (message)
+int print_explain_message_line(select_result_sink *result, 
+                               SELECT_LEX *select_lex,
+                               bool on_the_fly,
+                               uint8 options,
+                               const char *message)
 {
+  const CHARSET_INFO *cs= system_charset_info;
+  Item *item_null= new Item_null();
+  List<Item> item_list;
+
+  if (on_the_fly)
+    select_lex->set_explain_type(on_the_fly);
+
   item_list.push_back(new Item_int((int32)
-				     join->select_lex->select_number));
+                                   select_lex->select_number));
-    item_list.push_back(new Item_string(join->select_lex->type,
+  item_list.push_back(new Item_string(select_lex->type,
-					strlen(join->select_lex->type), cs));
+                                      strlen(select_lex->type), cs));
   for (uint i=0 ; i < 7; i++)
     item_list.push_back(item_null);
-    if (join->thd->lex->describe & DESCRIBE_PARTITIONS)
+  if (options & DESCRIBE_PARTITIONS)
     item_list.push_back(item_null);
-    if (join->thd->lex->describe & DESCRIBE_EXTENDED)
+  if (options & DESCRIBE_EXTENDED)
     item_list.push_back(item_null);
 
   item_list.push_back(new Item_string(message,strlen(message),cs));
+
   if (result->send_data(item_list))
-      join->error= 1;
+    return 1;
+  return 0;
 }
-  else if (join->select_lex == join->unit->fake_select_lex)
+
+
+int print_fake_select_lex_join(select_result_sink *result, bool on_the_fly,
+                               SELECT_LEX *select_lex, uint8 explain_flags)
 {
+  const CHARSET_INFO *cs= system_charset_info;
+  Item *item_null= new Item_null();
+  List<Item> item_list;
+  if (on_the_fly)
+    select_lex->set_explain_type(on_the_fly);
   /* 
     here we assume that the query will return at least two rows, so we
     show "filesort" in EXPLAIN. Of course, sometimes we'll be wrong
@@ -21376,12 +21623,12 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
   /* id */
   item_list.push_back(new Item_null);
   /* select_type */
-    item_list.push_back(new Item_string(join->select_lex->type,
+  item_list.push_back(new Item_string(select_lex->type,
-					strlen(join->select_lex->type),
+                                      strlen(select_lex->type),
                                       cs));
   /* table */
   {
-      SELECT_LEX *sl= join->unit->first_select();
+    SELECT_LEX *sl= select_lex->master_unit()->first_select();
     uint len= 6, lastop= 0;
     memcpy(table_name_buffer, STRING_WITH_LEN("<union"));
     for (; sl && len + lastop + 5 < NAME_LEN; sl= sl->next_select())
@@ -21403,7 +21650,7 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
     item_list.push_back(new Item_string(table_name_buffer, len, cs));
   }
   /* partitions */
-    if (join->thd->lex->describe & DESCRIBE_PARTITIONS)
+  if (explain_flags & DESCRIBE_PARTITIONS)
     item_list.push_back(item_null);
   /* type */
   item_list.push_back(new Item_string(join_type_str[JT_ALL],
@@ -21418,27 +21665,87 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
   /* ref */
   item_list.push_back(item_null);
   /* in_rows */
-    if (join->thd->lex->describe & DESCRIBE_EXTENDED)
+  if (explain_flags & DESCRIBE_EXTENDED)
     item_list.push_back(item_null);
   /* rows */
   item_list.push_back(item_null);
   /* extra */
-    if (join->unit->global_parameters->order_list.first)
+  if (select_lex->master_unit()->global_parameters->order_list.first)
     item_list.push_back(new Item_string("Using filesort",
                                         14, cs));
   else
     item_list.push_back(new Item_string("", 0, cs));
 
