MDEV-30143 Segfault on select query using index for group-by and filesort

The problem was trying to access JOIN_TAB::select which is set to NULL
when using the filesort. The correct way is accessing either
JOIN_TAB::select or JOIN_TAB::filesort->select depending on whether
the filesort is used.
This commit introduces member function JOIN_TAB::get_sql_select()
encapsulating that check so the code duplication is eliminated.

The new condition (s->table->quick_keys.is_set(best_key->key))
was added to  best_access_path() to eliminate a Valgrind error.
The cause of that error was using TRASH_ALLOC(quick_key_parts)
instead of bzero(quick_key_parts); hence, accessing
s->table->quick_key_parts[best_key->key]) without prior checking
for quick_keys.is_set() might have caused reading "dirty" memory

mysql-test/main/group_min_max_innodb.result

@@ -308,6 +308,28 @@ NULL	bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
 NULL	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
 NULL	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
 drop table t1,t2;
+#
+# MDEV-30143: Segfault on select query using index for group-by and filesort
+#
+CREATE TABLE t1 (a varchar(35), b varchar(4)) ENGINE=InnoDB;
+INSERT INTO t1 VALUES
+('Albania','AXA'),('Australia','AUS'),('American Samoa','AMSA'),('Bahamas','BS');
+CREATE TABLE t2 (a varchar(4), b varchar(50), PRIMARY KEY (b,a), KEY (a)) ENGINE=InnoDB;
+INSERT INTO t2 VALUES
+('BERM','African Methodist Episcopal'),('AUS','Anglican'),('BERM','Anglican'),('BS','Anglican'),('BS','Baptist'),('BS','Methodist');
+EXPLAIN SELECT t1.a
+FROM (SELECT a FROM t2 GROUP BY a ORDER BY COUNT(DISTINCT b) LIMIT 1) dt
+JOIN t1 ON dt.a=t1.b;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	#	
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	#	Using where; Using join buffer (flat, BNL join)
+2	DERIVED	t2	range	a	a	58	NULL	#	Using index for group-by (scanning); Using temporary; Using filesort
+SELECT t1.a
+FROM (SELECT a FROM t2 GROUP BY a ORDER BY COUNT(DISTINCT b) LIMIT 1) dt
+JOIN t1 ON dt.a=t1.b;
+a
+Australia
+DROP TABLES t1, t2;
 set global innodb_stats_persistent= @innodb_stats_persistent_save;
 set global innodb_stats_persistent_sample_pages=
 @innodb_stats_persistent_sample_pages_save;

mysql-test/main/group_min_max_innodb.test

@@ -251,7 +251,28 @@ insert into t2 values (1,repeat("a",1000)),(2,repeat("a",1000)),(3,repeat("b",10
 SELECT GROUP_CONCAT(t1.language_id SEPARATOR ',') AS `translation_resources`, `d`.`serialized_c` FROM t2 AS `d` LEFT JOIN t1 ON `d`.`voter_id` = t1.`voter_id` GROUP BY `d`.`voter_id` ORDER BY 10-d.voter_id+RAND()*0;
 drop table t1,t2;
 
+
+--echo #
+--echo # MDEV-30143: Segfault on select query using index for group-by and filesort
+--echo #
+CREATE TABLE t1 (a varchar(35), b varchar(4)) ENGINE=InnoDB;
+INSERT INTO t1 VALUES
+('Albania','AXA'),('Australia','AUS'),('American Samoa','AMSA'),('Bahamas','BS');
+
+CREATE TABLE t2 (a varchar(4), b varchar(50), PRIMARY KEY (b,a), KEY (a)) ENGINE=InnoDB;
+INSERT INTO t2 VALUES
+('BERM','African Methodist Episcopal'),('AUS','Anglican'),('BERM','Anglican'),('BS','Anglican'),('BS','Baptist'),('BS','Methodist');
+
+let query=
+SELECT t1.a
+FROM (SELECT a FROM t2 GROUP BY a ORDER BY COUNT(DISTINCT b) LIMIT 1) dt
+JOIN t1 ON dt.a=t1.b;
+--replace_column 9 #
+eval EXPLAIN $query;
+eval $query;
+
+DROP TABLES t1, t2;
+
 set global innodb_stats_persistent= @innodb_stats_persistent_save;
 set global innodb_stats_persistent_sample_pages=
              @innodb_stats_persistent_sample_pages_save;
-

sql/sql_select.cc

@@ -8120,6 +8120,7 @@ best_access_path(JOIN      *join,
   if ((records >= s->found_records || best > s->read_time) &&            // (1)
       !(best_key && best_key->key == MAX_KEY) &&                         // (2)
       !(s->quick && best_key && s->quick->index == best_key->key &&      // (2)
+        s->table->quick_keys.is_set(best_key->key) &&                    // (2)
         best_max_key_part >= s->table->quick_key_parts[best_key->key]) &&// (2)
       !((s->table->file->ha_table_flags() & HA_TABLE_SCAN_ON_INDEX) &&   // (3)
         ! s->table->covering_keys.is_clear_all() && best_key && !s->quick) &&// (3)
@@ -13791,7 +13792,7 @@ double JOIN_TAB::scan_time()
 ha_rows JOIN_TAB::get_examined_rows()
 {
   double examined_rows;
-  SQL_SELECT *sel= filesort? filesort->select : this->select;
+  const SQL_SELECT *sel= get_sql_select();
 
   if (sel && sel->quick && use_quick != 2)
     examined_rows= (double)sel->quick->records;
@@ -26858,13 +26859,12 @@ bool JOIN_TAB::save_explain_data(Explain_table_access *eta,
   eta->key.clear();
   eta->quick_info= NULL;
 
-  SQL_SELECT *tab_select;
   /* 
     We assume that if this table does pre-sorting, then it doesn't do filtering
     with SQL_SELECT.
   */
   DBUG_ASSERT(!(select && filesort));
-  tab_select= (filesort)? filesort->select : select;
+  const SQL_SELECT *tab_select= get_sql_select();
 
   if (filesort)
   {

sql/sql_select.h

@@ -546,14 +546,19 @@ typedef struct st_join_table {
   void cleanup();
   inline bool is_using_loose_index_scan()
   {
-    const SQL_SELECT *sel= filesort ? filesort->select : select;
+    const SQL_SELECT *sel= get_sql_select();
     return (sel && sel->quick &&
             (sel->quick->get_type() == QUICK_SELECT_I::QS_TYPE_GROUP_MIN_MAX));
   }
   bool is_using_agg_loose_index_scan ()
   {
+    const SQL_SELECT *sel= get_sql_select();
     return (is_using_loose_index_scan() &&
-            ((QUICK_GROUP_MIN_MAX_SELECT *)select->quick)->is_agg_distinct());
+            ((QUICK_GROUP_MIN_MAX_SELECT *)sel->quick)->is_agg_distinct());
+  }
+  const SQL_SELECT *get_sql_select()
+  {
+    return filesort ? filesort->select : select;
   }
   bool is_inner_table_of_semi_join_with_first_match()
   {