Mysql not using an index on where in with two columns

Question

I have the following query:

select * from my_table 
where (col1, col2) in ( (1000,1), (2000,2) )

I have indexes defined on col1, col2 and col1 and col2 together. If I run an explain on it, I see that mysql does not find an available index to use. Interestingly enough if I put only one pair in the IN clause, the explain finds all the indexes.

Tried force index, does not help

Any ideas, how can I make mysql recognize the index?

Answer 1

I managed to solve this without upgrading the mysql version, by adding an extra where in condition on one of the columns only:

select * from my_table 
where (col1, col2) in ( (1000,1), (2000,2) )
and col1 in (1000,2000)

This way, the optimizer will find the index on col1, and will search by it, which did the job for me.

Answer 2

You need MySql version 5.7.3 or above
Earlier versions do not implement an optimization for row constructor expressions

Please follow this link for details: https://dev.mysql.com/doc/refman/5.7/en/range-optimization.html#row-constructor-range-optimization

8.2.1.3.4 Range Optimization of Row Constructor Expressions

As of MySQL 5.7.3, the optimizer is able to apply the range scan access method to queries of this form:

SELECT ... FROM t1 WHERE ( col_1, col_2 ) IN (( 'a', 'b' ), ( 'c', 'd' ));

Previously, for range scans to be used it was necessary for the query to be written as:

SELECT ... FROM t1 WHERE ( col_1 = 'a' AND col_2 = 'b' ) OR ( col_1 = 'c' AND col_2 = 'd' );

Note: a row constructor is an expression of a form:
( value1, value2, ... ,valueN) or ROW( val1, val2, ..., valN).

Answer 3

I wasn't not aware about this; so tried out first (Fiddle http://sqlfiddle.com/#!2/f88bb/2) and yes it's correct; for a multi column IN clause it's using full table scan instead of using the index available.

After some more search found that it's a BUG in MySQL which is targeted to be released or fixed in MySQL 6.x.

See here for a full info Multi column IN does not use index

So for now you can transform your query to use single IN clause so that it recognizes the index; like

select * from my_table 
where col1 in ( 1000,2000) and col2 in (1,2)

Person who created the BUG/Optimization request has explained (OR) given reproduction steps here

Multi-Column IN clause – Unexpected MySQL Issue