The ordering of indexes group

Question

I read somewhere there is a different between these two indexes groups:

• ADD KEY id_user (id_user,seen);
• ADD KEY id_user (seen,id_user);

Well is there? If yes so their order should be based on what parameter?

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In reality I have these two queries on one table:

Query1:

SELECT * FROM mytable WHERE id_user = :id AND seen IS NULL

Query2:

SELECT * FROM mytable WHERE id_user = :id AND timestamp > :tm

So what kind of indexes would be proper in my case? Currently I have three separated indexed on id_user , seen , timestamp columns.

Answer 1

Both your queries have an equality condition on id_user . Hence, either can take advantage of an index where id_user is the first key in the index. So, I would recommend the first index you mention.

MySQL has good documentation on multi-column indexes. I would suggest you start there to learn about them.

Your query can take advantage of indexes on (id_user, seen) and (id_user, timestamp) . Probably, the first key is most important. You should try this different indexes and see which best meet your performance goals.

Answer 2

Most likely, an index on user_id is enough. But if you have many repetitions of user_id s, then you can make two index groups

ADD KEY id_user1 (id_user, seen);
ADD KEY id_user2 (id_user, timestamp);

The select will first use user_id to reduce the number or matches, then the second field in the index, either timestamp or seen .

For a select that looks for user_id and seen , these two indexes

ADD KEY id_user1 (id_user,seen);
ADD KEY id_user2 (seen,id_user);

will have mostly the same result.

In any case, you should use EXPLAIN SELECT * FROM ... to check the execution of slow queries, so you can see exactly where it is slow, and if your indexes are actually used as expected.