From: Tom Lane <tgl@sss.pgh.pa.us>
To: Richard Guo <guofenglinux@gmail.com>
cc: Pg Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Making Vars outer-join aware
In-reply-to: 
 <CAMbWs49KZO2VvMqo+1MQfaX4cun7vhF=5RgVdpA0gjJcj77HFQ@mail.gmail.com>
References: <830269.1656693747@sss.pgh.pa.us>
 <CAMbWs49KZO2VvMqo+1MQfaX4cun7vhF=5RgVdpA0gjJcj77HFQ@mail.gmail.com>
Comments: In-reply-to Richard Guo <guofenglinux@gmail.com>
	message dated "Tue, 05 Jul 2022 19:02:38 +0800"
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-ID: <1903933.1657031040.1@sss.pgh.pa.us>
Date: Tue, 05 Jul 2022 10:24:00 -0400
Message-ID: <1903934.1657031040@sss.pgh.pa.us>
Archived-At: 
 <https://www.postgresql.org/message-id/1903934.1657031040%40sss.pgh.pa.us>
Precedence: bulk

Richard Guo <guofenglinux@gmail.com> writes:
> For the query in the example

>     SELECT * FROM t1 LEFT JOIN t2 ON (t1.x = t2.y) WHERE foo(t2.z)

> (foo() is not strict.) We want to avoid pushing foo(t2.z) down to the t2
> scan level. Previously we do that with check_outerjoin_delay() by
> scanning all the outer joins below and check if the qual references any
> nullable rels of the OJ, and if so include the OJ's rels into the qual.
> So as a result we'd get that foo(t2.z) is referencing t1 and t2, and
> we'd put the qual into the join lists of t1 and t2.

> Now there is the 'varnullingrels' marker in the t2.z, which is the LEFT
> JOIN below (with RTI 3). So we consider the qual is referencing RTE 2
> (which is t2) and RTE 3 (which is the OJ). Do we still need to include
> RTE 1, i.e. t1 into the qual's required relids? How should we do that?

It seems likely to me that we could leave the qual's required_relids
as just {2,3} and not have to bother ORing any additional bits into
that.  However, in the case of a Var-free JOIN/ON clause it'd still
be necessary to artificially add some relids to its initially empty
relids.  Since I've not yet tried to rewrite distribute_qual_to_rels
I'm not sure how the details will shake out.

			regards, tom lane