MDEV-324: SHOW EXPLAIN: Plan produced by SHOW EXPLAIN for a query with TEMPTABLE view loses 'DERIVED' line

- Make SHOW EXPLAIN code take into account that st_select_lex object without joins can be
  a full-featured SELECTs which were already executed and cleaned up.

mysql-test/r/show_explain.result

@@ -153,10 +153,7 @@ set @show_explain_probe_select_id=1;
 set debug_dbug='d,show_explain_probe_join_exec_end';
 select a, (select max(a) from t0 b where b.a+a.a<10) from t0 a where a<1;
 show explain for $thr2;
-id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+ERROR HY000: Error when executing command SHOW EXPLAIN: Target is not running EXPLAINable command
-1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Query plan already deleted
-Warnings:
-Note	1003	select a, (select max(a) from t0 b where b.a+a.a<10) from t0 a where a<1
 a	(select max(a) from t0 b where b.a+a.a<10)
 0	9
 # Try to do SHOW EXPLAIN for a query that runs a  SET command:
@@ -308,12 +305,10 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	5	Using join buffer (flat, BNL join)
 set debug_dbug='d,show_explain_in_find_all_keys';
 SELECT alias.a FROM t2, ( SELECT * FROM t2 ) AS alias GROUP BY alias.a;
+# FIXED by "conservative assumptions about when QEP is available" fix:
 #    NOTE: current code will not show "Using join buffer": 
 show explain for $thr2;
-id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+ERROR HY000: Error when executing command SHOW EXPLAIN: Target is not running EXPLAINable command
-1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Query plan already deleted
-Warnings:
-Note	1003	SELECT alias.a FROM t2, ( SELECT * FROM t2 ) AS alias GROUP BY alias.a
 a
 1
 2
@@ -366,10 +361,7 @@ set @show_explain_probe_select_id=2;
 set debug_dbug='d,show_explain_probe_join_exec_end';
 SELECT * FROM v1, t2;
 show explain for $thr2;
-id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+ERROR HY000: Error when executing command SHOW EXPLAIN: Target is not running EXPLAINable command
-1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Not yet optimized
-Warnings:
-Note	1003	SELECT * FROM v1, t2
 a	b
 8	4
 8	5
@@ -401,10 +393,7 @@ set @show_explain_probe_select_id=1;
 set debug_dbug='d,show_explain_probe_join_exec_end';
 select * from t0 where 1>10;
 show explain for $thr2;
-id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+ERROR HY000: Error when executing command SHOW EXPLAIN: Target is not running EXPLAINable command
-1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Query plan already deleted
-Warnings:
-Note	1003	select * from t0 where 1>10
 a
 set debug_dbug='';
 #
@@ -416,10 +405,7 @@ set @show_explain_probe_select_id=1;
 set debug_dbug='d,show_explain_probe_join_exec_end';
 select * from t0,t3 where t3.a=112233;
 show explain for $thr2;
-id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+ERROR HY000: Error when executing command SHOW EXPLAIN: Target is not running EXPLAINable command
-1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Query plan already deleted
-Warnings:
-Note	1003	select * from t0,t3 where t3.a=112233
 a	a
 set debug_dbug='';
 drop table t3;
@@ -524,10 +510,7 @@ set @show_explain_probe_select_id=1;
 set debug_dbug='d,show_explain_probe_join_exec_end';
 SELECT * FROM t2 WHERE (5, 78) IN (SELECT `a1`, MAX(`a1`) FROM t2 GROUP BY `a1`);
 show explain for $thr2;
-id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+ERROR HY000: Error when executing command SHOW EXPLAIN: Target is not running EXPLAINable command
-1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Query plan already deleted
-Warnings:
-Note	1003	SELECT * FROM t2 WHERE (5, 78) IN (SELECT `a1`, MAX(`a1`) FROM t2 GROUP BY `a1`)
 pk	a1
 set debug_dbug='';
 DROP TABLE t2;
@@ -616,4 +599,30 @@ t1	1	b	1	b	A	NULL	NULL	NULL	YES	BTREE
 t1	1	c	1	c	A	NULL	NULL	NULL	YES	BTREE		
 set debug_dbug='';
 DROP TABLE t1;
+#
+# MDEV-324: SHOW EXPLAIN: Plan produced by SHOW EXPLAIN for a query with TEMPTABLE view 
+#           loses 'DERIVED' line on the way without saying that the plan was already deleted
+#
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1),(2);
+CREATE ALGORITHM=TEMPTABLE VIEW v1 AS SELECT * FROM t1;
+EXPLAIN SELECT a + 1 FROM v1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	2	
+2	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	2	
+set debug_dbug='d,show_explain_probe_join_tab_preread';
+set @show_explain_probe_select_id=1;
+SELECT a + 1 FROM v1;
+show explain for $thr2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	2	
+2	DERIVED	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Query plan already deleted
+Warnings:
+Note	1003	SELECT a + 1 FROM v1
+a + 1
+2
+3
+set debug_dbug='';
+DROP VIEW v1;
+DROP TABLE t1;
 drop table t0;

