MDEV-6442: Assertion `join->best_read < double(...)' failed with optimizer_use_condition_selectivity >=3

- Fix the crash by making get_column_range_cardinality() 
  to handle the special case where Column_stats objects 
  is an all-zeros object (the question of what is the point
  of having Field::read_stats point to such object remains a 
  mystery)

- Added a few comments. Learning the code still.

mysql-test/r/selectivity_innodb.result

@@ -1358,6 +1358,8 @@ set optimizer_use_condition_selectivity=@save_optimizer_use_condition_selectivit
 DROP TABLE t1,t2;
 set use_stat_tables=@save_use_stat_tables;
 set optimizer_switch=@save_optimizer_switch_for_selectivity_test;
+set @tmp_ust= @@use_stat_tables;
+set @tmp_oucs= @@optimizer_use_condition_selectivity;
 #
 # MDEV-6808: MariaDB 10.0.13 crash with optimizer_use_condition_selectivity > 1
 #
@@ -1393,4 +1395,22 @@ t2.subject_type in ('user')
 repost_id	subject_type	subject_id	object_type	object_id	is_private	event_id
 DROP TABLE t1, t2;
 set optimizer_use_condition_selectivity=@tmp_mdev6808;
+#
+# MDEV-6442: Assertion `join->best_read < double(...)' failed with optimizer_use_condition_selectivity >=3, ...
+#
+SET use_stat_tables = PREFERABLY;
+SET optimizer_use_condition_selectivity = 3;
+CREATE TABLE t1 ( a VARCHAR(3), b VARCHAR(8), KEY (a,b) ) ENGINE=InnoDB;
+INSERT INTO t1 VALUES ('USA','Chinese'),('USA','English');
+CREATE TABLE t2 (i INT) ENGINE=InnoDB;
+SELECT * FROM t1, t2 WHERE ( 't', 'o' ) IN ( 
+SELECT t1_2.b, t1_1.a FROM t1 AS t1_1 STRAIGHT_JOIN t1 AS t1_2 ON ( t1_2.a = t1_1.b ) 
+);
+a	b	i
+DROP TABLE t1,t2;
+#
+# End of 10.0 tests
+#
+set use_stat_tables= @tmp_ust;
+set optimizer_use_condition_selectivity= @tmp_oucs;
 SET SESSION STORAGE_ENGINE=DEFAULT;

mysql-test/t/selectivity_innodb.test

@@ -8,6 +8,9 @@ set optimizer_switch='extended_keys=on';
 --source selectivity.test
 
 set optimizer_switch=@save_optimizer_switch_for_selectivity_test;
+set @tmp_ust= @@use_stat_tables;
+set @tmp_oucs= @@optimizer_use_condition_selectivity;
+
 
 --echo #
 --echo # MDEV-6808: MariaDB 10.0.13 crash with optimizer_use_condition_selectivity > 1
@@ -46,4 +49,26 @@ WHERE
 DROP TABLE t1, t2;
 set optimizer_use_condition_selectivity=@tmp_mdev6808;
 
+--echo #
+--echo # MDEV-6442: Assertion `join->best_read < double(...)' failed with optimizer_use_condition_selectivity >=3, ...
+--echo #
+SET use_stat_tables = PREFERABLY;
+SET optimizer_use_condition_selectivity = 3;
+
+CREATE TABLE t1 ( a VARCHAR(3), b VARCHAR(8), KEY (a,b) ) ENGINE=InnoDB;
+INSERT INTO t1 VALUES ('USA','Chinese'),('USA','English');
+
+CREATE TABLE t2 (i INT) ENGINE=InnoDB;
+
+SELECT * FROM t1, t2 WHERE ( 't', 'o' ) IN ( 
+  SELECT t1_2.b, t1_1.a FROM t1 AS t1_1 STRAIGHT_JOIN t1 AS t1_2 ON ( t1_2.a = t1_1.b ) 
+);
+DROP TABLE t1,t2;
+
+--echo #
+--echo # End of 10.0 tests
+--echo #
+
+set use_stat_tables= @tmp_ust;
+set optimizer_use_condition_selectivity= @tmp_oucs;
 SET SESSION STORAGE_ENGINE=DEFAULT;

sql/sql_statistics.cc

@@ -3502,7 +3502,7 @@ double get_column_range_cardinality(Field *field,
                    !(range_flag & NEAR_MIN);
 
   if (col_non_nulls < 1)
-    res= 0;
+    res= 0; /* this is likely wrong, see MDEV-6843 */
   else if (min_endp && max_endp && min_endp->length == max_endp->length &&
            !memcmp(min_endp->key, max_endp->key, min_endp->length))
   { 
@@ -3515,6 +3515,15 @@ double get_column_range_cardinality(Field *field,
     {
       double avg_frequency= col_stats->get_avg_frequency();
       res= avg_frequency;   
+      /*
+        psergey-todo: what does check for min_value, max_value mean? 
+          min/max_value are set to NULL in alloc_statistics_for_table() and
+          alloc_statistics_for_table_share().  Both functions will immediately
+          call create_min_max_statistical_fields_for_table and 
+          create_min_max_statistical_fields_for_table_share() respectively,
+          which will set min/max_value to be valid pointers, unless OOM
+          occurs.
+      */
       if (avg_frequency > 1.0 + 0.000001 && 
           col_stats->min_value && col_stats->max_value)
       {
@@ -3530,6 +3539,11 @@ double get_column_range_cardinality(Field *field,
                                        avg_frequency / col_non_nulls);
         }
       }
+      else if (avg_frequency == 0.0)
+      {
+        /* This actually means there is no statistics data */
+        res= tab_records;
+      }
     }
   }  
   else 

sql/sql_statistics.h

@@ -280,7 +280,14 @@ public:
 };
 
 
-/* Statistical data on a column */
+/* 
+  Statistical data on a column 
+
+  Note: objects of this class may be "empty", where they have almost all fields
+  as zeros, for example, get_avg_frequency() will return 0.
+
+  objects are allocated in alloc_statistics_for_table[_share].
+*/
 
 class Column_statistics
 {
@@ -297,6 +304,7 @@ public:
   */
   uint32 column_stat_nulls;
   
+  /* For the below two, see comments in get_column_range_cardinality() */
   /* Minimum value for the column */
   Field *min_value; 
   /* Maximum value for the column */