Re: BUG #19684: Assertion in tuplesort_begin_heap() falsified by parallel plan with sort

Hi,

On Sat, 12 Sept 2026 at 20:33, PG Bug reporting form
<noreply(at)postgresql(dot)org> wrote:
>
> The following bug has been logged on the website:
>
> Bug reference: 19684
> Logged by: Alexander Lakhin
> Email address: exclusion(at)gmail(dot)com
> PostgreSQL version: 19beta3
> Operating system: Ubuntu 24.04
> Description:
>
> The following script:
> SET cpu_tuple_cost = 1000;
> SET min_parallel_table_scan_size = 1;
>
> CREATE TABLE t(i int);
> SELECT FROM t UNION SELECT FROM t;
>
> triggers:
> TRAP: failed Assert("nkeys > 0"), File: "tuplesortvariants.c", Line: 195,
> PID: 1465852
>
> EXPLAIN shows:
> QUERY PLAN
> -----------------------------------------------------------------------------------------------
> Unique (cost=3188844.07..3188856.82 rows=2 width=0)
> -> Sort (cost=3188844.07..3188856.82 rows=5100 width=0)
> -> Gather (cost=1000.00..3188530.00 rows=5100 width=0)
> Workers Planned: 2
> -> Parallel Append (cost=0.00..3187020.00 rows=2124
> width=0)
> -> Parallel Seq Scan on t (cost=0.00..1062510.00
> rows=1062 width=0)
> -> Parallel Seq Scan on t t_1 (cost=0.00..1062510.00
> rows=1062 width=0)
>
> Without asserts enabled, SELECT succeeds and EXPLAIN (ANALYZE) shows the
> same plan.
>
> Reproduced starting from 66c0185a3/12933dc60.

Thanks for the report!

It looks like the Gather path is missing the check already used for the
non-parallel Append path. For a zero-column UNION, groupList is NIL, but
the Gather path still creates a Sort and eventually calls
tuplesort_begin_heap() with zero sort keys.

I added the same "if (groupList != NIL)" condition around
create_sort_path() for the Gather path. With the patch, the reported test
and a variant using a populated table both complete successfully.

I wonder if we should also add an Assert(pathkeys != NIL) inside
create_sort_path()?

Regards,
Ayush