MDEV-6484: Assertion `tab->ref.use_count' failed on query with joins, constant...

MDEV-6484: Assertion `tab->ref.use_count' failed on query with joins, constant table, multi-part key

- test_if_skip_sort_order()/create_ref_for_key() may change table 
  access from EQ_REF(index1) to REF(index2). 
- Doing so doesn't make much sense from optimization POV, but since 
  they are doing it, they should update tab->read_record.unlock_row
  accordingly.

mysql-test/r/group_by.result

@@ -2482,3 +2482,14 @@ test
 1
 2
 SET sql_mode='';
+#
+# MDEV-6484: Assertion `tab->ref.use_count' failed on query with joins, constant table, multi-part key
+#
+CREATE TABLE t1 (i1 INT, c1 VARCHAR(1)) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (6,'b');
+CREATE TABLE t2 (pk2 INT, i2 INT, c2 VARCHAR(1), PRIMARY KEY(pk2), KEY(pk2,i2)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES (1,2,'s'),(2,4,'r'),(3,8,'m'),(4,4,'b'),(5,4,'x'),(6,7,'g'),(7,4,'p');
+SELECT i2 FROM t1 AS t1a STRAIGHT_JOIN ( t2 INNER JOIN t1 AS t1b ON (t1b.c1 = c2) ) ON (t1b.i1 = pk2 ) 
+WHERE t1a.c1 = c2 GROUP BY i2;
+i2
+DROP TABLE t1,t2;

mysql-test/t/group_by.test

@@ -1651,6 +1651,20 @@ SET sql_mode='ONLY_FULL_GROUP_BY';
 SELECT 1 AS test UNION SELECT 2 AS test ORDER BY test IS NULL ASC;
 SET sql_mode='';
 
+--echo #
+--echo # MDEV-6484: Assertion `tab->ref.use_count' failed on query with joins, constant table, multi-part key
+--echo #
+CREATE TABLE t1 (i1 INT, c1 VARCHAR(1)) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (6,'b');
+
+CREATE TABLE t2 (pk2 INT, i2 INT, c2 VARCHAR(1), PRIMARY KEY(pk2), KEY(pk2,i2)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES (1,2,'s'),(2,4,'r'),(3,8,'m'),(4,4,'b'),(5,4,'x'),(6,7,'g'),(7,4,'p');
+
+SELECT i2 FROM t1 AS t1a STRAIGHT_JOIN ( t2 INNER JOIN t1 AS t1b ON (t1b.c1 = c2) ) ON (t1b.i1 = pk2 ) 
+WHERE t1a.c1 = c2 GROUP BY i2;
+
+DROP TABLE t1,t2;
+
 #
 # End of MariaDB 5.5 tests
 #

sql/sql_select.cc

@@ -8178,6 +8178,9 @@ static bool create_ref_for_key(JOIN *join, JOIN_TAB *j,
   }
   else
     j->type=JT_EQ_REF;
+
+  j->read_record.unlock_row= (j->type == JT_EQ_REF)? 
+                             join_read_key_unlock_row : rr_unlock_row; 
   DBUG_RETURN(0);
 }