Deleting millions of rows in MySQL

Question

I recently found and fixed a bug in a site I was working on that resulted in millions of duplicate rows of data in a table that will be quite large even without them (still in the millions). I can easily find these duplicate rows and can run a single delete query to kill them all. The problem is that trying to delete this many rows in one shot locks up the table for a long time, which I would like to avoid if possible. The only ways I can see to get rid of these rows, without taking down the site (by locking up the table) are:

1. Write a script that will execute thousands of smaller delete queries in a loop. This will theoretically get around the locked table issue because other queries will be able to make it into the queue and run in between the deletes. But it will still spike the load on the database quite a bit and will take a long time to run.
2. Rename the table and recreate the existing table (it'll now be empty). Then do my cleanup on the renamed table. Rename the new table, name the old one back and merge the new rows into the renamed table. This is way takes considerably more steps, but should get the job done with minimal interruption. The only tricky part here is that the table in question is a reporting table, so once it's renamed out of the way and the empty one put in its place all historic reports go away until I put it back in place. Plus the merging process could be a bit of a pain because of the type of data being stored. Overall this is my likely choice right now.

I was just wondering if anyone else has had this problem before and, if so, how you dealt with it without taking down the site and, hopefully, with minimal if any interruption to the users? If I go with number 2, or a different, similar, approach, I can schedule the stuff to run late at night and do the merge early the next morning and just let the users know ahead of time, so that's not a huge deal. I'm just looking to see if anyone has any ideas for a better, or easier, way to do the cleanup.

Answer 1

DELETE FROM `table`
WHERE (whatever criteria)
ORDER BY `id`
LIMIT 1000

Wash, rinse, repeat until zero rows affected. Maybe in a script that sleeps for a second or three between iterations.

Answer 2

Do it in batches of lets say 2000 rows at a time. Commit in-between. A million rows isn't that much and this will be fast, unless you have many indexes on the table.

Answer 3

I suggest the following.

1. Create a public synonym (with the same name as the real table) that points to the real table.

2. Create a new, temporary table with the exact same content, constraints, keys, etc... as the real table.

3. Perform your cleanup on the temporary table however you wish.

-- begin transaction -- 4. Update the public synonym to point to the new, cleaned up temporary table.

1. Drop the old real table.

2. Rename the new table to the old name.

3. Drop the public synonym. -- commit transaction --

This should not affect anybody script's need to access the existing table while you are cleaning up.

Answer 4

I'd also recommend adding some constraints to your table to make sure that this doesn't happen to you again. A million rows, at 1000 per shot, will take 1000 repetitions of a script to complete. If the script runs once every 3.6 seconds you'll be done in an hour. No worries. Your clients are unlikely to notice.

Answer 5

I'd use mk-archiver from the excellent Maatkit utilities package (a bunch of Perl scripts for MySQL management) Maatkit is from Baron Schwartz, the author of the O'Reilly "High Performance MySQL" book.

The goal is a low-impact, forward-only job to nibble old data out of the table without impacting OLTP queries much. You can insert the data into another table, which need not be on the same server. You can also write it to a file in a format suitable for LOAD DATA INFILE. Or you can do neither, in which case it's just an incremental DELETE.

It's already built for archiving your unwanted rows in small batches and as a bonus, it can save the deleted rows to a file in case you screw up the query that selects the rows to remove.

No installation required, just grab http://www.maatkit.org/get/mk-archiver and run perldoc on it (or read the web site) for documentation.