   if (result->send_data(item_list))
-      join->error= 1;
+    return 1;
+  return 0;
+}
+
+
+/**
+  EXPLAIN handling.
+
+  Produce lines explaining execution of *this* select (not including children
+  selects)
+  @param on_the_fly TRUE <=> we're being executed on-the-fly, so don't make 
+                    modifications to any select's data structures
+*/
+
+int JOIN::print_explain(select_result_sink *result, uint8 explain_flags,
+                         bool on_the_fly,
+                         bool need_tmp_table, bool need_order,
+                         bool distinct, const char *message)
+{
+  List<Item> field_list;
+  List<Item> item_list;
+  JOIN *join= this; /* Legacy: this code used to be a non-member function */
+  THD *thd=join->thd;
+  Item *item_null= new Item_null();
+  CHARSET_INFO *cs= system_charset_info;
+  int quick_type;
+  int error= 0;
+  DBUG_ENTER("JOIN::print_explain");
+  DBUG_PRINT("info", ("Select 0x%lx, type %s, message %s",
+		      (ulong)join->select_lex, join->select_lex->type,
+		      message ? message : "NULL"));
+  DBUG_ASSERT(on_the_fly? have_query_plan == QEP_AVAILABLE: TRUE);
+  /* Don't log this into the slow query log */
+
+  if (!on_the_fly)
+  {
+    thd->server_status&= ~(SERVER_QUERY_NO_INDEX_USED | SERVER_QUERY_NO_GOOD_INDEX_USED);
+    join->unit->offset_limit_cnt= 0;
+  }
+
+  /* 
+    NOTE: the number/types of items pushed into item_list must be in sync with
+    EXPLAIN column types as they're "defined" in THD::send_explain_fields()
+  */
+  if (message)
+  {
+    if (print_explain_message_line(result, join->select_lex, on_the_fly, 
+                                   explain_flags, message))
+      error= 1;
+
+  }
+  else if (join->select_lex == join->unit->fake_select_lex)
+  {
+    if (print_fake_select_lex_join(result, on_the_fly, 
+                                   join->select_lex, 
+                                   explain_flags))
+      error= 1;
   }
   else if (!join->select_lex->master_unit()->derived ||
            join->select_lex->master_unit()->derived->is_materialized_derived())
   {
     table_map used_tables=0;
+    //if (!join->select_lex->type)
+    if (on_the_fly)
+      join->select_lex->set_explain_type(on_the_fly);
 
     bool printing_materialize_nest= FALSE;
     uint select_id= join->select_lex->select_number;
+    JOIN_TAB* const first_top_tab= first_breadth_first_tab(join, WALK_OPTIMIZATION_TABS);
 
     for (JOIN_TAB *tab= first_breadth_first_tab(join, WALK_OPTIMIZATION_TABS); tab;
          tab= next_breadth_first_tab(join, WALK_OPTIMIZATION_TABS, tab))
@@ -21473,6 +21780,8 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
       tmp3.length(0);
       tmp4.length(0);
       quick_type= -1;
+      QUICK_SELECT_I *quick= NULL;
+      JOIN_TAB *saved_join_tab= NULL;
 
       /* Don't show eliminated tables */
       if (table->map & join->eliminated_tables)
@@ -21481,6 +21790,13 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
         continue;
       }
 
+      if (join->table_access_tabs == join->join_tab &&
+          tab == (first_top_tab + join->const_tables) && pre_sort_join_tab)
+      {
+        saved_join_tab= tab;
+        tab= pre_sort_join_tab;
+      }
+
       item_list.empty();
       /* id */
       item_list.push_back(new Item_uint((uint32)select_id));
@@ -21489,17 +21805,19 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
                                                     join->select_lex->type;
       item_list.push_back(new Item_string(stype, strlen(stype), cs));
       
+      enum join_type tab_type= tab->type;
       if ((tab->type == JT_ALL || tab->type == JT_HASH) &&
-           tab->select && tab->select->quick)
+           tab->select && tab->select->quick && tab->use_quick != 2)
       {
+        quick= tab->select->quick;
         quick_type= tab->select->quick->get_type();
         if ((quick_type == QUICK_SELECT_I::QS_TYPE_INDEX_MERGE) ||
             (quick_type == QUICK_SELECT_I::QS_TYPE_INDEX_INTERSECT) ||
             (quick_type == QUICK_SELECT_I::QS_TYPE_ROR_INTERSECT) ||
             (quick_type == QUICK_SELECT_I::QS_TYPE_ROR_UNION))
-          tab->type= tab->type == JT_ALL ? JT_INDEX_MERGE : JT_HASH_INDEX_MERGE;
+          tab_type= tab->type == JT_ALL ? JT_INDEX_MERGE : JT_HASH_INDEX_MERGE;
         else
-	  tab->type= tab->type == JT_ALL ? JT_RANGE : JT_HASH_RANGE;
+	  tab_type= tab->type == JT_ALL ? JT_RANGE : JT_HASH_RANGE;
       }
 
       /* table */
@@ -21528,7 +21846,7 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
                                             strlen(real_table->alias), cs));
       }
       /* "partitions" column */
-      if (join->thd->lex->describe & DESCRIBE_PARTITIONS)
+      if (explain_flags & DESCRIBE_PARTITIONS)
       {
 #ifdef WITH_PARTITION_STORAGE_ENGINE
         partition_info *part_info;
@@ -21547,8 +21865,8 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
 #endif
       }
       /* "type" column */
-      item_list.push_back(new Item_string(join_type_str[tab->type],
+      item_list.push_back(new Item_string(join_type_str[tab_type],
-					  strlen(join_type_str[tab->type]),
+					  strlen(join_type_str[tab_type]),
 					  cs));
       /* Build "possible_keys" value and add it to item_list */
       if (!tab->keys.is_clear_all())
@@ -21572,7 +21890,7 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
 	item_list.push_back(item_null);
 
