Re: Make EXPLAIN generate a generic plan for a parameterized query

From: Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Christoph Berg <myon(at)debian(dot)org>
Cc: Andres Freund <andres(at)anarazel(dot)de>, Michel Pelletier <pelletier(dot)michel(at)gmail(dot)com>, Julien Rouhaud <rjuju123(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Make EXPLAIN generate a generic plan for a parameterized query
Date: 2023-03-28 06:35:57
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On Fri, 2023-03-24 at 16:41 -0400, Tom Lane wrote:
> Christoph Berg <myon(at)debian(dot)org> writes:
> > Re: Tom Lane
> > > I don't actually see why a postgres_fdw test case is needed at all?
> > > The tests in explain.sql seem sufficient.
> > When I asked Laurenz about that he told me that local tables wouldn't
> > exercise the code specific for EXEC_FLAG_EXPLAIN_GENERIC.
> But there isn't any ... or at least, I fail to see what isn't sufficiently
> exercised by that new explain.sql test case that's identical to this one
> except for being a non-foreign table.  Perhaps at some point this patch
> modified postgres_fdw code?  But it doesn't now.
> I don't mind having a postgres_fdw test if there's something for it
> to test, but it just looks duplicative to me.  Other things being
> equal, I'd prefer to test this feature in explain.sql, since (a) it's
> a core feature and (b) the core tests are better parallelized than the
> contrib tests, so the same test should be cheaper to run.
> > (Admittedly my knowledge of the planner wasn't deep enough to verify
> > that. Laurenz is currently traveling, so I don't know if he could
> > answer this himself now.)
> OK, thanks for the status update.  What I'll do to get this off my
> plate is to push the patch without the postgres_fdw test -- if
> Laurenz wants to advocate for that when he returns, we can discuss it
> more.

Thanks for committing this.

As Chrisoph mentioned, I found that I could not reproduce the problem
that was addressed by the EXEC_FLAG_EXPLAIN_GENERIC hack using local
partitioned tables. My optimizer knowledge is not deep enough to tell
why, and it might well be a bug lurking in the FDW part of the
optimizer code. It is not FDW specific, since I discovered it with
oracle_fdw and could reproduce it with postgres_fdw.

I was aware that it is awkward to add a test to a contrib module, but
I thought that I should add a test that exercises the new code path.
But I am fine without the postgres_fdw test.

Laurenz Albe

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