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Re: hash index improving v3

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Alex Hunsaker" <badalex(at)gmail(dot)com>
Cc: "Zdenek Kotala" <Zdenek(dot)Kotala(at)sun(dot)com>, "Xiao Meng" <mx(dot)cogito(at)gmail(dot)com>, pgsql-patches(at)postgresql(dot)org, "Jonah H(dot) Harris" <jonah(dot)harris(at)gmail(dot)com>
Subject: Re: hash index improving v3
Date: 2008-09-05 01:13:18
Message-ID: 10995.1220577198@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
"Alex Hunsaker" <badalex(at)gmail(dot)com> writes:
> Ok let me know if this is to naive of an approach or not hitting the
> right cases you want tested.

You have the unique-versus-not dimension, but I'm also wondering about
narrow vs wide index keys (say about 8 bytes vs 50-100 or so).  In the
former case we're not saving any index space by storing only the hash
code, so these could be expected to have different performance
behaviors.

As for specifics of the suggested scripts:

* might be better to do select count(*) not select 1, so that client
communication is minimized

* check that the queries actually use the indexes (not sure that the
proposed switch settings ensure this, not to mention you didn't create
the indexes)

* make sure the pgbench transaction counts are large enough to ensure
significant runtime

* the specific table sizes suggested are surely not large enough

			regards, tom lane

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