Bug#48295: explain extended crash with subquery and ONLY_FULL_GROUP_BY sql

If an outer query is broken, a subquery might not even get set up.
EXPLAIN EXTENDED did not expect this and merrily tried to de-ref all
of the half-setup info.

We now catch this case and print as much as we have, as it doesn't cost us
anything (doesn't make regular execution slower).

backport from 5.1

mysql-test/r/explain.result

@@ -167,3 +167,20 @@ EXPLAIN SELECT DISTINCT 1 FROM t1,
 WHERE t1.a = d1.a;
 ERROR 42S22: Unknown column 'd1.a' in 'where clause'
 DROP TABLE t1;
+#
+# Bug#48295:
+# explain extended crash with subquery and ONLY_FULL_GROUP_BY sql_mode
+#
+CREATE TABLE t1 (f1 INT);
+SELECT @@session.sql_mode INTO @old_sql_mode;
+SET SESSION sql_mode='ONLY_FULL_GROUP_BY';
+EXPLAIN EXTENDED SELECT 1 FROM t1
+WHERE f1 > ALL( SELECT t.f1 FROM t1,t1 AS t );
+ERROR 42000: Mixing of GROUP columns (MIN(),MAX(),COUNT(),...) with no GROUP columns is illegal if there is no GROUP BY clause
+SHOW WARNINGS;
+Level	Code	Message
+Error	1140	Mixing of GROUP columns (MIN(),MAX(),COUNT(),...) with no GROUP columns is illegal if there is no GROUP BY clause
+Note	1003	select 1 AS `1` from `test`.`t1` where <not>(<exists>(...))
+SET SESSION sql_mode=@old_sql_mode;
+DROP TABLE t1;
+End of 5.0 tests.

mysql-test/t/explain.test

@@ -147,4 +147,24 @@ EXPLAIN SELECT DISTINCT 1 FROM t1,
          WHERE t1.a = d1.a;
 DROP TABLE t1;
 
-# End of 5.0 tests.
+--echo #
+--echo # Bug#48295:
+--echo # explain extended crash with subquery and ONLY_FULL_GROUP_BY sql_mode
+--echo #
+
+CREATE TABLE t1 (f1 INT);
+
+SELECT @@session.sql_mode INTO @old_sql_mode;
+SET SESSION sql_mode='ONLY_FULL_GROUP_BY';
+
+# EXPLAIN EXTENDED (with subselect). used to crash. should give NOTICE.
+--error ER_MIX_OF_GROUP_FUNC_AND_FIELDS
+EXPLAIN EXTENDED SELECT 1 FROM t1
+                          WHERE f1 > ALL( SELECT t.f1 FROM t1,t1 AS t );
+SHOW WARNINGS;
+
+SET SESSION sql_mode=@old_sql_mode;
+
+DROP TABLE t1;
+
+--echo End of 5.0 tests.

sql/item_subselect.cc

@@ -262,9 +262,14 @@ void Item_subselect::update_used_tables()
 
 void Item_subselect::print(String *str)
 {
+  if (engine)
+  {
     str->append('(');
     engine->print(str);
     str->append(')');
+  }
+  else
+    str->append("(...)");
 }