func_gconcat.result, func_gconcat.test:

  Added a test case for bug #12095.
sql_class.h:
  Fixed bug #12095: a join query with GROUP_CONCAT over a single row table.
  Added a flag to the TMP_TABLE_PARAM class forcing to put constant
  items generated after elimination of a single row table into temp table
  in some cases (e.g. when GROUP_CONCAT is calculated over a single row
  table).
  bk ci sql/item_sum.cc
  Fixed bug #12095: a join query with GROUP_CONCAT over a single row table.
  If GROUP_CONCAT is calculated we always put its argument into a temp
  table, even when the argument is a constant item.
sql_select.cc:
  Fixed bug #12095: a join query with GROUP_CONCAT over one row table.
  If temp table is used to calculate GROUP_CONCAT the argument should
  be always put into this table, even when it is a constant item.

mysql-test/r/func_gconcat.result

@@ -517,3 +517,34 @@ a	group_concat(distinct b order by b)
 2	3,7
 NULL	1,2,3,4,7
 drop table t1;
+CREATE TABLE t1 (
+aID smallint(5) unsigned NOT NULL auto_increment,
+sometitle varchar(255) NOT NULL default '',
+bID smallint(5) unsigned NOT NULL,
+PRIMARY KEY  (aID),
+UNIQUE KEY sometitle (sometitle)
+);
+INSERT INTO t1 SET sometitle = 'title1', bID = 1;
+INSERT INTO t1 SET sometitle = 'title2', bID = 1;
+CREATE TABLE t2 (
+bID smallint(5) unsigned NOT NULL auto_increment,
+somename varchar(255) NOT NULL default '',
+PRIMARY KEY  (bID),
+UNIQUE KEY somename (somename)
+);
+INSERT INTO t2 SET somename = 'test';
+SELECT COUNT(*), GROUP_CONCAT(DISTINCT t2.somename SEPARATOR ' |')
+FROM t1 JOIN t2 ON t1.bID = t2.bID;
+COUNT(*)	GROUP_CONCAT(DISTINCT t2.somename SEPARATOR ' |')
+2	test
+INSERT INTO t2 SET somename = 'test2';
+SELECT COUNT(*), GROUP_CONCAT(DISTINCT t2.somename SEPARATOR ' |')
+FROM t1 JOIN t2 ON t1.bID = t2.bID;
+COUNT(*)	GROUP_CONCAT(DISTINCT t2.somename SEPARATOR ' |')
+2	test
+DELETE FROM t2 WHERE somename = 'test2';
+SELECT COUNT(*), GROUP_CONCAT(DISTINCT t2.somename SEPARATOR ' |')
+FROM t1 JOIN t2 ON t1.bID = t2.bID;
+COUNT(*)	GROUP_CONCAT(DISTINCT t2.somename SEPARATOR ' |')
+2	test
+DROP TABLE t1,t2;

mysql-test/t/func_gconcat.test

@@ -310,4 +310,37 @@ select a, group_concat(b order by b) from t1 group by a with rollup;
 select a, group_concat(distinct b order by b) from t1 group by a with rollup;
 drop table t1;
 
+#
+# Bug #12095: GROUP_CONCAT for one row table 
+#
+
+CREATE TABLE t1 (
+  aID smallint(5) unsigned NOT NULL auto_increment,
+  sometitle varchar(255) NOT NULL default '',
+  bID smallint(5) unsigned NOT NULL,
+  PRIMARY KEY  (aID),
+  UNIQUE KEY sometitle (sometitle)
+);
+INSERT INTO t1 SET sometitle = 'title1', bID = 1;
+INSERT INTO t1 SET sometitle = 'title2', bID = 1;
+
+CREATE TABLE t2 (
+  bID smallint(5) unsigned NOT NULL auto_increment,
+  somename varchar(255) NOT NULL default '',
+  PRIMARY KEY  (bID),
+  UNIQUE KEY somename (somename)
+);
+INSERT INTO t2 SET somename = 'test';
+
+SELECT COUNT(*), GROUP_CONCAT(DISTINCT t2.somename SEPARATOR ' |')
+  FROM t1 JOIN t2 ON t1.bID = t2.bID;
+INSERT INTO t2 SET somename = 'test2';
+SELECT COUNT(*), GROUP_CONCAT(DISTINCT t2.somename SEPARATOR ' |')
+  FROM t1 JOIN t2 ON t1.bID = t2.bID;
+DELETE FROM t2 WHERE somename = 'test2';
+SELECT COUNT(*), GROUP_CONCAT(DISTINCT t2.somename SEPARATOR ' |')
+  FROM t1 JOIN t2 ON t1.bID = t2.bID;
+
+DROP TABLE t1,t2;
+
 # End of 4.1 tests

sql/sql_class.h

@@ -1325,10 +1325,12 @@ public:
   bool  using_indirect_summary_function;
   /* If >0 convert all blob fields to varchar(convert_blob_length) */
   uint  convert_blob_length; 
+  bool need_const; /* <=> const items are saved in tmp table */
 
   TMP_TABLE_PARAM()
     :copy_field(0), group_parts(0),
-    group_length(0), group_null_parts(0), convert_blob_length(0)
+    group_length(0), group_null_parts(0), convert_blob_length(0),
+    need_const(0) 
   {}
   ~TMP_TABLE_PARAM()
   {

sql/sql_select.cc

@@ -5201,7 +5201,8 @@ create_tmp_table(THD *thd,TMP_TABLE_PARAM *param,List<Item> &fields,
 	param->using_indirect_summary_function=1;
 	continue;
       }
-      if (item->const_item() && (int) hidden_field_count <= 0)
+      if (item->const_item() && (int) hidden_field_count <= 0 &&
+          !param->need_const)
         continue; // We don't have to store this
     }
     if (type == Item::SUM_FUNC_ITEM && !group && !save_sum_fields)