Re: Parallel Seq Scan

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Noah Misch <noah(at)leadboat(dot)com>
Cc: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Kouhei Kaigai <kaigai(at)ak(dot)jp(dot)nec(dot)com>, Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>, Gavin Flower <GavinFlower(at)archidevsys(dot)co(dot)nz>, Jeff Davis <pgsql(at)j-davis(dot)com>, Andres Freund <andres(at)2ndquadrant(dot)com>, Amit Langote <amitlangote09(at)gmail(dot)com>, Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>, Fabrízio Mello <fabriziomello(at)gmail(dot)com>, Thom Brown <thom(at)linux(dot)com>, Stephen Frost <sfrost(at)snowman(dot)net>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Parallel Seq Scan
Date: 2015-10-28 00:04:12
Message-ID: CA+TgmoYL7HC2BuHyscDjAgjMt9PpC5EVowL345VZsN-V9cqimw@mail.gmail.com
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On Sat, Oct 24, 2015 at 6:31 PM, Noah Misch <noah(at)leadboat(dot)com> wrote:
> On Sat, Oct 24, 2015 at 07:49:07AM -0400, Robert Haas wrote:
>> On Fri, Oct 23, 2015 at 9:38 PM, Noah Misch <noah(at)leadboat(dot)com> wrote:
>> > Since that specification permits ParamListInfo consumers to ignore paramMask,
>> > the plpgsql_param_fetch() change from copy-paramlistinfo-fixes.patch is still
>> > formally required.
>>
>> So why am I not just doing that, then? Seems a lot more surgical.
>
> do $$
> declare
> param_unused text := repeat('a', 100 * 1024 * 1024);
> param_used oid := 403;
> begin
> perform count(*) from pg_am where oid = param_used;
> end
> $$;
>
> I expect that if you were to inspect the EstimateParamListSpace() return
> values when executing that, you would find that it serializes the irrelevant
> 100 MiB datum. No possible logic in plpgsql_param_fetch() could stop that
> from happening, because copyParamList() and SerializeParamList() call the
> paramFetch hook only for dynamic parameters. Cursors faced the same problem,
> which is the raison d'être for setup_unshared_param_list().

Well, OK. That's not strictly a correctness issue, but here's an
updated patch along the lines you suggested.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Attachment Content-Type Size
serialize-paramlistinfo-fixes-v2.patch application/x-patch 12.5 KB

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