       /* Build "key", "key_len", and "ref" values and add them to item_list */
-      if (tab->type == JT_NEXT)
+      if (tab_type == JT_NEXT)
       {
 	key_info= table->key_info+tab->index;
         key_len= key_info->key_length;
@@ -21591,22 +21909,30 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
         length= (longlong10_to_str(key_len, keylen_str_buf, 10) - 
                  keylen_str_buf);
         tmp3.append(keylen_str_buf, length, cs);
-        if (tab->ref.key_parts)
+        if (tab->ref.key_parts && tab_type != JT_FT)
 	{
-	  for (store_key **ref=tab->ref.key_copy ; *ref ; ref++)
+          store_key **ref=tab->ref.key_copy;
+          for (uint kp= 0; kp < tab->ref.key_parts; kp++)
 	  {
 	    if (tmp4.length())
 	      tmp4.append(',');
+
+            if ((key_part_map(1) << kp) & tab->ref.const_ref_part_map)
+              tmp4.append("const");
+            else
+            {
               tmp4.append((*ref)->name(), strlen((*ref)->name()), cs);
+              ref++;
             }
           }
         }
-      if (is_hj && tab->type != JT_HASH)
+      }
+      if (is_hj && tab_type != JT_HASH)
       {
         tmp2.append(':');
         tmp3.append(':');
       }
-      if (tab->type == JT_HASH_NEXT)
+      if (tab_type == JT_HASH_NEXT)
       {
         register uint length;
 	key_info= table->key_info+tab->index;
@@ -21616,9 +21942,9 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
                  keylen_str_buf);
         tmp3.append(keylen_str_buf, length, cs);
       }         
-      if (tab->type != JT_CONST && tab->select && tab->select->quick)
+      if (tab->type != JT_CONST && tab->select && quick)
         tab->select->quick->add_keys_and_lengths(&tmp2, &tmp3);
-      if (key_info || (tab->select && tab->select->quick))
+      if (key_info || (tab->select && quick))
       {
         if (tmp2.length())
           item_list.push_back(new Item_string(tmp2.ptr(),tmp2.length(),cs));
@@ -21628,7 +21954,7 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
           item_list.push_back(new Item_string(tmp3.ptr(),tmp3.length(),cs));
         else
           item_list.push_back(item_null);
-        if (key_info && tab->type != JT_NEXT)
+        if (key_info && tab_type != JT_NEXT)
           item_list.push_back(new Item_string(tmp4.ptr(),tmp4.length(),cs));
         else
           item_list.push_back(item_null);
@@ -21671,7 +21997,7 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
           table_list->schema_table)
       {
         /* in_rows */
-        if (join->thd->lex->describe & DESCRIBE_EXTENDED)
+        if (explain_flags & DESCRIBE_EXTENDED)
           item_list.push_back(item_null);
         /* rows */
         item_list.push_back(item_null);
@@ -21684,7 +22010,7 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
                                          MY_INT64_NUM_DECIMAL_DIGITS));
 
         /* Add "filtered" field to item_list. */
-        if (join->thd->lex->describe & DESCRIBE_EXTENDED)
+        if (explain_flags & DESCRIBE_EXTENDED)
         {
           float f= 0.0; 
           if (examined_rows)
@@ -21696,11 +22022,11 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
 
       /* Build "Extra" field and add it to item_list. */
       key_read=table->key_read;
-      if ((tab->type == JT_NEXT || tab->type == JT_CONST) &&
+      if ((tab_type == JT_NEXT || tab_type == JT_CONST) &&
           table->covering_keys.is_set(tab->index))
 	key_read=1;
       if (quick_type == QUICK_SELECT_I::QS_TYPE_ROR_INTERSECT &&
-          !((QUICK_ROR_INTERSECT_SELECT*)tab->select->quick)->need_to_fetch_row)
+          !((QUICK_ROR_INTERSECT_SELECT*)quick)->need_to_fetch_row)
         key_read=1;
         
       if (tab->info)
@@ -21728,8 +22054,8 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
         uint keyno= MAX_KEY;
         if (tab->ref.key_parts)
           keyno= tab->ref.key;
-        else if (tab->select && tab->select->quick)
+        else if (tab->select && quick)
-          keyno = tab->select->quick->index;
+          keyno = quick->index;
 
         if (keyno != MAX_KEY && keyno == table->file->pushed_idx_cond_keyno &&
             table->file->pushed_idx_cond)
@@ -21768,7 +22094,7 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
             {
               extra.append(STRING_WITH_LEN("; Using where with pushed "
                                            "condition"));
-              if (thd->lex->describe & DESCRIBE_EXTENDED)
+              if (explain_flags & DESCRIBE_EXTENDED)
               {
                 extra.append(STRING_WITH_LEN(": "));
                 ((COND *)pushed_cond)->print(&extra, QT_ORDINARY);
@@ -21861,7 +22187,7 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
           extra.append(STRING_WITH_LEN("; End temporary"));
         else if (tab->do_firstmatch)
         {
-          if (tab->do_firstmatch == join->join_tab - 1)
+          if (tab->do_firstmatch == /*join->join_tab*/ first_top_tab - 1)
             extra.append(STRING_WITH_LEN("; FirstMatch"));
           else
           {
@@ -21908,12 +22234,34 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
 	item_list.push_back(new Item_string(str, len, cs));
       }
       
