Bug #27807.

Non-correlated scalar subqueries may get executed
in EXPLAIN at the optimization phase if they are
part of a right hand sargable expression.
If the scalar subquery uses a temp table to 
materialize its results it will replace the 
subquery structure from the parser with a simple
select from the materialization table.
As a result the EXPLAIN will crash as the 
temporary materialization table is not to be shown
in EXPLAIN at all.
Fixed by preserving the original query structure
right after calling optimize() for scalar subqueries
with temp tables executed during EXPLAIN.

1 files changed, 15 insertions, 0 deletions

diff --git a/sql/item_subselect.cc b/sql/item_subselect.cc
index b3744d6eb96..a4d07e08473 100644
--- a/sql/item_subselect.cc
+++ b/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);