Description:
Given an index key(a, b) and a query like
select b from t where a = 1 group by b
We are not able to use loose index scan even though we should be able to. This can be worked around by querying for
select b from t where a = 1 group by a, b;
We get something similar when we use DISTINCT instead of GROUP BY.
This is issue is somewhat similar in spirit to this one (though fix is completely different): https://bugs.mysql.com/bug.php?id=94903
How to repeat:
Run this:
CREATE TABLE t (a INT, b INT, c INT, d INT, KEY k1(a, b, c, d)) ENGINE=innodb;
--disable_query_log
CREATE TEMPORARY TABLE a (a INT);
let $i=10;
while ($i)
{
--eval INSERT INTO a VALUES ($i)
dec $i;
}
CREATE TEMPORARY TABLE b (a INT);
let $i=5;
while ($i)
{
--eval INSERT INTO b VALUES ($i)
dec $i;
}
CREATE TEMPORARY TABLE c (a INT);
let $i=5;
while ($i)
{
--eval INSERT INTO c VALUES ($i)
dec $i;
}
CREATE TEMPORARY TABLE d (a INT);
let $i=10;
while ($i)
{
--eval INSERT INTO d VALUES ($i)
dec $i;
}
INSERT INTO t SELECT a.a, b.a, c.a, d.a FROM a, b, c, d;
INSERT INTO t VALUES (11, 2, 3, -10);
INSERT INTO t VALUES (11, 3, 3, 20);
INSERT INTO t VALUES (11, 15, NULL, -20);
INSERT INTO t VALUES (11, NULL, 40, 30);
INSERT INTO t VALUES (12, 15, 3, -15);
INSERT INTO t VALUES (12, 15, 40, -20);
INSERT INTO t VALUES (12, 2, 40, -23);
INSERT INTO t VALUES (12, 2, 3, NULL);
DROP TEMPORARY TABLE a, b, c, d;
--enable_query_log
ANALYZE TABLE t;
explain
select b from t where a = 1 group by b;
select b from t where a = 1 group by b;
explain
select b from t where a = 1 group by a, b;
select b from t where a = 1 group by a, b;
DROP TABLE t;
Relevant results:
explain
select b from t where a = 1 group by b;
id select_type table partitions type possible_keys key key_len ref rows filtered Extra
1 SIMPLE t NULL ref k1 k1 5 const 250 100.00 Using index
explain
select b from t where a = 1 group by a, b;
id select_type table partitions type possible_keys key key_len ref rows filtered Extra
1 SIMPLE t NULL range k1 k1 10 NULL 6 100.00 Using where; Using index for group-by
Suggested fix:
We should be able to make the check in group_attribute_not_prefix_in_index a little smarter, similar to what we do in in test_if_order_by_key when we examine const_key_parts.