+      if (saved_join_tab)
+        tab= saved_join_tab;
+
       // For next iteration
       used_tables|=table->map;
       if (result->send_data(item_list))
-	join->error= 1;
+	error= 1;
     }
   }
+  DBUG_RETURN(error);
+}
+
+
+/*
+  See st_select_lex::print_explain() for the SHOW EXPLAIN counterpart
+*/ 
+
+static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
+			    bool distinct,const char *message)
+{
+  THD *thd=join->thd;
+  select_result *result=join->result;
+  DBUG_ENTER("select_describe");
+  join->error= join->print_explain(result, thd->lex->describe, 
+                                   FALSE, /* Not on-the-fly */
+                                   need_tmp_table, need_order, distinct, 
+                                   message);
+
   for (SELECT_LEX_UNIT *unit= join->select_lex->first_inner_unit();
        unit;
        unit= unit->next_unit())
@@ -21956,7 +22304,7 @@ bool mysql_explain_union(THD *thd, SELECT_LEX_UNIT *unit, select_result *result)
 
   for (SELECT_LEX *sl= first; sl; sl= sl->next_select())
   {
-    sl->set_explain_type();
+    sl->set_explain_type(FALSE);
     sl->options|= SELECT_DESCRIBE;
   }
 

sql/sql_select.h

@@ -101,6 +101,13 @@ typedef struct st_table_ref
   uchar         *key_buff;                ///< value to look for with key
   uchar         *key_buff2;               ///< key_buff+key_length
   store_key     **key_copy;               //
+
+  /*
+    Bitmap of key parts which refer to constants. key_copy only has copiers for
+    non-const key parts.
+  */
+  key_part_map  const_ref_part_map;
+
   Item          **items;                  ///< val()'s for each keypart
   /*  
     Array of pointers to trigger variables. Some/all of the pointers may be
@@ -897,7 +904,6 @@ protected:
 
 public:
   JOIN_TAB *join_tab, **best_ref;
-
   /*
     For "Using temporary+Using filesort" queries, JOIN::join_tab can point to
     either: 
@@ -910,6 +916,15 @@ public:
   JOIN_TAB *table_access_tabs;
   uint     top_table_access_tabs_count;
   
+  /* 
+    Saved join_tab for pre_sorting. create_sort_index() will save here.. 
+  */
+  JOIN_TAB *pre_sort_join_tab;
+  uint pre_sort_index;
+  Item *pre_sort_idx_pushed_cond;
+  void clean_pre_sort_join_tab();
+
+
   JOIN_TAB **map2table;    ///< mapping between table indexes and JOIN_TABs
   JOIN_TAB *join_tab_save; ///< saved join_tab for subquery reexecution
 
@@ -1175,9 +1190,15 @@ public:
   const char *zero_result_cause; ///< not 0 if exec must return zero result
   
   bool union_part; ///< this subselect is part of union 
+
+  enum join_optimization_state { NOT_OPTIMIZED=0,
+                                 OPTIMIZATION_IN_PROGRESS=1,
+                                 OPTIMIZATION_DONE=2};
   bool optimized; ///< flag to avoid double optimization in EXPLAIN
   bool initialized; ///< flag to avoid double init_execution calls
   
+  enum { QEP_NOT_PRESENT_YET, QEP_AVAILABLE, QEP_DELETED} have_query_plan;
+
   /*
     Additional WHERE and HAVING predicates to be considered for IN=>EXISTS
     subquery transformation of a JOIN object.
@@ -1259,6 +1280,7 @@ public:
     ref_pointer_array_size= 0;
     zero_result_cause= 0;
     optimized= 0;
+    have_query_plan= QEP_NOT_PRESENT_YET;
     initialized= 0;
     cond_equal= 0;
     having_equal= 0;
@@ -1279,6 +1301,8 @@ public:
     outer_ref_cond= pseudo_bits_cond= NULL;
     in_to_exists_where= NULL;
     in_to_exists_having= NULL;
+
+    pre_sort_join_tab= NULL;
   }
 
   int prepare(Item ***rref_pointer_array, TABLE_LIST *tables, uint wind_num,
@@ -1287,9 +1311,11 @@ public:
 	      SELECT_LEX_UNIT *unit);
   bool prepare_stage2();
   int optimize();
+  int optimize_inner();
   int reinit();
   int init_execution();
   void exec();
+  void exec_inner();
   int destroy();
   void restore_tmp();
   bool alloc_func_list();
@@ -1403,6 +1429,11 @@ public:
   {
     return (unit->item && unit->item->is_in_predicate());
   }
+
+  int print_explain(select_result_sink *result, uint8 explain_flags,
+                     bool on_the_fly,
+                     bool need_tmp_table, bool need_order,
+                     bool distinct,const char *message);
 private:
   /**
     TRUE if the query contains an aggregate function but has no GROUP
@@ -1754,6 +1785,9 @@ inline bool optimizer_flag(THD *thd, uint flag)
   return (thd->variables.optimizer_switch & flag);
 }
 
