Question

I’ve been working on a mysql database, and one of the most important tables that have the DataBase will be between 900 and 1000 million records. Im able to do this query:

select MAX(SEN_ID) as SEN_ID from senal group by variable_VAR_ID

but it takes like 12 minutes. How can I optimize this query?

This query was working fine when the database was small, but now that there are millions of rows in the database, I am realizing I should have looked at optimizing this earlier.

The query execute within EXPLAIN show this:

id select_type table type  possible_keys key                key_len ref  rows     Extra
1  SIMPLE      senal index NULL          fk_senal_variable1 4       NULL 6333242  Using index

Answer 1

You need to create composite index variable_VAR_ID + SEN_ID

Note 1: not two separated indexes, but one composite
Note 2: yes, the order matters

PS: here is a syntax description of how to create indexes: http://dev.mysql.com/doc/refman/5.5/en/create-index.html