Re: ModifyTable overheads in generic plans

On Wed, Nov 4, 2020 at 11:32 AM Amit Langote <amitlangote09(at)gmail(dot)com> wrote:
> On Tue, Nov 3, 2020 at 9:05 PM Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
> > On 03/11/2020 10:27, Amit Langote wrote:
> > > Please check the attached if that looks better.
> >
> > Great, thanks! Yeah, I like that much better.
> >
> > This makes me a bit unhappy:
> >
> > >
> > > /* Also let FDWs init themselves for foreign-table result rels */
> > > if (resultRelInfo->ri_FdwRoutine != NULL)
> > > {
> > > if (resultRelInfo->ri_usesFdwDirectModify)
> > > {
> > > ForeignScanState *fscan = (ForeignScanState *) mtstate->mt_plans[i];
> > >
> > > /*
> > > * For the FDW's convenience, set the ForeignScanState node's
> > > * ResultRelInfo to let the FDW know which result relation it
> > > * is going to work with.
> > > */
> > > Assert(IsA(fscan, ForeignScanState));
> > > fscan->resultRelInfo = resultRelInfo;
> > > resultRelInfo->ri_FdwRoutine->BeginDirectModify(fscan, eflags);
> > > }
> > > else if (resultRelInfo->ri_FdwRoutine->BeginForeignModify != NULL)
> > > {
> > > List *fdw_private = (List *) list_nth(node->fdwPrivLists, i);
> > >
> > > resultRelInfo->ri_FdwRoutine->BeginForeignModify(mtstate,
> > > resultRelInfo,
> > > fdw_private,
> > > i,
> > > eflags);
> > > }
> > > }
> >
> > If you remember, I was unhappy with a similar assertion in the earlier
> > patches [1]. I'm not sure what to do instead though. A few options:
> >
> > A) We could change FDW API so that BeginDirectModify takes the same
> > arguments as BeginForeignModify(). That avoids the assumption that it's
> > a ForeignScan node, because BeginForeignModify() doesn't take
> > ForeignScanState as argument. That would be consistent, which is nice.
> > But I think we'd somehow still need to pass the ResultRelInfo to the
> > corresponding ForeignScan, and I'm not sure how.
>
> Maybe ForeignScan doesn't need to contain any result relation info
> then? ForeignScan.operation != CMD_SELECT is enough to tell it to
> call IterateDirectModify() as today.

Hmm, I misspoke. We do still need ForeignScanState.resultRelInfo,
because the IterateDirectModify() API uses it to return the remotely
inserted/updated/deleted tuple for the RETURNING projection performed
by ExecModifyTable().

> > B) Look up the ResultRelInfo, and call BeginDirectModify(), on the first
> > call to ForeignNext().
> >
> > C) Accept the Assertion. And add an elog() check in the planner for that
> > with a proper error message.
> >
> > I'm leaning towards B), but maybe there's some better solution I didn't
> > think of? Perhaps changing the API would make sense in any case, it is a
> > bit weird as it is. Backwards-incompatible API changes are not nice, but
> > I don't think there are many FDWs out there that implement the
> > DirectModify functions. And those functions are pretty tightly coupled
> > with the executor and ModifyTable node details anyway, so I don't feel
> > like we can, or need to, guarantee that they stay unchanged across major
> > versions.
>
> B is not too bad, but I tend to prefer doing A too.

On second thought, it seems A would amount to merely a cosmetic
adjustment of the API, nothing more. B seems to get the job done for
me and also doesn't unnecessarily break compatibility, so I've updated
0001 to implement B. Please give it a look.

--
Amit Langote
EDB: http://www.enterprisedb.com