Bug#50539: Wrong result when loose index scan is used for an aggregate

           function with distinct.
Loose index scan is used to find MIN/MAX values using appropriate index and
thus allow to avoid grouping. For each found row it updates non-aggregated
fields with values from row with found MIN/MAX value.
Without loose index scan non-aggregated fields are copied by end_send_group
function. With loose index scan there is no need in end_send_group and
end_send is used instead. Non-aggregated fields still need to be copied and
this was wrongly implemented in QUICK_GROUP_MIN_MAX_SELECT::get_next.
WL#3220 added a case when loose index scan can be used with end_send_group to
optimize calculation of aggregate functions with distinct. In this case
the row found by QUICK_GROUP_MIN_MAX_SELECT::get_next might belong to a next
group and copying it will produce wrong result.

Update of non-aggregated fields is moved to the end_send function from
QUICK_GROUP_MIN_MAX_SELECT::get_next.

Author: Evgeny Potemkin

mysql-test/r/group_min_max.result

@@ -2686,7 +2686,7 @@ a	c	COUNT(DISTINCT c, a, b)
 1	1	1
 1	1	1
 1	1	1
-2	1	1
+1	1	1
 2	1	1
 2	1	1
 2	1	1
@@ -2714,7 +2714,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	range	NULL	a	10	NULL	9	Using index for group-by
 SELECT a, COUNT(DISTINCT b), SUM(DISTINCT b) FROM t2 GROUP BY a;
 a	COUNT(DISTINCT b)	SUM(DISTINCT b)
-2	8	36
+1	8	36
 2	8	36
 EXPLAIN SELECT COUNT(DISTINCT b), SUM(DISTINCT b) FROM t2 GROUP BY a;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
@@ -2761,7 +2761,7 @@ SELECT 42 * (a + c + COUNT(DISTINCT c, a, b)) FROM t2 GROUP BY a, b, c;
 126
 126
 126
-168
+126
 168
 168
 168
@@ -2779,3 +2779,24 @@ SELECT (SUM(DISTINCT a) + MAX(b)) FROM t2 GROUP BY a;
 10
 DROP TABLE t1,t2;
 # end of WL#3220 tests
+#
+# Bug#50539: Wrong result when loose index scan is used for an aggregate
+#            function with distinct
+#
+CREATE TABLE t1 (
+f1 int(11) NOT NULL DEFAULT '0',
+f2 char(1) NOT NULL DEFAULT '',
+PRIMARY KEY (f1,f2)
+) ;
+insert into t1 values(1,'A'),(1 , 'B'), (1, 'C'), (2, 'A'), 
+(3, 'A'), (3, 'B'), (3, 'C'), (3, 'D');
+SELECT f1, COUNT(DISTINCT f2) FROM t1 GROUP BY f1;
+f1	COUNT(DISTINCT f2)
+1	3
+2	1
+3	4
+explain SELECT f1, COUNT(DISTINCT f2) FROM t1 GROUP BY f1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	NULL	PRIMARY	5	NULL	9	Using index for group-by (scanning)
+drop table t1;
+# End of test#50539.

mysql-test/t/group_min_max.test

@@ -1166,3 +1166,22 @@ SELECT (SUM(DISTINCT a) + MAX(b)) FROM t2 GROUP BY a;
 DROP TABLE t1,t2;
 
 --echo # end of WL#3220 tests
+
+--echo #
+--echo # Bug#50539: Wrong result when loose index scan is used for an aggregate
+--echo #            function with distinct
+--echo #
+CREATE TABLE t1 (
+  f1 int(11) NOT NULL DEFAULT '0',
+  f2 char(1) NOT NULL DEFAULT '',
+  PRIMARY KEY (f1,f2)
+) ;
+insert into t1 values(1,'A'),(1 , 'B'), (1, 'C'), (2, 'A'), 
+(3, 'A'), (3, 'B'), (3, 'C'), (3, 'D');
+
+SELECT f1, COUNT(DISTINCT f2) FROM t1 GROUP BY f1;
+explain SELECT f1, COUNT(DISTINCT f2) FROM t1 GROUP BY f1;
+ 
+drop table t1;
+--echo # End of test#50539.
+

sql/opt_range.cc

@@ -10959,17 +10959,7 @@ int QUICK_GROUP_MIN_MAX_SELECT::get_next()
   } while ((result == HA_ERR_KEY_NOT_FOUND || result == HA_ERR_END_OF_FILE) &&
            is_last_prefix != 0);
 
-  if (result == 0)
-  {
-    /*
-      Partially mimic the behavior of end_select_send. Copy the
-      field data from Item_field::field into Item_field::result_field
-      of each non-aggregated field (the group fields, and optionally
-      other fields in non-ANSI SQL mode).
-    */
-    copy_fields(&join->tmp_table_param);
-  }
-  else if (result == HA_ERR_KEY_NOT_FOUND)
+  if (result == HA_ERR_KEY_NOT_FOUND)
     result= HA_ERR_END_OF_FILE;
 
   DBUG_RETURN(result);

sql/sql_select.cc

@@ -12255,6 +12255,12 @@ end_send(JOIN *join, JOIN_TAB *join_tab __attribute__((unused)),
   if (!end_of_records)
   {
     int error;
+    if (join->tables &&
+        join->join_tab->is_using_loose_index_scan())
+    {
+      /* Copy non-aggregated fields when loose index scan is used. */
+      copy_fields(&join->tmp_table_param);
+    }
     if (join->having && join->having->val_int() == 0)
       DBUG_RETURN(NESTED_LOOP_OK);               // Didn't match having
     error=0;