Fix for bug #34928: Confusion by having Primary Key and Index

The bug is a regression introduced in 5.1 by the patch for bug28404.

Under some circumstances test_if_skip_sort_order() could leave some
data structures in an inconsistent state so that some parts of code
could assume the selected execution strategy for GROUP BY/DISTINCT as
a loose index scan (e.g. JOIN_TAB::is_using_loose_index_scan()), while
the actual strategy chosen was an ordered index scan, which led to
wrong data being returned.

Fixed test_if_skip_sort_order() so that when changing the type for a
join table, select->quick is reset not only for EXPLAIN, but for the 
actual join execution as well, to not confuse code that depends on its
value to determine the chosen GROUP BY/DISTINCT strategy.

mysql-test/r/distinct.result

@@ -682,3 +682,26 @@ a	a	b
 1	1	3
 DROP TABLE t1;
 End of 5.0 tests
+CREATE TABLE t1(a INT, b INT, c INT, d INT, e INT,
+PRIMARY KEY(a,b,c,d,e),
+KEY(a,b,d,c)
+);
+INSERT INTO t1(a, b, c) VALUES (1, 1, 1),
+(1, 1, 2),
+(1, 1, 3),
+(1, 2, 1),
+(1, 2, 2),
+(1, 2, 3);
+EXPLAIN SELECT DISTINCT a, b, d, c FROM t1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index	NULL	a	16	NULL	6	Using index
+SELECT DISTINCT a, b, d, c FROM t1;
+a	b	d	c
+1	1	0	1
+1	1	0	2
+1	1	0	3
+1	2	0	1
+1	2	0	2
+1	2	0	3
+DROP TABLE t1;
+End of 5.1 tests

mysql-test/t/distinct.test

@@ -553,3 +553,26 @@ SELECT DISTINCT a, a, b FROM t1;
 DROP TABLE t1;
 
 --echo End of 5.0 tests
+
+#
+# Bug #34928: Confusion by having Primary Key and Index
+#
+CREATE TABLE t1(a INT, b INT, c INT, d INT, e INT,
+                PRIMARY KEY(a,b,c,d,e),
+                KEY(a,b,d,c)
+);
+
+INSERT INTO t1(a, b, c) VALUES (1, 1, 1),
+                               (1, 1, 2),
+                               (1, 1, 3),
+                               (1, 2, 1),
+                               (1, 2, 2),
+                               (1, 2, 3);
+
+EXPLAIN SELECT DISTINCT a, b, d, c FROM t1;
+
+SELECT DISTINCT a, b, d, c FROM t1;
+
+DROP TABLE t1;
+
+--echo End of 5.1 tests

sql/sql_select.cc

@@ -13158,6 +13158,11 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
           tab->read_first_record= best_key_direction > 0 ?
                                   join_read_first:join_read_last;
           tab->type=JT_NEXT;           // Read with index_first(), index_next()
+          if (select && select->quick)
+          {
+            delete select->quick;
+            select->quick= 0;
+          }
           if (table->covering_keys.is_set(best_key))
           {
             table->key_read=1;
@@ -13168,11 +13173,6 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
           {
             tab->ref.key= -1;
             tab->ref.key_parts= 0;
-            if (select && select->quick)
-            {
-              delete select->quick;
-              select->quick= 0;
-            }
             if (select_limit < table_records) 
               tab->limit= select_limit;
           }