Re: Teaching planner to short-circuit empty UNION/EXCEPT/INTERSECT inputs

On Tue, Nov 4, 2025 at 5:00 PM Alexander Lakhin <exclusion(at)gmail(dot)com> wrote:
> Please look at a new anomaly, introduced with 03d40e4b5:
> CREATE TABLE t(i integer);
> CREATE TABLE pt(i integer) PARTITION BY LIST(i);
>
> SET enable_seqscan = 'off';
> SELECT * FROM t UNION SELECT * FROM t
> UNION ALL
> SELECT * FROM pt;
> produces:
> ERROR: XX000: unrecognized node type: 0
> LOCATION: create_plan_recurse, createplan.c:538

I looked into this. The child relation with relid 3 (the scan on the
partitioned table) is a dummy, so it is skipped in
generate_union_paths(). As a result, the final setop relation ends up
the same as the child relation with relid set to (1, 2). Then,
generate_union_paths() creates an Append path using this relation's
cheapest path as its subpath. Somehow, add_path() determines that
this new Append path dominates the original cheapest path, causing the
original cheapest path to be freed. This leads to the final Append
path referencing a subpath that has already been freed.

- Richard