Fixed bug #28728: a crash when executing EXPLAIN EXTENDED for a query

using a derived table over a grouping subselect.

This crash happens only when materialization of the derived tables 
requires creation of auxiliary temporary table, for example when
a grouping operation is carried out with usage of a temporary table.

The crash happened because EXPLAIN EXTENDED when printing the query
expression made an attempt to use the objects created in the mem_root
of the temporary table which has been already freed by the moment
when printing is called.

This bug appeared after the method Item_field::print() had been 
introduced.    

mysql-test/r/subselect.result

@@ -4071,4 +4071,14 @@ id	st
 2	GA
 4	FL
 DROP TABLE t1,t2;
+CREATE TABLE t1 (a int);
+INSERT INTO t1 VALUES (1), (2);
+EXPLAIN EXTENDED
+SELECT * FROM (SELECT count(*) FROM t1 GROUP BY a) as res;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	2	
+2	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	2	Using temporary; Using filesort
+Warnings:
+Note	1003	select `res`.`count(*)` AS `count(*)` from (select count(0) AS `count(*)` from `test`.`t1` group by `test`.`t1`.`a`) `res`
+DROP TABLE t1;
 End of 5.0 tests.

mysql-test/t/subselect.test

@@ -2906,4 +2906,18 @@ SELECT id, st FROM t1
 
 DROP TABLE t1,t2;
 
+#
+# Bug #28728: crash with EXPLAIN EXTENDED for a query with a derived table
+#             over a grouping subselect
+# 
+
+CREATE TABLE t1 (a int);
+
+INSERT INTO t1 VALUES (1), (2);
+
+EXPLAIN EXTENDED
+SELECT * FROM (SELECT count(*) FROM t1 GROUP BY a) as res;
+
+DROP TABLE t1;
+
 --echo End of 5.0 tests.

sql/sql_select.cc

@@ -2075,6 +2075,17 @@ JOIN::exec()
   thd->examined_row_count+= curr_join->examined_rows;
   DBUG_PRINT("counts", ("thd->examined_row_count: %lu",
                         (ulong) thd->examined_row_count));
+
+  /* 
+    With EXPLAIN EXTENDED we have to restore original ref_array
+    for a derived table which is always materialized.
+    Otherwise we would not be able to print the query  correctly.
+  */ 
+  if (items0 &&
+      (thd->lex->describe & DESCRIBE_EXTENDED) &&
+      select_lex->linkage == DERIVED_TABLE_TYPE)      
+    set_items_ref_array(items0);
+
   DBUG_VOID_RETURN;
 }