Re: Foreign join pushdown vs EvalPlanQual

On Fri, Nov 6, 2015 at 9:42 AM, Kouhei Kaigai <kaigai(at)ak(dot)jp(dot)nec(dot)com> wrote:
> This patch needs to be rebased.
> One thing different from the latest version is fdw_recheck_quals of
> ForeignScan was added. So, ...
>
> (1) Principle is that FDW driver knows what qualifiers were pushed down
> and how does it kept in the private field. So, fdw_recheck_quals is
> redundant and to be reverted.
>
> (2) Even though the principle is as described in (1), however,
> wired logic in ForeignRecheck() and fdw_recheck_quals are useful
> default for most of FDW drivers. So, it shall be kept and valid
> only if RecheckForeignScan callback is not defined.
>
> Which is better approach for the v3 patch?
> My preference is (1), because fdw_recheck_quals is a new feature,
> thus, FDW driver has to be adjusted in v9.5 more or less, even if
> it already supports qualifier push-down.
> In general, interface becomes more graceful to stick its principle.

fdw_recheck_quals seems likely to be very convenient for FDW authors,
and I think ripping it out would be a terrible decision.

I think ForeignRecheck should first call ExecQual to test
fdw_recheck_quals. If it returns false, return false. If it returns
true, then give the FDW callback a chance, if one is defined. If that
returns false, return false. If we haven't yet returned false,
return true.

--
Robert Haas
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