From: | Stephen Robert Norris <srn(at)commsecure(dot)com(dot)au> |
---|---|
To: | Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> |
Cc: | pgsql-general(at)postgresql(dot)org, Mark Greenaway <mark(at)commsecure(dot)com(dot)au> |
Subject: | Re: USING HASH considered harmful? |
Date: | 2001-08-17 02:45:28 |
Message-ID: | 20010817124528.E28987@ws12.commsecure.com.au |
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Lists: | pgsql-general |
On Thu, Aug 16, 2001 at 09:59:02PM -0400, Bruce Momjian wrote:
>
> Checking application/pgp-signature: FAILURE
> -- Start of PGP signed section.
> > We've just discovered a rather nasty feature of hashes, namely that
> > simultaneous reads & writes to a single row will deadlock if there
> > is a hash index on the table.
> >
> > I guess this is because PG really has to lock the hash table entry in
> > both cases. It does, however, make HASH indices completely useless for
> > any table that you might want to update.
> >
> > Is this a known feature?
>
> Yes, I have heard about this problem. Would you test btree vs hash and
> report back which is faster. I have requested this from >20 people and
> no one reported back.
I changed to hash because it was marginally faster for the queries
we were doing - maybe 5% or so. It was very marginal, but we
need every bit of performance we can get.
Stephen
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