Indexes Matter (or: Memcache Will Only Take You So Far)

28 Jan

About a week ago, I was doing some work on the DB in the middle of the night and noticed that my simple queries were running a bit sluggish. I dropped out of mysql and ran top, and noticed that load averages were way higher than I was used to seeing. I ran SHOW FULL PROCESSLIST a bunch of times, and noticed two queries popping up frequently, one was a backend processing query which did not belong on the production database, and the other was the query used to build Widget objects. My first suspect was the backend process, since it did not belong, so we took that off and moved it to a more appropriate server, which brought down the load average by 1; a significant improvement, but the load averages were still pretty high, however the server was usable and responsive enough again, so I forgot about it.

A couple of days later, I noticed our load averages were still pretty high and the main recurring query was still the widget one, so I ran an explain on it, and although the query looked innocent enough, it was missing an index, so instead of a quick lookup it was a full table scan across millions of rows. Ouch.

I knew we wouldn’t have a chance to have some downtime to run the necessary ALTERs to get the indexes in there until after the weekend, so I asked Chanel to put in memcache support so that widgets would only need to be loaded once from SQL. Chanel got that done on Sunday, and on Monday night we were able to get the proper indexes added.

Because of the time span involved, combined with the fact that we monitor server metrics with Zabbix, means that we can look back at a nice little graph of our performance before and after each of the changes.

The days with the grey background are Saturday and Sunday, before memcache was added. The next day, with memcache added the peak load is cut in half. The day after that, with proper indexes, the peak load is barely perceptible, roughly 1/4 of what the load was with just memecache.

The lesson to be learned from this is that while memcache can help quite a bit, there’s a lot to be said for making sure your SQL queries are optimized.


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  1. chanel

    January 28, 2009 at 1:33 pm

    Nice find Jay. A lot can be said for building your application to be ready made to scale from day one. Thank goodness we acted quickly once the culprit was found before it took down the databases.