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Re: Memory usage - indexes

From: Mark Kirkwood <mark(dot)kirkwood(at)catalyst(dot)net(dot)nz>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Memory usage - indexes
Date: 2010-09-23 22:12:53
Message-ID: 4C9BD0E5.7070403@catalyst.net.nz (view raw or flat)
Thread:
Lists: pgsql-performance
On 24/09/10 09:50, Tobias Brox wrote:
> We've come to a tipping point with one of our database servers, it's
> generally quite loaded but up until recently it was handling the load
> well - but now we're seeing that it struggles to process all the
> selects fast enough.  Sometimes we're observing some weird lock-like
> behaviour (see my other post on that), but most of the time the
> database server is just not capable of handling the load fast enough
> (causing the queries to pile up in the pg_stat_activity-view).
>
> My main hypothesis is that all the important indexes would fit snuggly
> into the memory before, and now they don't.  We'll eventually get the
> server moved over to new and improved hardware, but while waiting for
> that to happen we need to do focus on reducing the memory footprint of
> the database.  I have some general questions now ...
>
> 1) Are there any good ways to verify my hypothesis?  Some months ago I
> thought of running some small memory-gobbling program on the database
> server just to see how much memory I could remove before we would see
> indications of the database being overloaded.  It seems a bit radical,
> but I think the information learned from such an experiment would be
> very useful ... and we never managed to set up any testing environment
> that faithfully replicates production traffic.  Anyway, it's sort of
> too late now that we're already observing performance problems even
> without the memory gobbling script running.
>
> 2) I've seen it discussed earlier on this list ... shared_buffers vs
> OS caches.  Some claims that it has very little effect to adjust the
> size of the shared buffers.  Anyway, isn't it a risk that memory is
> wasted because important data is stored both in the OS cache and the
> shared buffers?  What would happen if using almost all the available
> memory for shared buffers?  Or turn it down to a bare minimum and let
> the OS do almost all the cache handling?
>
> 3) We're discussing to drop some overlapping indexes ... i.e. to drop
> one out of two indexes looking like this:
>
> some_table(a)
> some_table(a,b)
>
> Would the query "select * from some_table where a=?" run slower if we
> drop the first index?  Significantly?
>
> (in our situation I found that the number of distinct b's for each a
> is low and that the usage stats on the second index is quite low
> compared with the first one, so I think we'll drop the second index).
>
> 4) We're discussing to drop other indexes.  Does it make sense at all
> as long as we're not experiencing problems with inserts/updates?  I
> suppose that if the index isn't used it will remain on disk and won't
> affect the memory usage ... but what if the index is rarely used ...
> wouldn't it be better to do a seqscan on a table that is frequently
> accessed and mostly in memory than to consult an index that is stored
> on the disk?
>
> Sorry for all the stupid questions ;-)
>
>    


All good questions! Before (or maybe as well as) looking at index sizes 
vs memory I'd check to see if any of your commonly run queries have 
suddenly started to use different plans due to data growth, e.g:

- index scan to seq scan (perhaps because effective_cache_size is too 
small now)
- hash agg to sort (work_mem too small now)

We had a case of the 1st point happen here a while ago, symptoms looked 
very like what you are describing.

Re index size, you could try indexes like:

some_table(a)
some_table(b)

which may occupy less space, and the optimizer can bitmap and/or them to 
work like the compound index some_table(a,b).

regards

Mark

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Subject: Re: locking issue on simple selects?
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