From: Tom Lane <tgl@sss.pgh.pa.us>
To: dastapov@gmail.com
cc: pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #18234: Nested Loop joint strategy is ignored for a tiny
 table joined with UNION ALL of two filtered parts
In-reply-to: <18234-571586e70bd0658d@postgresql.org>
References: <18234-571586e70bd0658d@postgresql.org>
Comments: In-reply-to PG Bug reporting form <noreply@postgresql.org>
	message dated "Thu, 07 Dec 2023 18:50:01 +0000"
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Date: Thu, 07 Dec 2023 16:53:07 -0500
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PG Bug reporting form <noreply@postgresql.org> writes:
> Summary of the issue: for a (5-row recordset) JOIN (massive partitioned
> recordset indexed by id) USING (id), the (Nested Loop over 5 values)
> strategy is completely ignored, and Hash Join or Merge Join is done instead,
> which does SeqScan over the "massive recordset".

> Reproduction in DB Fiddle:
> https://www.db-fiddle.com/f/sJUUWNgW7pqPWcJwihVoj5/1 (this demonstrates both
> the bad behaviour and a way to work around it)

We are generally not too happy with non-self-contained bug reports.
Once that DB Fiddle entry disappears, this bug report will be useless.
However ...

> 1)The massive recordset on the right side of the JOIN must come from the
> UNION ALL of two parts, both of which have a filter, like this view in my
> reproduction:

> create view vw_broken as
>   select id from huge where filter_out
>   union all
>   select id from medium where filter_out;

I suspect the WHERE clauses trigger the problem because the resulting
sub-selects can't be pulled up to become an "appendrel", per
is_safe_append_member:

     * It's only safe to pull up the child if its jointree contains exactly
     * one RTE, else the AppendRelInfo data structure breaks. The one base RTE
     * could be buried in several levels of FromExpr, however.  Also, if the
     * child's jointree is completely empty, we can pull up because
     * pull_up_simple_subquery will insert a single RTE_RESULT RTE instead.
     *
     * Also, the child can't have any WHERE quals because there's no place to
     * put them in an appendrel.  (This is a bit annoying...)

That means the sub-selects will be planned independently and there's
no chance to consider the nestloop-with-inner-indexscan plan you are
hoping for.

This is a longstanding wart, but improving matters would require some
fairly painstaking work.  The "appendrel" mechanism is core to both
traditional inheritance and partitioning; I don't recommend trying
to blow it up and start over.  I vaguely recall previous discussions
that identified some semantic issues with trying to just attach
WHERE clauses to appendrel members, but it was a long time ago and
the details escape me.

			regards, tom lane