Bug#52711 Segfault when doing EXPLAIN SELECT with union...order by (select... where...)

backport from 5.1

mysql-test/r/subselect.result

@@ -4527,4 +4527,20 @@ pk	int_key
 3	3
 7	3
 DROP TABLE t1,t2;
+#
+# Bug #52711: Segfault when doing EXPLAIN SELECT with 
+#  union...order by (select... where...)
+#
+CREATE TABLE t1 (a VARCHAR(10), FULLTEXT KEY a (a));
+INSERT INTO t1 VALUES (1),(2);
+CREATE TABLE t2 (b INT);
+INSERT INTO t2 VALUES (1),(2);
+# Should not crash
+EXPLAIN
+SELECT * FROM t2 UNION SELECT * FROM t2
+ORDER BY (SELECT * FROM t1 WHERE MATCH(a) AGAINST ('+abc' IN BOOLEAN MODE));
+# Should not crash
+SELECT * FROM t2 UNION SELECT * FROM t2
+ORDER BY (SELECT * FROM t1 WHERE MATCH(a) AGAINST ('+abc' IN BOOLEAN MODE));
+DROP TABLE t1,t2;
 End of 5.0 tests.

mysql-test/t/subselect.test

@@ -3506,5 +3506,26 @@ ORDER BY outr.pk;
 
 DROP TABLE t1,t2;
 
+--echo #
+--echo # Bug #52711: Segfault when doing EXPLAIN SELECT with 
+--echo #  union...order by (select... where...)
+--echo #
+
+CREATE TABLE t1 (a VARCHAR(10), FULLTEXT KEY a (a));
+INSERT INTO t1 VALUES (1),(2);
+CREATE TABLE t2 (b INT);
+INSERT INTO t2 VALUES (1),(2);
+
+--echo # Should not crash
+--disable_result_log
+EXPLAIN
+SELECT * FROM t2 UNION SELECT * FROM t2
+  ORDER BY (SELECT * FROM t1 WHERE MATCH(a) AGAINST ('+abc' IN BOOLEAN MODE));
+
+--echo # Should not crash
+SELECT * FROM t2 UNION SELECT * FROM t2
+  ORDER BY (SELECT * FROM t1 WHERE MATCH(a) AGAINST ('+abc' IN BOOLEAN MODE));
+DROP TABLE t1,t2;
+--enable_result_log
 
 --echo End of 5.0 tests.

sql/sql_select.cc

@@ -501,7 +501,7 @@ JOIN::prepare(Item ***rref_pointer_array,
     thd->lex->allow_sum_func= save_allow_sum_func;
   }
 
-  if (!thd->lex->view_prepare_mode)
+  if (!thd->lex->view_prepare_mode && !(select_options & SELECT_DESCRIBE))
   {
     Item_subselect *subselect;
     /* Is it subselect? */
@@ -6861,7 +6861,8 @@ remove_const(JOIN *join,ORDER *first_order, COND *cond,
       *simple_order=0;				// Must do a temp table to sort
     else if (!(order_tables & not_const_tables))
     {
-      if (order->item[0]->with_subselect)
+      if (order->item[0]->with_subselect &&
+          !(join->select_lex->options & SELECT_DESCRIBE))
         order->item[0]->val_str(&order->item[0]->str_value);
       DBUG_PRINT("info",("removing: %s", order->item[0]->full_name()));
       continue;					// skip const item