bug#28273: GROUP_CONCAT and ORDER BY: No warning when result gets truncated.

When using GROUP_CONCAT with ORDER BY, a tree is used for the sorting, as 
opposed to normal nested loops join used when there is no ORDER BY. 

The tree traversal that generates the result counts the lines that have been 
cut down. (as they get cut down to the field's max_size)
But the check of that count was before the tree traversal, so no 
warning was generated if the output is truncated.

Fixed by moving the check to after the tree traversal.

mysql-test/r/func_gconcat.result

@@ -737,4 +737,30 @@ SELECT GROUP_CONCAT(DISTINCT UCASE(b)) FROM t1;
 GROUP_CONCAT(DISTINCT UCASE(b))
 ONE.1,TWO.2,ONE.3
 DROP TABLE t1;
+CREATE TABLE t1( a VARCHAR( 10 ), b INT );
+INSERT INTO t1 VALUES ( repeat( 'a', 10 ), 1), 
+( repeat( 'b', 10 ), 2);
+SET group_concat_max_len = 20;
+SELECT GROUP_CONCAT( a ) FROM t1;
+GROUP_CONCAT( a )
+aaaaaaaaaa,bbbbbbbbb
+Warnings:
+Warning	1260	1 line(s) were cut by GROUP_CONCAT()
+SELECT GROUP_CONCAT( DISTINCT a ) FROM t1;
+GROUP_CONCAT( DISTINCT a )
+aaaaaaaaaa,bbbbbbbbb
+Warnings:
+Warning	1260	1 line(s) were cut by GROUP_CONCAT()
+SELECT GROUP_CONCAT( a ORDER BY b ) FROM t1;
+GROUP_CONCAT( a ORDER BY b )
+aaaaaaaaaa,bbbbbbbbb
+Warnings:
+Warning	1260	1 line(s) were cut by GROUP_CONCAT()
+SELECT GROUP_CONCAT( DISTINCT a ORDER BY b ) FROM t1;
+GROUP_CONCAT( DISTINCT a ORDER BY b )
+aaaaaaaaaa,bbbbbbbbb
+Warnings:
+Warning	1260	1 line(s) were cut by GROUP_CONCAT()
+SET group_concat_max_len = DEFAULT;
+DROP TABLE t1;
 End of 5.0 tests

mysql-test/t/func_gconcat.test

@@ -507,4 +507,18 @@ SELECT GROUP_CONCAT(DISTINCT UCASE(a)) FROM t1;
 SELECT GROUP_CONCAT(DISTINCT UCASE(b)) FROM t1;
 DROP TABLE t1;
 
+#
+# Bug #28273: GROUP_CONCAT and ORDER BY: No warning when result gets truncated.
+#
+CREATE TABLE t1( a VARCHAR( 10 ), b INT );
+INSERT INTO t1 VALUES ( repeat( 'a', 10 ), 1), 
+                      ( repeat( 'b', 10 ), 2);
+SET group_concat_max_len = 20;
+SELECT GROUP_CONCAT( a ) FROM t1;
+SELECT GROUP_CONCAT( DISTINCT a ) FROM t1;  
+SELECT GROUP_CONCAT( a ORDER BY b ) FROM t1;          
+SELECT GROUP_CONCAT( DISTINCT a ORDER BY b ) FROM t1; 
+SET group_concat_max_len = DEFAULT;
+DROP TABLE t1;
+
 --echo End of 5.0 tests

sql/item_sum.cc

@@ -3339,6 +3339,10 @@ String* Item_func_group_concat::val_str(String* str)
   DBUG_ASSERT(fixed == 1);
   if (null_value)
     return 0;
+  if (!result.length() && tree)
+    /* Tree is used for sorting as in ORDER BY */
+    tree_walk(tree, (tree_walk_action)&dump_leaf_key, (void*)this,
+              left_root_right);
   if (count_cut_values && !warning)
   {
     /*
@@ -3350,11 +3354,6 @@ String* Item_func_group_concat::val_str(String* str)
                           ER_CUT_VALUE_GROUP_CONCAT,
                           ER(ER_CUT_VALUE_GROUP_CONCAT));
   }
-  if (result.length())
-    return &result;
-  if (tree)
-    tree_walk(tree, (tree_walk_action)&dump_leaf_key, (void*)this,
-              left_root_right);
   return &result;
 }