mysql-test/t/show_explain.test

@@ -191,6 +191,7 @@ set debug_dbug='d,show_explain_probe_join_exec_end';
 send select a, (select max(a) from t0 b where b.a+a.a<10) from t0 a where a<1;
 connection default;
 --source include/wait_condition.inc
+--error ER_ERROR_WHEN_EXECUTING_COMMAND
 evalp show explain for $thr2;
 connection con1;
 reap;
@@ -325,7 +326,9 @@ send SELECT alias.a FROM t2, ( SELECT * FROM t2 ) AS alias GROUP BY alias.a;
 
 connection default;
 --source include/wait_condition.inc
+--echo # FIXED by "conservative assumptions about when QEP is available" fix:
 --echo #    NOTE: current code will not show "Using join buffer": 
+--error ER_ERROR_WHEN_EXECUTING_COMMAND
 evalp show explain for $thr2;
 connection con1;
 reap;
@@ -374,6 +377,7 @@ send SELECT * FROM v1, t2;
 
 connection default;
 --source include/wait_condition.inc
+--error ER_ERROR_WHEN_EXECUTING_COMMAND
 evalp show explain for $thr2;
 connection con1;
 reap;
@@ -403,6 +407,7 @@ set debug_dbug='d,show_explain_probe_join_exec_end';
 send select * from t0 where 1>10;
 connection default;
 --source include/wait_condition.inc
+--error ER_ERROR_WHEN_EXECUTING_COMMAND
 evalp show explain for $thr2;
 connection con1;
 reap;
@@ -418,6 +423,7 @@ set debug_dbug='d,show_explain_probe_join_exec_end';
 send select * from t0,t3 where t3.a=112233;
 connection default;
 --source include/wait_condition.inc
+--error ER_ERROR_WHEN_EXECUTING_COMMAND
 evalp show explain for $thr2;
 connection con1;
 reap;
@@ -513,6 +519,7 @@ send
   SELECT * FROM t2 WHERE (5, 78) IN (SELECT `a1`, MAX(`a1`) FROM t2 GROUP BY `a1`);
 connection default;
 --source include/wait_condition.inc
+--error ER_ERROR_WHEN_EXECUTING_COMMAND
 evalp show explain for $thr2;
 connection con1;
 reap;
@@ -601,6 +608,32 @@ set debug_dbug='';
 
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-324: SHOW EXPLAIN: Plan produced by SHOW EXPLAIN for a query with TEMPTABLE view 
+--echo #           loses 'DERIVED' line on the way without saying that the plan was already deleted
+--echo #
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1),(2);
+CREATE ALGORITHM=TEMPTABLE VIEW v1 AS SELECT * FROM t1;
+
+EXPLAIN SELECT a + 1 FROM v1;
+
+set debug_dbug='d,show_explain_probe_join_tab_preread';
+set @show_explain_probe_select_id=1;
+
+send
+  SELECT a + 1 FROM v1;
+connection default;
+--source include/wait_condition.inc
+evalp show explain for $thr2;
+connection con1;
+reap;
+set debug_dbug='';
+
+DROP VIEW v1;
+DROP TABLE t1;
+
+
 
 ## TODO: Test this: have several SHOW EXPLAIN requests be queued up for a
 ##       thread and served together.

sql/sql_class.cc

@@ -2356,13 +2356,14 @@ int select_result_explain_buffer::send_data(List<Item> &items)
 }
 
 
+/* Write all strings out to the output, and free them. */
+
 void select_result_explain_buffer::flush_data()
 {
   List_iterator<String> it(data_rows);
   String *str;
   while ((str= it++))
   {
-    /* TODO: write out the lines. */
     protocol->set_packet(str->ptr(), str->length());
     protocol->write();
     delete str;
@@ -2370,6 +2371,20 @@ void select_result_explain_buffer::flush_data()
   data_rows.empty();
 }
 
