Fix bug#13293 Wrongly used index results in endless loop.

Loose index scan using only second part of multipart index was choosen, which
results in creating wrong keys and endless loop.

get_best_group_min_max() now allows loose index scan for distinct only if used
keyparts forms a prefix of the index.

mysql-test/r/group_min_max.result

@@ -2002,3 +2002,13 @@ a	count(a)
 1	1
 NULL	1
 drop table t1;
+create table t1 (f1 int, f2 char(1), primary key(f1,f2)) engine=innodb;
+insert into t1 values ( 1,"e"),(2,"a"),( 3,"c"),(4,"d");
+alter table t1 drop primary key, add primary key (f2, f1);
+explain select distinct f1 a, f1 b from t1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index	NULL	PRIMARY	5	NULL	4	Using index; Using temporary
+explain select distinct f1, f2 from t1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	NULL	PRIMARY	5	NULL	3	Using index for group-by; Using temporary
+drop table t1;

mysql-test/t/group_min_max.test

@@ -693,3 +693,13 @@ create table t1(a int, key(a)) engine=innodb;
 insert into t1 values(1);
 select a, count(a) from t1 group by a with rollup;
 drop table t1;
+
+#
+# Bug #13293 Wrongly used index results in endless loop.
+#
+create table t1 (f1 int, f2 char(1), primary key(f1,f2)) engine=innodb;
+insert into t1 values ( 1,"e"),(2,"a"),( 3,"c"),(4,"d");
+alter table t1 drop primary key, add primary key (f2, f1);
+explain select distinct f1 a, f1 b from t1;
+explain select distinct f1, f2 from t1;
+drop table t1;

sql/opt_range.cc

@@ -7197,6 +7197,7 @@ get_best_group_min_max(PARAM *param, SEL_TREE *tree)
     {
       select_items_it.rewind();
       cur_used_key_parts.clear_all();
+      uint max_key_part= 0;
       while ((item= select_items_it++))
       {
         item_field= (Item_field*) item; /* (SA5) already checked above. */
@@ -7214,7 +7215,19 @@ get_best_group_min_max(PARAM *param, SEL_TREE *tree)
         cur_group_prefix_len+= cur_part->store_length;
         cur_used_key_parts.set_bit(key_part_nr);
         ++cur_group_key_parts;
+        max_key_part= max(max_key_part,key_part_nr);
       }
+      /*
+        Check that used key parts forms a prefix of the index.
+        To check this we compare bits in all_parts and cur_parts.
+        all_parts have all bits set from 0 to (max_key_part-1).
+        cur_parts have bits set for only used keyparts.
+      */
+      ulonglong all_parts, cur_parts;
+      all_parts= (1<<max_key_part) - 1;
+      cur_parts= cur_used_key_parts.to_ulonglong() >> 1;
+      if (all_parts != cur_parts)
+        goto next_index;
     }
     else
       DBUG_ASSERT(FALSE);