Re: tuplesort_gettuple_common() and *should_free argument

From: Peter Geoghegan <pg(at)bowt(dot)ie>
To: Peter Geoghegan <pg(at)heroku(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>, hlinnaka <hlinnaka(at)iki(dot)fi>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: tuplesort_gettuple_common() and *should_free argument
Date: 2017-03-14 01:14:07
Message-ID: CAH2-Wzn-EFddFairDVtJSopb4N5YwKO69E8F=iOW5V9for3srg@mail.gmail.com
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On Wed, Jan 25, 2017 at 3:11 PM, Peter Geoghegan <pg(at)heroku(dot)com> wrote:
> On Wed, Jan 25, 2017 at 3:11 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Please. You might want to hit the existing ones with a separate patch,
>> but it doesn't much matter; I'd be just as happy with a patch that did
>> both things.
>
> Got it.

Attached is a patch that does both things at once. The internal
function tuplesort_gettuple_common() still mentions repeated fetches,
since that context matters to its internal callers. The various
external-facing functions have a simpler, stricter contract, as
discussed.

I didn't end up adding a line like "copy=FALSE is recommended only
when the next tuplesort manipulation will be another
tuplesort_gettupleslot fetch into the same slot", which you suggested.
When the tuplesort state machine is in any state following
"performing" a sort, there are very few remaining sane manipulations.
Just things like skipping or seeking around for tuples, and actually
ending the tuplesort and releasing resources.

--
Peter Geoghegan

Attachment Content-Type Size
0001-Avoid-copying-within-tuplesort_gettupleslot.patch text/x-patch 8.4 KB

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