Fixed bug #21727.

This is a performance issue for queries with subqueries evaluation
of which requires filesort.
Allocation of memory for the sort buffer at each evaluation of a
subquery may take a significant amount of time if the buffer is rather big.
With the fix we allocate the buffer at the first evaluation of the
subquery and reuse it at each subsequent evaluation.

1 files changed, 17 insertions, 0 deletions

diff --git a/sql/table.cc b/sql/table.cc
index 4dd3494f834..851b747dc83 100644
--- a/sql/table.cc
+++ b/sql/table.cc
@@ -3018,6 +3018,23 @@ void st_table_list::reinit_before_use(THD *thd)
          embedding->nested_join->join_list.head() == embedded);
 }
 
+/*
+  Return subselect that contains the FROM list this table is taken from
+
+  SYNOPSIS
+    st_table_list::containing_subselect()
+ 
+  RETURN
+    Subselect item for the subquery that contains the FROM list
+    this table is taken from if there is any
+    0 - otherwise
+
+*/
+
+Item_subselect *st_table_list::containing_subselect()
+{    
+  return (select_lex ? select_lex->master_unit()->item : 0);
+}
 
 /*****************************************************************************
 ** Instansiate templates