+
+/* Just free all of the accumulated strings */
+
+void select_result_explain_buffer::discard_data()
+{
+  List_iterator<String> it(data_rows);
+  String *str;
+  while ((str= it++))
+  {
+    delete str;
+  }
+  data_rows.empty();
+}
+
 //////////////////////////////////////////////////////////////////////////////
 
 
@@ -3288,6 +3303,7 @@ void Show_explain_request::get_explain_data(void *arg)
   //      Actually, change the ARENA, because we're going to allocate items!
   Query_arena backup_arena;
   THD *target_thd= req->target_thd;
+  bool printed_anything= FALSE;
 
   target_thd->set_n_backup_active_arena((Query_arena*)req->request_thd,
                                         &backup_arena);
@@ -3296,7 +3312,11 @@ void Show_explain_request::get_explain_data(void *arg)
                       target_thd->query_length(),
                       &my_charset_bin);
 
-  if (target_thd->lex->unit.print_explain(req->explain_buf, 0 /* explain flags */))
+  if (target_thd->lex->unit.print_explain(req->explain_buf, 0 /* explain flags*/,
+                                          &printed_anything))
+    req->failed_to_produce= TRUE;
+
+  if (!printed_anything)
     req->failed_to_produce= TRUE;
 
   target_thd->restore_active_arena((Query_arena*)req->request_thd, 

sql/sql_class.h

@@ -3328,6 +3328,8 @@ public:
   /* this will be called in the parent thread: */
   void flush_data();
 
+  void discard_data();
+
   List<String> data_rows;
 };
 

sql/sql_lex.cc

@@ -4074,7 +4074,8 @@ int print_explain_message_line(select_result_sink *result,
 
 
 int st_select_lex::print_explain(select_result_sink *output, 
-                                 uint8 explain_flags)
+                                 uint8 explain_flags,
+                                 bool *printed_anything)
 {
   int res;
   if (join && join->have_query_plan == JOIN::QEP_AVAILABLE)
@@ -4083,6 +4084,7 @@ int st_select_lex::print_explain(select_result_sink *output,
       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 */
@@ -4112,7 +4114,8 @@ int st_select_lex::print_explain(select_result_sink *output,
       */
       if (!(unit->item && unit->item->eliminated))
       {
-        unit->print_explain(output, explain_flags);
+        if ((res= unit->print_explain(output, explain_flags, printed_anything)))
+          goto err;
       }
     }
   }
@@ -4120,7 +4123,9 @@ int st_select_lex::print_explain(select_result_sink *output,
   {
     const char *msg;
     if (!join)
-      DBUG_ASSERT(0); // psergey: TODO: can this happen or not?
+      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
@@ -4132,12 +4137,12 @@ int st_select_lex::print_explain(select_result_sink *output,
                                     0, msg);
   }
 err:
-  return 0;
+  return res;
 }
 
 
 int st_select_lex_unit::print_explain(select_result_sink *output, 
-                                      uint8 explain_flags)
+                                      uint8 explain_flags, bool *printed_anything)
 {
   int res= 0;
   SELECT_LEX *first= first_select();
@@ -4148,12 +4153,15 @@ int st_select_lex_unit::print_explain(select_result_sink *output,
       If there is only one child, 'first', and it has join==NULL, emit "not in
       EXPLAIN state" error.
     */
-    return 1;
+    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)))
+    if ((res= sl->print_explain(output, explain_flags, printed_anything)))
       break;
   }
 

sql/sql_lex.h

@@ -718,7 +718,8 @@ public:
   friend int subselect_union_engine::exec();
 
   List<Item> *get_unit_column_types();
-  int print_explain(select_result_sink *output, uint8 explain_flags);
+  int print_explain(select_result_sink *output, uint8 explain_flags,
+                    bool *printed_anything);
 };
 
 typedef class st_select_lex_unit SELECT_LEX_UNIT;
@@ -1039,7 +1040,8 @@ public:
   bool save_prep_leaf_tables(THD *thd);
 
   bool is_merged_child_of(st_select_lex *ancestor);
-  int print_explain(select_result_sink *output, uint8 explain_flags);
+  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.

sql/sql_select.cc

@@ -10365,9 +10365,18 @@ 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;
 }
 

sql/sql_show.cc

@@ -2085,6 +2085,7 @@ void mysqld_show_explain(THD *thd, ulong thread_id)
                  "Target is not running EXPLAINable command");
       }
       bres= TRUE;
+      explain_buf->discard_data();
     }
     else
     {