From: Tom Lane <tgl@sss.pgh.pa.us>
To: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
cc: pgsql-hackers@lists.postgresql.org
Subject: Re: Missing MaterialPath support in reparameterize_path_by_child
In-reply-to: 
 <CAExHW5sCvu58sthGkXRRbrgT2QZkAvaaa2mWjzEHyDZ-iahBxQ@mail.gmail.com>
References: <1854233.1669949723@sss.pgh.pa.us>
 <CAExHW5sCvu58sthGkXRRbrgT2QZkAvaaa2mWjzEHyDZ-iahBxQ@mail.gmail.com>
Comments: In-reply-to Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
	message dated "Fri, 02 Dec 2022 16:51:27 +0530"
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-ID: <1935225.1669995790.1@sss.pgh.pa.us>
Content-Transfer-Encoding: quoted-printable
Date: Fri, 02 Dec 2022 10:43:10 -0500
Message-ID: <1935226.1669995790@sss.pgh.pa.us>
Archived-At: 
 <https://www.postgresql.org/message-id/1935226.1669995790%40sss.pgh.pa.us>
Precedence: bulk

Ashutosh Bapat <ashutosh.bapat.oss@gmail.com> writes:
> On Fri, Dec 2, 2022 at 8:25 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Unfortunately, I don't have an example that produces such a
>> failure against HEAD.  It seems certain to me that such cases
>> exist, though, so I'd like to apply and back-patch the attached.

> From this comment, that I wrote back when I implemented that function,
> I wonder if we thought MaterialPath wouldn't appear on the inner side
> of nestloop join. But that can't be the case. Or probably we didn't
> find MaterialPath being there from our tests.
>      * This function is currently only applied to the inner side of a ne=
stloop
>      * join that is being partitioned by the partitionwise-join code.  H=
ence,
>      * we need only support path types that plausibly arise in that cont=
ext.
> But I think it's good to have MaterialPath there.

So thinking about this a bit: the reason it is okay if reparameterize_path
fails is that it's not fatal.  We just go on our way without making
a parameterized path for that appendrel.  However, if
reparameterize_path_by_child fails for every available child path,
we end up with "could not devise a query plan", because the
partitionwise-join code is brittle and won't tolerate failure
to build a parent-join path.  Seems like we should be willing to
fall back to a non-partitionwise join in that case.

			regards, tom lane