* Bug #20792: Incorrect results from aggregate subquery

When processing aggregate functions all tables values are reset
to NULLs at the end of each group. 
When doing that if there are no rows found for a group
the const tables must not be reset as they are not recalculated 
by do_select()/sub_select() for each group.

mysql-test/r/subselect2.result

@@ -130,3 +130,15 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 5	DEPENDENT SUBQUERY	t3	unique_subquery	PRIMARY,FFOLDERID_IDX	PRIMARY	32	func	1	Using index; Using where
 6	DEPENDENT SUBQUERY	t3	unique_subquery	PRIMARY,FFOLDERID_IDX,CMFLDRPARNT_IDX	PRIMARY	32	func	1	Using index; Using where
 drop table t1, t2, t3, t4;
+CREATE TABLE t1 (a int(10) , PRIMARY KEY (a)) Engine=InnoDB;
+INSERT INTO t1 VALUES (1),(2);
+CREATE TABLE t2 (a int(10), PRIMARY KEY (a)) Engine=InnoDB;
+INSERT INTO t2 VALUES (1);
+CREATE TABLE t3 (a int(10), b int(10), c int(10),
+PRIMARY KEY (a)) Engine=InnoDB;
+INSERT INTO t3 VALUES (1,2,1);
+SELECT t1.* FROM t1 WHERE (SELECT COUNT(*) FROM t3,t2 WHERE t3.c=t2.a 
+and t2.a='1' AND t1.a=t3.b) > 0;
+a
+2
+DROP TABLE t1,t2,t3;

mysql-test/t/subselect2.test

@@ -150,3 +150,21 @@ EXPLAIN SELECT t2.*, t4.DOCTYPENAME, t1.CONTENTSIZE,t1.MIMETYPE FROM t2 INNER JO
 
 drop table t1, t2, t3, t4;
 # End of 4.1 tests
+
+#
+# Bug #20792: Incorrect results from aggregate subquery
+#
+CREATE TABLE t1 (a int(10) , PRIMARY KEY (a)) Engine=InnoDB;
+INSERT INTO t1 VALUES (1),(2);
+
+CREATE TABLE t2 (a int(10), PRIMARY KEY (a)) Engine=InnoDB;
+INSERT INTO t2 VALUES (1);
+
+CREATE TABLE t3 (a int(10), b int(10), c int(10),
+                PRIMARY KEY (a)) Engine=InnoDB;
+INSERT INTO t3 VALUES (1,2,1);
+
+SELECT t1.* FROM t1 WHERE (SELECT COUNT(*) FROM t3,t2 WHERE t3.c=t2.a 
+                           and t2.a='1' AND t1.a=t3.b) > 0;
+
+DROP TABLE t1,t2,t3;

sql/sql_select.cc

@@ -4452,10 +4452,16 @@ return_zero_rows(JOIN *join, select_result *result,TABLE_LIST *tables,
   DBUG_RETURN(0);
 }
 
-
+/*
+  used only in JOIN::clear
+*/
 static void clear_tables(JOIN *join)
 {
-  for (uint i=0 ; i < join->tables ; i++)
+  /* 
+    must clear only the non-const tables, as const tables
+    are not re-calculated.
+  */
+  for (uint i=join->const_tables ; i < join->tables ; i++)
     mark_as_null_row(join->table[i]);		// All fields are NULL
 }