Re: HashAggregate loses worker spill stats on rescan

On Sun, 18 Oct 2026 at 22:55, Ayush Tiwari <ayushtiwari(dot)slg01(at)gmail(dot)com>
wrote:

> Hi,
>
> HashAgg seems to lose the worker's spill stats when Gather is rescanned.
> In the example below, Worker 0 spills on the first scan, but EXPLAIN shows
> one batch and no disk usage after the second:
>
> CREATE TABLE hashagg_stats AS
> SELECT value AS item, value % 10000 AS bucket
> FROM generate_series(1, 1000000) AS value;
> ALTER TABLE hashagg_stats SET (parallel_workers = 2);
> ANALYZE hashagg_stats;
>
> SET max_parallel_workers_per_gather = 2;
> SET max_parallel_workers = 1;
> SET parallel_leader_participation = off;
> SET parallel_setup_cost = 0;
> SET parallel_tuple_cost = 0;
> SET min_parallel_table_scan_size = 0;
> SET work_mem = '64kB';
> SET hash_mem_multiplier = 2;
> SET enable_sort = off;
>
> EXPLAIN (ANALYZE, VERBOSE, COSTS OFF, TIMING OFF, SUMMARY OFF, BUFFERS OFF)
> SELECT limits.maximum,
> (SELECT count(*)
> FROM (SELECT bucket, count(*) FROM hashagg_stats
> WHERE item <= (SELECT limits.maximum)
> GROUP BY bucket) AS grouped)
> FROM (VALUES (1000000), (10)) AS limits(maximum);
>
> AFAICS, ExecEndAgg() clobbers the shared slot on each worker exit, rather
> than combining the stats from successive scans.
>
> Patch attached to keep the memory/disk peaks and add the spill batches.
> I think we want to count the initial batch only once, as in the
> non-parallel case. Otherwise, wouldn't even a non-spilling rescan add
> another batch?
>
> Regards,
> Ayush
>

Hi Ayush,

Thanks for the report and patch.

I'm going to reproduce the issue and review the changes. I had a couple of
questions about the batch-counting logic:

-

Is the assumption that hash_batches_used is always at least 1 when
ExecEndAgg() records the worker's statistics guaranteed by the
surrounding code?
-

Could the si->hash_batches_used == 0 check be affected by any rescan or
worker-reuse scenario that would cause the initial batch to be counted more
than once?

I'll also check how the existing tests cover parallel HashAggregate rescans
and whether a regression test would be appropriate.
Regards,
Osama Abdul Qader