Fixed bug #28728: a crash when executing EXPLAIN EXTENDED for a query

using a derived table over a grouping subselect.

This crash happens only when materialization of the derived tables 
requires creation of auxiliary temporary table, for example when
a grouping operation is carried out with usage of a temporary table.

The crash happened because EXPLAIN EXTENDED when printing the query
expression made an attempt to use the objects created in the mem_root
of the temporary table which has been already freed by the moment
when printing is called.

This bug appeared after the method Item_field::print() had been 
introduced.    


mysql-test/r/subselect.result:
  Added a test case for bug #28728.
mysql-test/t/subselect.test:
  Added a test case for bug #28728.
sql/sql_select.cc:
  Fixed bug #28728: a crash when executing EXPLAIN EXTENDED for a query
  using a derived table over a grouping subselect.
  The crash happened because EXPLAIN EXTENDED when printing the query
  expression made an attempt to use the objects created in the mem_root
  of the temporary table which has been already freed by the moment
  when printing is accomplished.
  The fix in JOIN::exec() ensures using existing objects when printing 
  subselects for a derived tables by EXPLAIN EXTENDED.

1 files changed, 11 insertions, 0 deletions

diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index 78d213a45c4..41688794721 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -2075,6 +2075,17 @@ JOIN::exec()
   thd->examined_row_count+= curr_join->examined_rows;
   DBUG_PRINT("counts", ("thd->examined_row_count: %lu",
                         (ulong) thd->examined_row_count));
+
+  /* 
+    With EXPLAIN EXTENDED we have to restore original ref_array
+    for a derived table which is always materialized.
+    Otherwise we would not be able to print the query  correctly.
+  */ 
+  if (items0 &&
+      (thd->lex->describe & DESCRIBE_EXTENDED) &&
+      select_lex->linkage == DERIVED_TABLE_TYPE)      
+    set_items_ref_array(items0);
+
   DBUG_VOID_RETURN;
 }