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Re: Proposed Patch to Improve Performance of Multi-BatchHash Join for Skewed Data Sets

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Joshua Tolley <eggyknap(at)gmail(dot)com>, "Lawrence, Ramon" <ramon(dot)lawrence(at)ubc(dot)ca>, Bryce Cutt <pandasuit(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Proposed Patch to Improve Performance of Multi-BatchHash Join for Skewed Data Sets
Date: 2009-02-26 13:22:52
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Lists: pgsql-hackers
On Thu, Feb 26, 2009 at 4:22 AM, Heikki Linnakangas
<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> I haven't been following this thread closely, so pardon if this has been
> discussed already.
> The patch doesn't seem to change the cost estimates in the planner at all.
> Without that, I'd imagine that the planner rarely chooses a multi-batch hash
> join to begin with.

AFAICS, a multi-batch hash join happens when you are joining two big,
unsorted paths.  The planner essentially compares the cost of sorting
the two paths and then merge-joining them versus the cost of a hash
join.  It doesn't seem to be unusual for the hash join to come out the
winner, although admittedly I haven't played with it a ton.  You
certainly could try to model it in the costing algorithm, but I'm not
sure how much benefit you'd get out of it: if you're doing this a lot
you're probably better off creating indices.

> Joshua, in the tests that you've been running, did you have to rig the
> planner with "enable_mergjoin=off" or similar, to get the queries to use
> hash joins?

I didn't have to fiddle anything, but Josh's tests were more exhaustive.


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