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

On Mon, Oct 16, 2023 at 12:31 PM Michael Christofides
<michael@pgmustard.com> wrote:
> According to the docs[1]: "In a parallel bitmap heap scan, one process is chosen as the leader. That process performs
ascan of one or more indexes and builds a bitmap indicating which table blocks need to be visited. These blocks are
thendivided among the cooperating processes as in a parallel sequential scan." 
>
> My understanding is that the "Heap Blocks" statistic is only reporting blocks for the bitmap (i.e. not the subsequent
scan).As such, I think it is correct that the workers do not report additional exact heap blocks. 

I think you're wrong about that. The bitmap index scans are what scan
the indexes and build the bitmap. The bitmap heap scan node is what
scans the heap i.e. the table, and that is what is divided across the
workers.

On the patch itself, snapshot_and_stats doesn't strike me as a great
name. If we added three more variable-length things would we call the
member snapshot_and_stats_and_pink_and_orange_and_blue? Probably
better to pick a name that is somehow more generic.

--
Robert Haas
EDB: http://www.enterprisedb.com