From f1fb30a15f6de6260337beae8e4c6a4230ef4a29 Mon Sep 17 00:00:00 2001
From: "evgen@moonbone.local" <>
Date: Wed, 31 Aug 2005 18:26:50 +0400
Subject: [PATCH] Fix bug #12861 client hang with group_concat insubquery FROM
 DUAL.

Item_func_group_concat::fix_fields() set maybe_null flag to 0, and set it to
1 only if some of it's arguments may be null. When used in subquery in tmp
table created field which can't be null. When no data retireved result field
have to be set to null and error mentioned in bug report occurs. Also this
bug can occur if selecting from not null field in empty table.

Function group_concat now marked maybe_null from the very beginning not only
if some of it's argument may be null.
---
 mysql-test/r/func_gconcat.result | 3 +++
 mysql-test/t/func_gconcat.test   | 5 +++++
 sql/item_sum.cc                  | 4 +---
 3 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/mysql-test/r/func_gconcat.result b/mysql-test/r/func_gconcat.result
index 9faee3cbc01..a6c25fcd26c 100644
--- a/mysql-test/r/func_gconcat.result
+++ b/mysql-test/r/func_gconcat.result
@@ -548,3 +548,6 @@ FROM t1 JOIN t2 ON t1.bID = t2.bID;
 COUNT(*)	GROUP_CONCAT(DISTINCT t2.somename SEPARATOR ' |')
 2	test
 DROP TABLE t1,t2;
+select * from (select group_concat('c') from DUAL) t;
+group_concat('c')
+NULL
diff --git a/mysql-test/t/func_gconcat.test b/mysql-test/t/func_gconcat.test
index 9793d0d0a2c..827a2813718 100644
--- a/mysql-test/t/func_gconcat.test
+++ b/mysql-test/t/func_gconcat.test
@@ -343,4 +343,9 @@ SELECT COUNT(*), GROUP_CONCAT(DISTINCT t2.somename SEPARATOR ' |')
 
 DROP TABLE t1,t2;
 
+#
+# Bug #12861 hang with group_concat insubquery FROM DUAL
+#
+select * from (select group_concat('c') from DUAL) t;
+
 # End of 4.1 tests
diff --git a/sql/item_sum.cc b/sql/item_sum.cc
index 74a7fee113e..000dcdb4997 100644
--- a/sql/item_sum.cc
+++ b/sql/item_sum.cc
@@ -1916,7 +1916,7 @@ Item_func_group_concat::fix_fields(THD *thd, TABLE_LIST *tables, Item **ref)
   }
   
   thd->allow_sum_func= 0;
-  maybe_null= 0;
+  maybe_null= 1;
   item_thd= thd;
 
   /*
@@ -1929,8 +1929,6 @@ Item_func_group_concat::fix_fields(THD *thd, TABLE_LIST *tables, Item **ref)
          args[i]->fix_fields(thd, tables, args + i)) ||
         args[i]->check_cols(1))
       return 1;
-    if (i < arg_count_field)
-      maybe_null|= args[i]->maybe_null;
   }
 
   if (agg_item_charsets(collation, func_name(),