| From: | Simon Riggs <simon(at)2ndQuadrant(dot)com> |
|---|---|
| To: | Greg Stark <gsstark(at)mit(dot)edu> |
| Cc: | pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: cost_hashjoin |
| Date: | 2010-08-30 13:49:11 |
| Message-ID: | 1283176151.1800.2360.camel@ebony |
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| Lists: | pgsql-hackers |
On Mon, 2010-08-30 at 13:34 +0100, Greg Stark wrote:
> On Mon, Aug 30, 2010 at 10:18 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> > cost_hashjoin() has some treatment of what occurs when numbatches > 1
> > but that additional cost is not proportional to numbatches.
>
> Because that's not how our hash batching works. We generate two temp
> files for each batch, one for the outer and one for the inner. So if
> we're batching then every tuple of both the inner and outer tables
> (except for ones in the first batch) need to be written once and read
> once regardless of the number of batches.
Thanks for explaining. For some reason I thought we rewound the outer at
the start of each batch, which is better for avoiding cache spoiling.
--
Simon Riggs www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Training and Services
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