Re: Surprising SeqScan of appendRel that can't contribute any rows to the result

Thank you for the very detailed answer, much appreciated!

For the benefit of people who might find this in the future via search:  so
far the best workaround seems to be something along the lines of:

alter table partB add column always_null bigint;
create index on partB(always_null);

And then I change vw so that instead of constant NULL I expose this column
instead. Thenat instead of seqscan I get index scan on always_null which
(relatively) quickly yields zero rows.



On Wed, 18 Dec 2024, 00:08 David Rowley, <dgrowleyml@gmail.com> wrote:

> On Wed, 18 Dec 2024 at 12:17, Dmytro Astapov <dastapov@gmail.com> wrote:
> > Surprisingly, this does SeqScan on partB in NestedLoops over some_ids
> with a filter `some_ids.id = NULL::bigint`:
> >
> > Nested Loop (cost=0.29..359.16 rows=200 width=24)
> >   -> Seq Scan on some_ids i (cost=0.00..1.02 rows=2 width=8)
> >   -> Append (cost=0.29..178.56 rows=51 width=16)
> >         -> Index Scan using parta_payload_idx on parta (cost=0.29..8.30
> rows=1 width=16)
> >               Index Cond: (payload = i.id)
> >         -> Seq Scan on partb (cost=0.00..170.00 rows=50 width=16)
> >               Filter: (i.id = NULL::bigint)
> >
> > At the same time `explain select * from vw where payload = 1` correctly
> skips over partB entirely (the node is eliminated from execution plan), and
> so does:
> > explain select * from vw where payload in (1,2);
> >
> > However, any query that does not use explicit literal values still leads
> to SeqScan access on partB, such as:
> > explain select * from vw where payload in (select id from some_ids);
> > explain select * from vw where payload = ANY(ARRAY(select id from
> some_ids));
> > or various forms of joins
> >
> > Do you know if this is expected/documented, or is this a bug?
>
> TL;DR is it's not a bug and expected behaviour.
>
> We tend not to do much in terms of documentation about which
> optimisations the query planner does, so it's probably not documented
> anywhere aside from perhaps the source code.  It might be possible for
> us to eliminate the scan to "partb" for the first of the plans shown
> above. However, the code that applies in your example case where the
> planner does manage to eliminate the scan does so using "base" quals,
> i.e. quals that are pushed down into the scan level.  See
> apply_child_basequals().  For the Nested Loop example, the i.id =
> NULL::bigint isn't a base qual, so it does not work for that case.
> When we're building paramerised paths, as per what's used in your
> Nested Loop example above, we've already done the work to eliminate
> non-matching union children. We don't really have any concept of "this
> union child does not match for this specific parameterisation", so
> we'd need to invent something to do that (which perhaps is just
> removing or not adding the particular unneeded subpath from the Append
> pathlist.)
>
> For the other cases that depend on the results from subqueries, it's
> more tricky and in many cases not possible to eliminate the scans
> during query planner for those cases as the planner does not have
> information to know what will be returned by the subqueries. There
> might be something very limited we can do in terms of looking to see
> if the operator is strict or not so that we at least know that NULLs
> will never match, but that might be quite a corner case that it might
> not be worth the complexity to make that work.  Someone might need to
> write it and see how complex it is to implement before we'd know if it
> was a worthwhile optimisation or not.
>
> David
>