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Re: [GENERAL] Perfomance of IN-clause with many elements and possible solutions - Mailing list pgsql-general

From Jeff Janes
Subject Re: [GENERAL] Perfomance of IN-clause with many elements and possible solutions
Date
Msg-id CAMkU=1we8pXTzUaxY7d7T7ww9s74sjU+mje=yHi=8gH7Zix6Pg@mail.gmail.com
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In response to Re: [GENERAL] Perfomance of IN-clause with many elements and possiblesolutions  (Dmitry Lazurkin <dilaz03@gmail.com>)
Responses Re: [GENERAL] Perfomance of IN-clause with many elements and possiblesolutions  (Dmitry Lazurkin <dilaz03@gmail.com>)
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On Mon, Jul 24, 2017 at 3:12 PM, Dmitry Lazurkin <dilaz03@gmail.com> wrote:
On 25.07.2017 00:31, David G. Johnston wrote:

Basically you want to write something like:

SELECT *
FROM ids
JOIN ( :values_clause ) vc (vid) ON (vc.vid = ids.id)​

or 

WITH vc AS (SELECT vid FROM .... ORDER BY ... LIMIT )
SELECT *
FROM ids
JOIN vc ON (vid = ids.id)


 This query uses JOIN plan node as IN (VALUES ...).

And I have one question. I don't understand why IN-VALUES doesn't use Semi-Join? PostgreSQL has Hash Semi-Join...  For which task the database has node of this type?


I think it is simply because no one has gotten around to implementing it that way.  When you can just write it as a values list instead, the incentive to make the regular in-list work better is not all that strong.

Cheers,

Jeff

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