Re: Parallel Seq Scan

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Cc: David Rowley <dgrowleyml(at)gmail(dot)com>, Andres Freund <andres(at)2ndquadrant(dot)com>, Kouhei Kaigai <kaigai(at)ak(dot)jp(dot)nec(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: 2015-04-21 21:20:08
Message-ID: CA+TgmoaUz3f-rujFpzB4EyQ8FgC2sdDAZpY=fpBvCXMk6y2-KQ@mail.gmail.com
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On Tue, Apr 21, 2015 at 9:38 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> On Mon, Apr 20, 2015 at 10:08 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>>
>> On Tue, Apr 7, 2015 at 11:58 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
>> wrote:
>> > One disadvantage of retaining parallel-paths could be that it can
>> > increase the number of combinations planner might need to evaluate
>> > during planning (in particular during join path evaluation) unless we
>> > do some special handling to avoid evaluation of such combinations.
>>
>> Yes, that's true. But the overhead might not be very much. In the
>> common case, many baserels and joinrels will have no parallel paths
>> because the non-parallel paths is known to be better anyway. Also, if
>> parallelism does seem to be winning, we're probably planning a query
>> that involves accessing a fair amount of data,
>
> Am I understanding right that by above you mean to say that retain the
> parallel and non-parallel path only if parallel-path wins over non-parallel
> path?

Yes.

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

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