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Re: Query performance over a large proportion ofdata

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: "Steve McLellan" <smclellan(at)mintel(dot)com>,<pgsql-performance(at)postgresql(dot)org>
Subject: Re: Query performance over a large proportion ofdata
Date: 2009-03-10 22:06:38
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Lists: pgsql-performance
>>> "Steve McLellan" <smclellan(at)mintel(dot)com> wrote: 
> The server itself is a dual-core 3.7GHz Xeon Dell (each core
> reporting 2 logical CPUs) running an amd64 build of FreeBSD 6.2, and
> postgres 8.3.5 built from source. It's got 400GB storage in RAID-5
> (on 5 disks). It has 8GB of physical RAM. I'm able to use about 6GB
> of that for my own purposes; the server doesn't do much else but
> replicates a very low-usage mysql database.
> shared_buffers=1200MB
You might want to play with this -- that's not a bad starting point,
but your best performance with your load could be on either side of
that value.
> work_mem = 100MB
Probably kinda high, especially if you expect a lot of connections. 
This much memory can be concurrently used, possibly more than once, by
each active connection.
> fsync = off
Don't use this setting unless you can afford to lose your entire
database cluster.  We use it for initial (repeatable) loads, but not
much else.
> enable_seqscan = off
Not a good idea; some queries will optimize better with seqscans.
You can probably get the behavior you want using other adjustments.
> effective_cache_size = 2000MB
>From what you said above, I'd bump this up to 5GB or more.
You probably need to reduce random_page_cost.  If your caching is
complete enough, you might want to set it equal to seq_page_cost
(never set it lower that seq_page_cost!) and possibly reduce both of
these to 0.1.
Some people have had good luck with boosting cpu_tuple_cost and
cpu_index_tuple_cost.  (I've seen 0.5 for both recommended.)  I've
never had to do that, but if the earlier suggestions don't get good
plans, you might try that.
I hope that helps.

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