+int print_fake_select_lex_join(select_result_sink *result, bool on_the_fly,
+                               SELECT_LEX *select_lex, uint8 select_options);
+
 uint get_index_for_order(ORDER *order, TABLE *table, SQL_SELECT *select,
                          ha_rows limit, bool *need_sort, bool *reverse);
 ORDER *simple_remove_const(ORDER *order, COND *where);

sql/sql_show.cc

@@ -2003,6 +2003,166 @@ void mysqld_list_processes(THD *thd,const char *user, bool verbose)
   DBUG_VOID_RETURN;
 }
 
+
+/*
+  Produce EXPLAIN data.
+
+  This function is APC-scheduled to be run in the context of the thread that
+  we're producing EXPLAIN for.
+*/
+
+void Show_explain_request::call_in_target_thread()
+{
+  Query_arena backup_arena;
+  bool printed_anything= FALSE;
+
+  /* 
+    Change the arena because JOIN::print_explain and co. are going to allocate
+    items. Let them allocate them on our arena.
+  */
+  target_thd->set_n_backup_active_arena((Query_arena*)request_thd,
+                                        &backup_arena);
+  
+  query_str.copy(target_thd->query(), 
+                 target_thd->query_length(),
+                 target_thd->query_charset());
+
+  if (target_thd->lex->unit.print_explain(explain_buf, 0 /* explain flags*/,
+                                          &printed_anything))
+  {
+    failed_to_produce= TRUE;
+  }
+
+  if (!printed_anything)
+    failed_to_produce= TRUE;
+
+  target_thd->restore_active_arena((Query_arena*)request_thd, &backup_arena);
+}
+
+
+int select_result_explain_buffer::send_data(List<Item> &items)
+{
+  fill_record(thd, dst_table->field, items, TRUE, FALSE);
+  if ((dst_table->file->ha_write_tmp_row(dst_table->record[0])))
+    return 1;
+  return 0;
+}
+
+
+/*
+  Store the SHOW EXPLAIN output in the temporary table.
+*/
+
+int fill_show_explain(THD *thd, TABLE_LIST *table, COND *cond)
+{
+  const char *calling_user;
+  THD *tmp;
+  my_thread_id  thread_id;
+  DBUG_ENTER("fill_show_explain");
+
+  DBUG_ASSERT(cond==NULL);
+  thread_id= thd->lex->value_list.head()->val_int();
+  calling_user= (thd->security_ctx->master_access & PROCESS_ACL) ?  NullS :
+                 thd->security_ctx->priv_user;
+
+  if ((tmp= find_thread_by_id(thread_id)))
+  {
+    Security_context *tmp_sctx= tmp->security_ctx;
+    /*
+      If calling_user==NULL, calling thread has SUPER or PROCESS
+      privilege, and so can do SHOW EXPLAIN on any user.
+      
+      if calling_user!=NULL, he's only allowed to view SHOW EXPLAIN on
+      his own threads.
+    */
+    if (calling_user && (!tmp_sctx->user || strcmp(calling_user, 
+                                                   tmp_sctx->user)))
+    {
+      my_error(ER_SPECIFIC_ACCESS_DENIED_ERROR, MYF(0), "PROCESS");
+      mysql_mutex_unlock(&tmp->LOCK_thd_data);
+      DBUG_RETURN(1);
+    }
+
+    if (tmp == thd)
+    {
+      mysql_mutex_unlock(&tmp->LOCK_thd_data);
+      my_error(ER_TARGET_NOT_EXPLAINABLE, MYF(0));
+      DBUG_RETURN(1);
+    }
+
+    bool bres;
+    /* 
+      Ok we've found the thread of interest and it won't go away because 
+      we're holding its LOCK_thd data. Post it a SHOW EXPLAIN request.
+    */
+    bool timed_out;
+    int timeout_sec= 30;
+    Show_explain_request explain_req;
+    select_result_explain_buffer *explain_buf;
+    
+    explain_buf= new select_result_explain_buffer(thd, table->table);
+
+    explain_req.explain_buf= explain_buf;
+    explain_req.target_thd= tmp;
+    explain_req.request_thd= thd;
+    explain_req.failed_to_produce= FALSE;
+    
+    /* Ok, we have a lock on target->LOCK_thd_data, can call: */
+    bres= tmp->apc_target.make_apc_call(thd, &explain_req, timeout_sec, &timed_out);
+
+    if (bres || explain_req.failed_to_produce)
+    {
+      if (thd->killed)
+        thd->send_kill_message();
+      else if (timed_out)
+        my_error(ER_LOCK_WAIT_TIMEOUT, MYF(0));
+      else
+        my_error(ER_TARGET_NOT_EXPLAINABLE, MYF(0));
+
+      bres= TRUE;
+    }
+    else
+    {
+      /*
+        Push the query string as a warning. The query may be in a different
+        charset than the charset that's used for error messages, so, convert it
+        if needed.
+      */
+      CHARSET_INFO *fromcs= explain_req.query_str.charset();
+      CHARSET_INFO *tocs= error_message_charset_info;
+      char *warning_text;
+      if (!my_charset_same(fromcs, tocs))
+      {
+        uint conv_length= 1 + tocs->mbmaxlen * explain_req.query_str.length() / 
+                              fromcs->mbminlen;
+        uint dummy_errors;
+        char *to, *p;
+        if (!(to= (char*)thd->alloc(conv_length + 1)))
+          DBUG_RETURN(1);
+        p= to;
+        p+= copy_and_convert(to, conv_length, tocs,
+                             explain_req.query_str.c_ptr(), 
+                             explain_req.query_str.length(), fromcs,
+                             &dummy_errors);
+        *p= 0;
+        warning_text= to;
+      }
+      else
+        warning_text= explain_req.query_str.c_ptr_safe();
+
+      push_warning(thd, MYSQL_ERROR::WARN_LEVEL_NOTE,
+                   ER_YES, warning_text);
+    }
+    DBUG_RETURN(bres);
+  }
+  else
+  {
+    my_error(ER_NO_SUCH_THREAD, MYF(0), thread_id);
+    DBUG_RETURN(1);
+  }
+}
+
+
 int fill_schema_processlist(THD* thd, TABLE_LIST* tables, COND* cond)
 {
   TABLE *table= tables->table;
@@ -8333,6 +8493,32 @@ ST_FIELD_INFO keycache_fields_info[]=
 };
 
