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

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).

mysql-test/r/explain.result

@@ -194,4 +194,20 @@ dt
 2001-01-01 01:01:01
 2001-01-01 01:01:01
 drop tables t1, t2;
+#
+# 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 5.1 tests.

mysql-test/t/explain.test

@@ -167,4 +167,24 @@ flush tables;
 SELECT OUTR.dt FROM t1 AS OUTR WHERE OUTR.dt IN ( SELECT INNR.dt FROM t2 AS INNR WHERE OUTR.t < '2005-11-13 7:41:31' );
 drop tables t1, t2;
 
+--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 5.1 tests.

sql/item_subselect.cc

@@ -311,9 +311,14 @@ void Item_subselect::update_used_tables()
 
 void Item_subselect::print(String *str, enum_query_type query_type)
 {
-  str->append('(');
-  engine->print(str, query_type);
-  str->append(')');
+  if (engine)
+  {
+    str->append('(');
+    engine->print(str, query_type);
+    str->append(')');
+  }
+  else
+    str->append("(...)");
 }