Re: Tuplesort merge pre-reading

From: Greg Stark <stark(at)mit(dot)edu>
To: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Cc: Peter Geoghegan <pg(at)heroku(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, Claudio Freire <klaussfreire(at)gmail(dot)com>
Subject: Re: Tuplesort merge pre-reading
Date: 2016-09-09 12:25:29
Message-ID: CAM-w4HPuQDr1UgowfBnnvJz0eWbgpHPU9JA6U_pRuT397XfFvg@mail.gmail.com
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On Fri, Sep 9, 2016 at 1:01 PM, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
> I'm happy with what it looks like. We are in fact getting a more sequential
> access pattern with these patches, because we're not expanding the pre-read
> tuples into SortTuples. Keeping densely-packed blocks in memory, instead of
> SortTuples, allows caching more data overall.

Wow, this is really cool. We should do something like this for query
execution too.

I still didn't follow exactly why removing the prefetching allows more
sequential i/o. I thought the whole point of prefetching was to reduce
the random i/o from switching tapes.

--
greg

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