Re: Parallel Bitmap Heap Scan reports per-worker stats in EXPLAIN ANALYZE

On Fri, Apr 3, 2026 at 3:20 PM Tomas Vondra <tomas(at)vondra(dot)me> wrote:
>
> I'm working on adding information about prefetching for scans [1], which
> includes BitmapHeapScan. I realized the instrumentation added by this
> thread may not be quite right, resulting in missing instrumentation for
> non-parallel-aware scans in a parallel query.
>
> A better description / explanation of the issue is posted here [2]. I've
> posted a proposed fix in the following message [3], in a patch:
>
> v8-0002-Show-Bitmap-Heap-Scan-stats-for-non-parallel-awar.patch
>
> I wonder if someone from this thread could review my analysis, and
> confirm this is not intentional. I don't see it discussed in the thread,
> so I assume no one noticed this behavior. I'd also appreciate a review
> of the proposed fix, or suggestions for alternative fixes.

I can't imagine this was intentional.

I reviewed your approach and suggest we aim for even lower impact by
always allocating the ParallelBitmapHeapState. That means the DSM
layout won't differ such that pcxt->toc has to point to the
instrumentation in the parallel-oblivious case and the pstate in the
parallel-aware case. Attached is a patch that does this.

- Melanie