matt wrote:
Are you sure?  Have you tested the overall application to see if possibly
you gain more on insert performance than you lose on select performanc?

Unfortunately dropping any of the indexes results in much worse select performance that is not remotely clawed back by the improvement in insert performance.

Bummer. It was just a thought: never assume dropping indexes will hurt performance. But, since you've obviously tested ...

Actually there doesn't really seem to *be* that much improvement in
insert performance when going from 3 indexes to 2.  I guess indexes must
be fairly cheap for PG to maintain?

Don't know how "cheap" they are.

I have an app that does large batch updates. I found that if I dropped
the indexes, did the updates and recreated the indexes, it was faster
than doing the updates while the indexes were intact.

It doesn't sound like your app can use that approach, but I thought I'd
throw it out there.

It's possible that compiling Postgres manually with proper optimizations
could yield some improvements, as well as building a custom kernel in

Also, you don't mention which filesystem you're using:

Yeah, I can imagine getting 5% extra from a slim kernel and super-optimised PG.

The FS is ext3, metadata journaling (the default), mounted noatime.

ext3 is more reliable than ext2, but it's 1.1x slower. You can squeeze a little performance by using Reiser or JFS, if you're not willing to take the risk of ext2, either way, it's a pretty minor improvement.

Does noatime make much difference on a PostgreSQL database?  I haven't
tested that yet.

But if you're in the situation where you have more time than money,
you may find that an overall audit of your app is worthwhile.  Consider
taking parts that are in perl (for example) and recoding them into C
(that is, unless you've already identified that all the bottlenecks are
at the PostgreSQL server)

I can pretty cheaply add more CPU horsepower for the app servers, as they scale horizontally, so I can chuck in a couple (or 3, or 4, or ...) more dual-cpu boxen with a gig of ram and tell the load balancer about them. The problem with the DB is that that approach simply won't work - the box just has to get bigger!

Can you split it onto multiple boxes? Some database layouts lend themselves to this, others don't. Obviously you can't do joins from one server to another, so you may lose more in multiple queries than you gain by having multiple servers. It's worth looking into though.

I know my answers aren't quite the ones you were looking for, but my
experience is that many people try to solve poor application design
by simply throwing bigger hardware at the problem.  It appears as though
you've already done your homework, though.

Hope this has been _some_ help.

Bill Moran
Potential Technologies

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