Fix bug lp:806943

Analysis:
This bug is yet another incarnation of the generic problem
where optimization of the outer query triggers evaluation
of a subquery, and this evaluation performs a destructive
change to the subquery plan. Specifically a temp table is
created for the DISTINCT operation that replaces the
original subquery table. Later, select_describe() attempts
to print the table name, however, there is no corresponding
TABLE_LIST object to the internal temp table, so we get a
crash. Execution works fine because it is not interested in
the corresponding TABLE_LIST object (or its name).

Solution:
Similar to other such bugs, block the evaluation of expensive
Items in convert_const_to_int().

1 files changed, 33 insertions, 0 deletions

diff --git a/mysql-test/t/subselect4.test b/mysql-test/t/subselect4.test
index 69ef5ffbd21..1cc3a7314a3 100644
--- a/mysql-test/t/subselect4.test
+++ b/mysql-test/t/subselect4.test
@@ -1461,4 +1461,37 @@ WHERE t2.f2 = (SELECT f2 FROM t3
 
 drop table t1,t2,t3,t4;
 
+--echo #
+--echo # LP BUG#806943 Second crash with select_describe with nested subqueries in maria-5.3
+--echo #
+
+CREATE TABLE t1 ( f4 int) ;
+INSERT INTO t1 VALUES (0),(0);
+
+CREATE TABLE t2 ( f2 int) ;
+
+CREATE TABLE t3 ( f1 int NOT NULL );
+
+CREATE TABLE t4 ( f2 int, f3 int) ;
+INSERT INTO t4 VALUES (8,0),(3,0);
+
+EXPLAIN SELECT *
+FROM t2, t3
+WHERE t3.f1 = (
+        SELECT SUM( f2 )
+        FROM t4
+        WHERE EXISTS (
+                SELECT DISTINCT f4
+                FROM t1));
+SELECT *
+FROM t2, t3
+WHERE t3.f1 = (
+        SELECT SUM( f2 )
+        FROM t4
+        WHERE EXISTS (
+                SELECT DISTINCT f4
+                FROM t1));
+
+drop table t1, t2, t3, t4;
+
 set optimizer_switch=@subselect4_tmp;