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Comment by chrismoos | Hacker News Reader

I had indexes on id, user_id, device_id, and date_added, but maybe I was doing something else wrong. I'm not a database expert :(

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MySQL can only use one index per query. If you need it be on two or more columns you need a composite index with both columns in, populated from the left. If MySQL finds a column in the index you not keying off (WHERE/SORT etc.) then it will stop processing the index at that point.

The key_len column in the EXPLAIN shows you how far it got through the index, based on the size of the columns. In your case it says 4 which if device id is of type INT it just read one column in the index (though there is probably only one column in that index)

Were those single indexes like CREATE INDEX index_1 ON Locations(id) CREATE INDEX index_2 ON Locations(user_id) ...?

If you have multiple single column indexes it won't help you here. The mysql query planner probably correctly guessed the table scan was a better option than hash joining the results of two seeks given the machine's limited memory. It also probably underestimated the cost of disk IO on a virtual machine though.

What SQL was running, and what was the query plan?

ActiveRecord is notorious for generating terrible, terrible SQL.

Edit: What I would expect to see would be index scan on device id, then sort + limit. So the important factor would be rows per device, not total rows.

ActiveRecord...

The SQL was pretty simple I believe..select * from location where device_id = ? order by date_added desc limit 6...something like that.

Edit: Also, I don't know how much it matters, but MySQL probably only has ~256mb memory available to it (its hosted on a Xen box).

Queries like that should be very, very fast even with low amounts of ram. Good article, but consider using EXPLAIN a few times first before embarking on such adventures. :)

Definitely want to second the advice to use EXPLAIN.

MySQL has a lot of very specific limitations about when it will and will not use the available indices. It also matters how you created the indices (one index on multiple columns vs multiple indices on individual columns).

For example, if you created a single index with the columns (date_added, device_id) MySQL would not be able to use the index since device_id is the second part of the index, not the first and thus not available for use in the WHERE clause.

See http://dev.mysql.com/doc/refman/5.1/en/mysql-indexes.html for more limitations.

if you had an index on device_id there shouldn't be a difference in performance between the original table and the partioned one. the index is used to reduce the rows that need to be scanned so there wouldn't be a full table scan. btw, why do you use a table lock?

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