| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Kouhei Kaigai <kaigai(at)ak(dot)jp(dot)nec(dot)com> |
| Cc: | Jeevan Chalke <jeevan(dot)chalke(at)enterprisedb(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp>, Andres Freund <andres(at)anarazel(dot)de> |
| Subject: | Re: PassDownLimitBound for ForeignScan/CustomScan |
| Date: | 2016-09-15 15:38:52 |
| Message-ID: | CA+TgmoZePc5Tnj975G5WZR=JruVGnagiWJDyaTOUVztduDZRLQ@mail.gmail.com |
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On Tue, Sep 13, 2016 at 9:07 PM, Kouhei Kaigai <kaigai(at)ak(dot)jp(dot)nec(dot)com> wrote:
> In the current implementation calls recompute_limits() on the first
> invocation of ExecLimit and ExecReScanLimit. Do we expect the
> ps->numTuples will be also passed down to the child nodes on the same
> timing?
Sure, unless we find some reason why that's not good.
> I also think this new executor contract shall be considered as a hint
> (but not a requirement) for the child nodes, because it allows the
> parent nodes to re-distribute the upper limit regardless of the type
> of the child nodes as long as the parent node can work correctly and
> has benefit even if the child node returns a part of tuples. It makes
> the decision whether the upper limit should be passed down much simple.
> The child node "can" ignore the hint but can utilize for more optimization.
+1.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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