Fix for bug #6462 "Same request on same data returns different

results." a.k.a. "Proper cleanup of subqueries is missing for SET and DO
statements". (Version #2 with after-review fixes).

To perform proper cleanup for statements that can contain subqueries 
but don't have main select we must call free_undelaid_joins().

mysql-test/r/subselect.result

@@ -1990,3 +1990,18 @@ ac
 700
 NULL
 drop tables t1,t2;
+create table t1 (a int not null, b int not null, c int, primary key (a,b));
+insert into t1 values (1,1,1), (2,2,2), (3,3,3);
+set @b:= 0;
+explain select sum(a) from t1 where b > @b;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index	NULL	PRIMARY	8	NULL	3	Using where; Using index
+set @a:= (select sum(a) from t1 where b > @b);
+explain select a from t1 where c=2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	Using where
+do @a:= (select sum(a) from t1 where b > @b);
+explain select a from t1 where c=2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	Using where
+drop table t1;

mysql-test/t/subselect.test

@@ -1282,3 +1282,22 @@ INSERT INTO `t2` VALUES (6,5,12,7,'a'),(12,0,0,7,'a'),(12,1,0,7,'a'),(12,5,5,7,'
 SELECT b.sc FROM (SELECT (SELECT a.access FROM t1 a WHERE a.map = op.map AND a.slave = op.pid AND a.master = 1) ac FROM t2 op WHERE op.id = 12 AND op.map = 0) b;
 SELECT b.ac FROM (SELECT (SELECT a.access FROM t1 a WHERE a.map = op.map AND a.slave = op.pid AND a.master = 1) ac FROM t2 op WHERE op.id = 12 AND op.map = 0) b;
 drop tables t1,t2;
+
+#
+# Test for bug #6462. "Same request on same data returns different
+# results." a.k.a. "Proper cleanup of subqueries is missing for 
+# SET and DO statements".
+#
+create table t1 (a int not null, b int not null, c int, primary key (a,b));
+insert into t1 values (1,1,1), (2,2,2), (3,3,3);
+set @b:= 0;
+# Let us check that subquery will use covering index
+explain select sum(a) from t1 where b > @b;
+# This should not crash -debug server due to failing assertion
+set @a:= (select sum(a) from t1 where b > @b);
+# And this should not falsely report index usage
+explain select a from t1 where c=2;
+# Same for DO statement
+do @a:= (select sum(a) from t1 where b > @b);
+explain select a from t1 where c=2;
+drop table t1;

sql/set_var.cc

@@ -2703,13 +2703,18 @@ int sql_set_variables(THD *thd, List<set_var_base> *var_list)
   while ((var=it++))
   {
     if ((error=var->check(thd)))
-      DBUG_RETURN(error);
+      goto err;
   }
-  if (thd->net.report_error)
-    DBUG_RETURN(1);
+  if (!thd->net.report_error)
+  {
     it.rewind();
-  while ((var=it++))
+    while ((var= it++))
       error|= var->update(thd);         // Returns 0, -1 or 1
+  }
+  else
+    error= 1;
+err:
+  free_underlaid_joins(thd, &thd->lex->select_lex);
   DBUG_RETURN(error);
 }
 

sql/sql_do.cc

@@ -29,6 +29,7 @@ int mysql_do(THD *thd, List<Item> &values)
     DBUG_RETURN(-1);
   while ((value = li++))
     value->val_int();
+  free_underlaid_joins(thd, &thd->lex->select_lex);
   thd->clear_error(); // DO always is OK
   send_ok(thd);
   DBUG_RETURN(0);