Question

I’d like to expand a question I posted a while ago:

I’m querying a table for rows in which pairs of columns are in a specific set. For example, consider the following table:

    id | f1  | f2
    -------------
    1  | 'a' | 20
    2  | 'b' | 20
    3  | 'a' | 30
    4  | 'b' | 20
    5  | 'c' | 20

And I wish to extract rows in which the pair (f1, f2) are in a specified set of pairs, e.g. ((‘a’,30), (‘b’, 20),…). In the original question, Mark answered correctly that I can use the following syntax:

SELECT * FROM my_table WHERE (f1,f2) IN (('a',30), ('b',20))

This works fine, but I see some unexpected behavior regarding indexes:
I’ve defined a multi-column index for f1, f2, named IndexF1F2. Using the EXPLAIN phrase, I see that MySql uses the index for a single comparison, e.g.:

SELECT * FROM my_table WHERE (f1,f2) = ('a',30)

but not when using the ‘IN’ clause, as in the example above. Giving hints, e.g. USE INDEX(IndexF1F2) or even FORCE INDEX(IndexF1F2), does not seem to make any difference.

Any ideas?

Answer 1

This is a known bug in MySQL.

Use this syntax:

SELECT  *
FROM    composite
WHERE   (f1, f2) = ('a', 30)
        OR (f1, f2) = ('b', 20)