Question

I am trying to get several records by composite index from a table having PRIMARY KEY (a, b)

SELECT * FROM table WHERE (a, b) IN ((1,2), (2,4), (1,3))

The problem is, that MySQL is not using index, even if I FORCE INDEX (PRIMARY).
EXPLAIN SELECT shows null possible_keys.

Why there are no possible_keys?

What is the best way to retrieve multiple rows by composite key:

• using OR
• using UNION ALL
• using WHERE () IN ((),())

P.S. Query is equal by result to

SELECT * FROM table WHERE (a = 1 AND b = 2) OR (a = 2 AND b = 4) OR (a = 1 AND b = 3)

Thanks

Answer 1

If query selects only fields from index (or if table has no other fields) by composite WHERE ... IN, index will be used:

SELECT a,b FROM `table` WHERE (a, b) IN ((1,2), (2,4), (1,3))

Otherwise it will not be used.
The workaround is to use derived query:

SELECT t.* FROM (SELECT a, b FROM `table` WHERE (a, b) IN ((1,2), (2,4), (1,3))) AS o INNER JOIN `table` AS t ON (t.a = o.a AND t.b = o.b)

EXPLAIN SELECT:

id  select_type table   type    possible_keys   key key_len ref rows    Extra
1   PRIMARY <derived2>  ALL NULL    NULL    NULL    NULL    2   
1   PRIMARY t   eq_ref  PRIMARY PRIMARY 2   o.a,o.b 1   
2   DERIVED table   index   NULL    PRIMARY 2   NULL    6   Using where; Using index