Merge gkodinov@bk-internal.mysql.com:/home/bk/mysql-5.0-opt

into  magare.gmz:/home/kgeorge/mysql/autopush/B27807-5.0-opt

mysql-test/r/subselect.result

@@ -4034,4 +4034,11 @@ SUM( (SELECT AVG( (SELECT COUNT(*) FROM t1 t HAVING XXA < 12) ) FROM t2) )
 FROM t1;
 ERROR HY000: Invalid use of group function
 DROP TABLE t1,t2;
+CREATE TABLE t1 (a int, b int, KEY (a));
+INSERT INTO t1 VALUES (1,1),(2,1);
+EXPLAIN SELECT 1 FROM t1 WHERE a = (SELECT COUNT(*) FROM t1 GROUP BY b);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ref	a	a	5	const	1	Using where; Using index
+2	SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	2	Using temporary; Using filesort
+DROP TABLE t1;
 End of 5.0 tests.

mysql-test/t/subselect.test

@@ -2874,4 +2874,12 @@ FROM t1;
 
 DROP TABLE t1,t2;
 
+#
+# Bug #27807: Server crash when executing subquery with EXPLAIN
+#  
+CREATE TABLE t1 (a int, b int, KEY (a)); 
+INSERT INTO t1 VALUES (1,1),(2,1);
+EXPLAIN SELECT 1 FROM t1 WHERE a = (SELECT COUNT(*) FROM t1 GROUP BY b);
+DROP TABLE t1;
+
 --echo End of 5.0 tests.

sql/item_subselect.cc

@@ -1774,6 +1774,21 @@ int subselect_single_select_engine::exec()
       thd->lex->current_select= save_select;
       DBUG_RETURN(join->error ? join->error : 1);
     }
+    if (!select_lex->uncacheable && thd->lex->describe && 
+        !(join->select_options & SELECT_DESCRIBE) && 
+        join->need_tmp && item->const_item())
+    {
+      /*
+        Force join->join_tmp creation, because this subquery will be replaced
+        by a simple select from the materialization temp table by optimize()
+        called by EXPLAIN and we need to preserve the initial query structure
+        so we can display it.
+       */
+      select_lex->uncacheable|= UNCACHEABLE_EXPLAIN;
+      select_lex->master_unit()->uncacheable|= UNCACHEABLE_EXPLAIN;
+      if (join->init_save_join_tab())
+        DBUG_RETURN(1);
+    }
     if (item->engine_changed)
     {
       DBUG_RETURN(1);

sql/sql_select.cc

@@ -1426,14 +1426,13 @@ JOIN::optimize()
       }
     }
 
-    if (select_lex->uncacheable && !is_top_level_join())
-    {
-      /* If this join belongs to an uncacheable subquery */
-      if (!(tmp_join= (JOIN*)thd->alloc(sizeof(JOIN))))
+    /* 
+      If this join belongs to an uncacheable subquery save 
+      the original join 
+    */
+    if (select_lex->uncacheable && !is_top_level_join() &&
+        init_save_join_tab())
       DBUG_RETURN(-1);
-      error= 0;				// Ensure that tmp_join.error= 0
-      restore_tmp();
-    }
   }
 
   error= 0;
@@ -1495,6 +1494,27 @@ JOIN::reinit()
   DBUG_RETURN(0);
 }
 
+/**
+   @brief Save the original join layout
+      
+   @details Saves the original join layout so it can be reused in 
+   re-execution and for EXPLAIN.
+             
+   @return Operation status
+   @retval 0      success.
+   @retval 1      error occurred.
+*/
+
+bool
+JOIN::init_save_join_tab()
+{
+  if (!(tmp_join= (JOIN*)thd->alloc(sizeof(JOIN))))
+    return 1;
+  error= 0;				       // Ensure that tmp_join.error= 0
+  restore_tmp();
+  return 0;
+}
+
 
 bool
 JOIN::save_join_tab()

sql/sql_select.h

@@ -434,6 +434,7 @@ public:
   void cleanup(bool full);
   void clear();
   bool save_join_tab();
+  bool init_save_join_tab();
   bool send_row_on_empty_set()
   {
     return (do_send_rows && tmp_table_param.sum_func_count != 0 &&