 
+ST_FIELD_INFO show_explain_fields_info[]=
+{
+  /* field_name, length, type, value, field_flags, old_name*/
+  {"id", 3, MYSQL_TYPE_LONGLONG, 0 /*value*/, MY_I_S_MAYBE_NULL, "id", 
+    SKIP_OPEN_TABLE},
+  {"select_type", 19, MYSQL_TYPE_STRING, 0 /*value*/, 0, "select_type", 
+    SKIP_OPEN_TABLE},
+  {"table", NAME_CHAR_LEN, MYSQL_TYPE_STRING, 0 /*value*/, MY_I_S_MAYBE_NULL,
+   "table", SKIP_OPEN_TABLE},
+  {"type", 15, MYSQL_TYPE_STRING, 0, MY_I_S_MAYBE_NULL, "type", SKIP_OPEN_TABLE},
+  {"possible_keys", NAME_CHAR_LEN*MAX_KEY, MYSQL_TYPE_STRING, 0/*value*/,
+    MY_I_S_MAYBE_NULL, "possible_keys", SKIP_OPEN_TABLE},
+  {"key", NAME_CHAR_LEN*MAX_KEY, MYSQL_TYPE_STRING, 0/*value*/, 
+    MY_I_S_MAYBE_NULL, "key", SKIP_OPEN_TABLE},
+  {"key_len", NAME_CHAR_LEN*MAX_KEY, MYSQL_TYPE_STRING, 0/*value*/, 
+    MY_I_S_MAYBE_NULL, "key_len", SKIP_OPEN_TABLE},
+  {"ref", NAME_CHAR_LEN*MAX_REF_PARTS, MYSQL_TYPE_STRING, 0/*value*/,
+    MY_I_S_MAYBE_NULL, "ref", SKIP_OPEN_TABLE},
+  {"rows", 10, MYSQL_TYPE_LONGLONG, 0/*value*/, MY_I_S_MAYBE_NULL, "rows", 
+    SKIP_OPEN_TABLE},
+  {"Extra", 255, MYSQL_TYPE_STRING, 0/*value*/, 0 /*flags*/, "Extra", 
+    SKIP_OPEN_TABLE},
+  {0, 0, MYSQL_TYPE_STRING, 0, 0, 0, SKIP_OPEN_TABLE}
+};
+
+
 /*
   Description of ST_FIELD_INFO in table.h
 
