Re: Understanding Hash Join performance

From: Claudio Freire <klaussfreire(at)gmail(dot)com>
To: Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>
Cc: srobertjames <srobertjames(at)gmail(dot)com>, pgsql-performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Understanding Hash Join performance
Date: 2011-06-02 18:56:42
Message-ID: BANLkTi=sPMDFd4qyFtPVhtUnsPquWNYC9A@mail.gmail.com
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On Thu, Jun 2, 2011 at 4:57 PM, Kevin Grittner
<Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:
> And the
> planner does take the size of work_mem and the expected data set
> into consideration when estimating the cost of the hash join.

And shouldn't it?

In a gross mode, when hash joins go to disk, they perform very poorly.
Maybe the planner should take that into account.

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