Bug #27531: the 4.1 fix.

When checking for applicability of join cache
we must disable its usage only if there is no
temp table in use.
When a temp table is used we can use join
cache (and it will not make the result-set 
unordered) to fill the temp table. The filesort() 
operation is then applied to the data in the temp 
table and hence is not affected by join cache
usage.
Fixed by narrowing the condition for disabling 
join cache to exclude the case where temp table
is used.

mysql-test/r/join.result

@@ -376,3 +376,53 @@ i	i	i
 2	NULL	4
 2	2	2
 drop table t1,t2,t3;
+CREATE TABLE t1 (a int, b int default 0, c int default 1);
+INSERT INTO t1 (a) VALUES (1),(2),(3),(4),(5),(6),(7),(8);
+INSERT INTO t1 (a) SELECT a + 8 FROM t1;
+INSERT INTO t1 (a) SELECT a + 16 FROM t1;
+CREATE TABLE t2 (a int, d int, e int default 0);
+INSERT INTO t2 (a, d) VALUES (1,1),(2,2),(3,3),(4,4);
+INSERT INTO t2 (a, d) SELECT a+4, a+4 FROM t2;
+INSERT INTO t2 (a, d) SELECT a+8, a+8 FROM t2;
+EXPLAIN
+SELECT STRAIGHT_JOIN t2.e FROM t1,t2 WHERE t2.d=1 AND t1.b=t2.e
+ORDER BY t1.b, t1.c;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	32	Using temporary; Using filesort
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	16	Using where
+SELECT STRAIGHT_JOIN t2.e FROM t1,t2 WHERE t2.d=1 AND t1.b=t2.e
+ORDER BY t1.b, t1.c;
+e
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+DROP TABLE t1,t2;

mysql-test/t/join.test

@@ -326,4 +326,28 @@ select t1.i,t2.i,t3.i from t2 natural right join t3,t1 order by t1.i,t2.i,t3.i;
 select t1.i,t2.i,t3.i from t2 right join t3 on (t2.i=t3.i),t1 order by t1.i,t2.i,t3.i;
 drop table t1,t2,t3;
 
+#
+# Bug #27531: Query performance degredation in 4.1.22 and greater
+#
+CREATE TABLE t1 (a int, b int default 0, c int default 1);
+
+INSERT INTO t1 (a) VALUES (1),(2),(3),(4),(5),(6),(7),(8);
+INSERT INTO t1 (a) SELECT a + 8 FROM t1;
+INSERT INTO t1 (a) SELECT a + 16 FROM t1;
+
+CREATE TABLE t2 (a int, d int, e int default 0);
+
+INSERT INTO t2 (a, d) VALUES (1,1),(2,2),(3,3),(4,4);
+INSERT INTO t2 (a, d) SELECT a+4, a+4 FROM t2;
+INSERT INTO t2 (a, d) SELECT a+8, a+8 FROM t2;
+
+# should use join cache
+EXPLAIN
+SELECT STRAIGHT_JOIN t2.e FROM t1,t2 WHERE t2.d=1 AND t1.b=t2.e
+  ORDER BY t1.b, t1.c;
+SELECT STRAIGHT_JOIN t2.e FROM t1,t2 WHERE t2.d=1 AND t1.b=t2.e
+  ORDER BY t1.b, t1.c;
+
+DROP TABLE t1,t2;
+
 # End of 4.1 tests

sql/sql_select.cc

@@ -3939,14 +3939,17 @@ make_join_readinfo(JOIN *join, uint options)
       disable join cache because it will change the ordering of the results.
       Code handles sort table that is at any location (not only first after 
       the const tables) despite the fact that it's currently prohibited.
+      We must disable join cache if the first non-const table alone is
+      ordered. If there is a temp table the ordering is done as a last
+      operation and doesn't prevent join cache usage.
     */
-    if (!ordered_set && 
-        (table == join->sort_by_table &&
+    if (!ordered_set && !join->need_tmp &&
+        ((table == join->sort_by_table &&
          (!join->order || join->skip_sort_order ||
           test_if_skip_sort_order(tab, join->order, join->select_limit,
                                   1))
         ) ||
-        (join->sort_by_table == (TABLE *) 1 && i != join->const_tables))
+        (join->sort_by_table == (TABLE *) 1 && i != join->const_tables)))
       ordered_set= 1;
 
     switch (tab->type) {