@@ -8364,6 +8550,8 @@ ST_SCHEMA_TABLE schema_tables[]=
   {"EVENTS", events_fields_info, create_schema_table,
    0, make_old_format, 0, -1, -1, 0, 0},
 #endif
+  {"EXPLAIN", show_explain_fields_info, create_schema_table, fill_show_explain,
+  make_old_format, 0, -1, -1, TRUE /*hidden*/ , 0},
   {"FILES", files_fields_info, create_schema_table,
    hton_fill_schema_table, 0, 0, -1, -1, 0, 0},
   {"GLOBAL_STATUS", variables_fields_info, create_schema_table,

sql/sql_show.h

@@ -19,6 +19,7 @@
 #include "sql_list.h"                           /* List */
 #include "handler.h"                            /* enum_schema_tables */
 #include "table.h"                              /* enum_schema_table_state */
+#include "my_apc.h"
 
 /* Forward declarations */
 class JOIN;
@@ -131,5 +132,30 @@ enum enum_schema_tables get_schema_table_idx(ST_SCHEMA_TABLE *schema_table);
 
 /* These functions were under INNODB_COMPATIBILITY_HOOKS */
 int get_quote_char_for_identifier(THD *thd, const char *name, uint length);
+THD *find_thread_by_id(ulong id);
+
+class select_result_explain_buffer;
+/*
+  SHOW EXPLAIN request object. 
+*/
+
+class Show_explain_request : public Apc_target::Apc_call
+{
+public:
+  THD *target_thd;  /* thd that we're running SHOW EXPLAIN for */
+  THD *request_thd; /* thd that run SHOW EXPLAIN command */
+  
+  /* If true, there was some error when producing EXPLAIN output. */
+  bool failed_to_produce;
+   
+  /* SHOW EXPLAIN will be stored here */
+  select_result_explain_buffer *explain_buf;
+  
+  /* Query that we've got SHOW EXPLAIN for */
+  String query_str;
+  
+  /* Overloaded virtual function */
+  void call_in_target_thread();
+};
 
 #endif /* SQL_SHOW_H */

sql/sql_union.cc

@@ -722,6 +722,8 @@ bool st_select_lex_unit::exec()
     }
   }
 
+  DBUG_EXECUTE_IF("show_explain_probe_union_read", 
+                   dbug_serve_apcs(thd, 1););
   /* Send result to 'result' */
   saved_error= TRUE;
   {

sql/sql_yacc.yy

@@ -11692,6 +11692,14 @@ show_param:
             Lex->spname= $3;
             Lex->sql_command = SQLCOM_SHOW_CREATE_EVENT;
           }
+        | describe_command FOR_SYM expr
+          {
+            THD *thd= YYTHD;
+            Lex->sql_command= SQLCOM_SHOW_EXPLAIN;
+            if (prepare_schema_table(thd, Lex, 0, SCH_EXPLAIN))
+              MYSQL_YYABORT;
+            add_value_to_list(thd, $3);
+          }
         ;
 
 show_engine_param:

sql/table.h

@@ -1146,7 +1146,12 @@ public:
     See TABLE_LIST::process_index_hints().
   */
   bool force_index_group;
-  bool distinct,const_table,no_rows, used_for_duplicate_elimination;
+  /*
+    TRUE<=> this table was created with create_tmp_table(... distinct=TRUE..)
+    call
+  */
+  bool distinct;
+  bool const_table,no_rows, used_for_duplicate_elimination;
 
