Overhead of a Round-trip to MySql?

Question

So I've been building django applications for a while now, and drinking the cool-aid and all: only using the ORM and never writing custom SQL.

The main page of the site (the primary interface where users will spend 80% - 90% of their time) was getting slow once you have a large amount of user specific content (ie photos, friends, other data, etc)

So I popped in the sql logger (was pre-installed with pinax, I just enabled it in the settings) and imagine my surprise when it reported over 500 database queries!! With hand coded sql I hardly ever ran more than 50 on the most complex pages.

In hindsight it's not all together surprising, but it seems that this can't be good.

...even if only a dozen or so of the queries take 1ms+

So I'm wondering, how much overhead is there on a round trip to mysql? django and mysql are running on the same server so there shouldn't be any networking related overhead.

Answer 1

Just because you are using an ORM doesn't mean that you shouldn't do performance tuning.

I had - like you - a home page of one of my applications that had low performance. I saw that I was doing hundreds of queries to display that page. I went looking at my code and realized that with some careful use of select_related() my queries would bring more of the data I needed - I went from hundreds of queries to tens.

You can also run a SQL profiler and see if there aren't indices that would help your most common queries - you know, standard database stuff.

Caching is also your friend, I would think. If a lot of a page is not changing, do you need to query the database every single time?

If all else fails, remember: the ORM is great, and yes - you should try to use it because it is the Django philosophy; but you are not married to it.

If you really have a usecase where studying and tuning the ORM navigation didn't help, if you are sure that you could do it much better with a standard query: use raw sql for that case.

Answer 2

The overhead of each queries is only part of the picture. The actual round trip time between your Django and Mysql servers is probably very small since most of your queries are coming back in less than a one millisecond. The bigger problem is that the number of queries issued to your database can quickly overwhelm it. 500 queries for a page is way to much, even 50 seems like a lot to me. If ten users view complicated pages you're now up to 5000 queries.

The round trip time to the database server is more of a factor when the caller is accessing the database from a Wide Area Network, where roundtrips can easily be between 20ms and 100ms.

I would definitely look into using some kind of caching.