Bug#49487: crash with explain extended and group_concat in a derived table

When EXPLAIN EXTENDED tries to print column names, it checks whether the
referenced table is CONST (in which case, the column's value rather than
its name will be printed). If no proper table is reference (i.e. because
a derived table was used that has since gone out of scope), this will fail
spectacularly.

This ports an equivalent of the fix for Bug 43354.

mysql-test/r/func_gconcat.result:
  Show that EXPLAIN EXTENDED on a GROUP_CONCAT() on a derived table
  no longer crashes the server.
mysql-test/t/func_gconcat.test:
  Show that EXPLAIN EXTENDED on a GROUP_CONCAT() on a derived table
  no longer crashes the server.
sql/item_sum.cc:
  Do not de-ref what cannot be, that is, temp-tables that have gone away.
  This is of questionable utility anyway, since our deref has the sole
  purpose of checking whether the table is const (in which case, we'll
  substitute the column with its value in EXPLAIN EXTENDED - that is all).

1 files changed, 15 insertions, 0 deletions

diff --git a/mysql-test/t/func_gconcat.test b/mysql-test/t/func_gconcat.test
index 816ac9c2959..1cbf045e95d 100644
--- a/mysql-test/t/func_gconcat.test
+++ b/mysql-test/t/func_gconcat.test
@@ -693,4 +693,19 @@ SELECT 1 FROM t1 WHERE t1.a NOT IN
 
 DROP TABLE t1, t2;
 
+#
+# Bug #49487: crash with explain extended and group_concat in a derived table
+#
+
+CREATE TABLE t1 (f1 INT);
+INSERT INTO t1 VALUES (),();
+
+EXPLAIN EXTENDED SELECT 1 FROM
+ (SELECT DISTINCT GROUP_CONCAT(td.f1) FROM t1,t1 AS td GROUP BY td.f1) AS d,t1;
+
+SELECT 1 FROM
+ (SELECT DISTINCT GROUP_CONCAT(td.f1) FROM t1,t1 AS td GROUP BY td.f1) AS d,t1;
+
+DROP TABLE t1;
+
 --echo End of 5.0 tests