   /**
      If set, the optimizer has found that row retrieval should access index 

unittest/sql/CMakeLists.txt

@@ -0,0 +1,3 @@
+
+MY_ADD_TESTS(my_apc LINK_LIBRARIES mysys EXT cc)
+

unittest/sql/my_apc-t.cc

@@ -0,0 +1,227 @@
+/*
+   Copyright (c) 2012, Monty Program Ab
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; version 2 of the License.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */
+
+/*
+  This file does standalone APC system tests.
+*/
+#include <stdio.h>
+#include <my_global.h>
+#include <my_pthread.h>
+#include <my_sys.h>
+
+#include <tap.h>
+
+/*
+  A fake THD with enter_cond/exit_cond and some other members.
+*/
+class THD 
+{
+  mysql_mutex_t* thd_mutex; 
+public:
+  bool killed;
+
+  THD() : killed(FALSE) {}
+  inline const char* enter_cond(mysql_cond_t *cond, mysql_mutex_t* mutex,
+                                const char* msg)
+  {
+    mysql_mutex_assert_owner(mutex);
+    thd_mutex= mutex;
+    return NULL;
+  }
+  inline void exit_cond(const char* old_msg)
+  {
+    mysql_mutex_unlock(thd_mutex);
+  }
+};
+
+#include "../sql/my_apc.h"
+
+#define MY_APC_STANDALONE 1
+#include "../sql/my_apc.cc"
+
+volatile bool started= FALSE;
+volatile bool service_should_exit= FALSE;
+volatile bool requestors_should_exit=FALSE;
+
+/*  Counters for APC calls */
+int apcs_served= 0;
+int apcs_missed=0;
+int apcs_timed_out=0;
+mysql_mutex_t apc_counters_mutex;
+
+inline void increment_counter(int *var)
+{
+  mysql_mutex_lock(&apc_counters_mutex);
+  *var= *var+1;
+  mysql_mutex_unlock(&apc_counters_mutex);
+}
+
+volatile bool have_errors= false;
+
+Apc_target apc_target;
+mysql_mutex_t target_mutex;
+
+int int_rand(int size)
+{
+  return (int) (0.5 + ((double)rand() / RAND_MAX) * size);
+}
+
+/* 
+  APC target thread (the one that will serve the APC requests). We will have
+  one target.
+*/
+void *test_apc_service_thread(void *ptr)
+{
+  my_thread_init();
+  mysql_mutex_init(0, &target_mutex, MY_MUTEX_INIT_FAST);
+  apc_target.init(&target_mutex);
+  apc_target.enable();
+  started= TRUE;
+  diag("test_apc_service_thread started");
+  while (!service_should_exit)
+  {
+    //apc_target.disable();
+    my_sleep(10000);
+    //apc_target.enable();
+    for (int i = 0; i < 10 && !service_should_exit; i++)
+    {
+      apc_target.process_apc_requests();
+      my_sleep(int_rand(30));
+    }
+  }
+  apc_target.disable();
+  apc_target.destroy();
+  mysql_mutex_destroy(&target_mutex);
+  my_thread_end();
+  pthread_exit(0);
+  return NULL;
+}
+
+
+/*
+  One APC request (to write 'value' into *where_to)
+*/
+class Apc_order : public Apc_target::Apc_call
+{
+public:
+  int value;   // The value 
+  int *where_to;  // Where to write it
+  Apc_order(int a, int *b) : value(a), where_to(b) {}
+
+  void call_in_target_thread()
+  {
+    my_sleep(int_rand(1000));
+    *where_to = value;
+    increment_counter(&apcs_served);
+  }
+};
+
+
+/*
+  APC requestor thread. It makes APC requests, and checks if they were actually
+  executed.
+*/
+void *test_apc_requestor_thread(void *ptr)
+{
+  my_thread_init();
+  diag("test_apc_requestor_thread started");
+  THD my_thd;
+
+  while (!requestors_should_exit)
+  {
+    int dst_value= 0;
+    int src_value= int_rand(4*1000*100);
+    /* Create an APC to do "dst_value= src_value" assignment */
+    Apc_order apc_order(src_value, &dst_value);
+    bool timed_out;
+
+    mysql_mutex_lock(&target_mutex);
+    bool res= apc_target.make_apc_call(&my_thd, &apc_order, 60, &timed_out);
+    if (res)
+    {
+      if (timed_out)
+        increment_counter(&apcs_timed_out);
+      else
+        increment_counter(&apcs_missed);
+
+      if (dst_value != 0)
+      {
+        diag("APC was done even though return value says it wasnt!");
+        have_errors= true;
+      }
+    }
+    else
+    {
+      if (dst_value != src_value)
+      {
+        diag("APC was not done even though return value says it was!");
+        have_errors= true;
+      }
+    }
+    //my_sleep(300);
+  }
+  diag("test_apc_requestor_thread exiting");
+  my_thread_end();
+  return NULL;
+}
+
+/* Number of APC requestor threads */
+const int N_THREADS=23;
+
+
+int main(int args, char **argv)
+{
+  pthread_t service_thr;
+  pthread_t request_thr[N_THREADS];
+  int i;
+
+  my_thread_global_init();
+
+  mysql_mutex_init(0, &apc_counters_mutex, MY_MUTEX_INIT_FAST);
+
+  plan(1);
+  diag("Testing APC delivery and execution");
+
+  pthread_create(&service_thr, NULL, test_apc_service_thread, (void*)NULL);
+  while (!started)
+    my_sleep(1000);
+  for (i = 0; i < N_THREADS; i++)
+    pthread_create(&request_thr[i], NULL, test_apc_requestor_thread, (void*)NULL);
+  
+  for (i = 0; i < 15; i++)
+  {
+    my_sleep(500*1000);
+    diag("%d APCs served %d missed", apcs_served, apcs_missed);
+  }
+  diag("Shutting down requestors");
+  requestors_should_exit= TRUE;
+  for (i = 0; i < N_THREADS; i++)
+    pthread_join(request_thr[i], NULL);
+  
+  diag("Shutting down service");
+  service_should_exit= TRUE;
+  pthread_join(service_thr, NULL);
+
+  mysql_mutex_destroy(&apc_counters_mutex);
+
+  diag("Done");
+  my_thread_end();
+  my_thread_global_end();
+
+  ok1(!have_errors);
+  return exit_status();
+}
+