Re: [v9.5] Custom Plan API

Kouhei Kaigai <kaigai(at)ak(dot)jp(dot)nec(dot)com> writes:
> Let assume a custom-scan provider that runs on a materialized-view
> (or, something like a query cache in memory) instead of join.
> In this case, a reasonable design is to fetch a tuple from the
> materialized-view then put it on the ecxt_scantuple of ExprContext
> prior to evaluation of qualifier or tlist, unlike usual join takes
> two slots - ecxt_innertuple and ecxt_outertuple.
> Also, it leads individual varnode has to reference exct_scantuple,
> neither ecxt_innertuple nor ecxt_outertuple.

OK, that's possibly a reasonable way to do it at runtime. You don't
*have* to do it that way of course. It would be only marginally
less efficient to reconstruct two tuples that match the shapes of the
original join inputs.

> I intended to use the SetCustomScanRef callback to adjust varno
> and varattno of the varnode that references the custom-scan node;
> as if set_join_references() doing.

I think this is really fundamentally misguided. setrefs.c has no
business doing anything "interesting" like making semantically important
substitutions; those decisions need to be made much earlier. An example
in the context of your previous proposal is that getting rid of expensive
functions without any adjustment of cost estimates is just wrong; and
I don't mean that you forgot to have your setrefs.c hook hack up the
Plan's cost fields. The cost estimates need to change at the Path stage,
or the planner might not even select the right path at all.

I'm not sure where would be an appropriate place to deal with the kind of
thing you're thinking about here. But I'm really not happy with the
concept of exposing the guts of setrefs.c in order to enable
single-purpose kluges like this. We have fairly general problems to solve
in this area, and we should be working on solving them, not on freezing
relevant planner APIs to support marginally-useful external plugins.

regards, tom lane