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Re: [HACKERS] [PATCH] Push limit to sort through a subquery - Mailing list pgsql-hackers

From Robert Haas
Subject Re: [HACKERS] [PATCH] Push limit to sort through a subquery
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Msg-id CA+Tgmobw5goj6aURp+owc-uwreUAzvyNf_YSwv-zkZ_K=JzQiQ@mail.gmail.com
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In response to Re: [HACKERS] [PATCH] Push limit to sort through a subquery  (Douglas Doole <dougdoole@gmail.com>)
Responses Re: [HACKERS] [PATCH] Push limit to sort through a subquery  (Douglas Doole <dougdoole@gmail.com>)
Re: [HACKERS] [PATCH] Push limit to sort through a subquery  (Konstantin Knizhnik <k.knizhnik@postgrespro.ru>)
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On Thu, Aug 17, 2017 at 11:36 AM, Douglas Doole <dougdoole@gmail.com> wrote:
I completely agree. The further a limit can be pushed down, the better.

The patch looks good to me.

It seems like a somewhat ad-hoc approach; it supposes that we can take any query produced by deparseSelectStmtForRel() and stick a LIMIT clause onto the very end and all will be well.  Maybe that's not a problematic assumption, not sure.  The grammar happens to allow both FOR UPDATE LIMIT n and LIMIT n FOR UPDATE even though only the latter syntax is documented.

Regarding the other patch on this thread, you mentioned upthread that "If it is possible to get more than one SubqueryScanState and/or ResultState between the limit and sort, then the first block of code could be placed in a while loop."  I think that's not possible for a ResultState, but I think it *is* possible for a SubqueryScanState.
 
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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