Re: BUG #18234: Nested Loop joint strategy is ignored for a tiny table joined with UNION ALL of two filtered parts

PG Bug reporting form <noreply(at)postgresql(dot)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