Thread

1. Pre-PG18 parallel scan estimation: accessing planned worker count from amestimateparallelscan()

   Sivaprasad <sivaprasad.off@gmail.com> — 2026-06-25T01:46:07Z

   Hello,
   
   While adding parallel scan support to the Biscuit
   <https://github.com/CrystallineCore/Biscuit> index access method, I ran
   into a question about the pre-PG18 parallel scan API.
   
   Biscuit partitions scan work into a fixed number of segments at estimation
   time and assigns those segments to parallel participants via a
   shared-memory descriptor in the DSM segment. Because the segment layout
   depends on the number of participants, we need the planned worker count
   during amestimateparallelscan() — the only callback that both sizes the DSM
   allocation and runs in the same process immediately before
   aminitparallelscan().
   
   The callback signatures across versions are:
   -
   
      -
   
      PG16: Size (*)(void)
      -
   
      PG17: Size (*)(int nkeys, int norderbys)
      -
   
      PG18+: Size (*)(Relation, int *nworkers*, int nchunks)
   
   
   In PG18+, nworkers is available directly. In PG16 and PG17 it is not.
   
   My question: prior to PG18, was there any supported mechanism by which an
   index AM could determine the planner's intended parallel worker count
   during scan estimation?
   
   Thanks,
   Sivaprasad Murali
   Biscuit index access method
   

2. Re: Pre-PG18 parallel scan estimation: accessing planned worker count from amestimateparallelscan()

   Peter Geoghegan <pg@bowt.ie> — 2026-06-25T05:42:47Z

   On Wed, Jun 24, 2026 at 9:46 PM Sivaprasad <sivaprasad.off@gmail.com> wrote:
   > My question: prior to PG18, was there any supported mechanism by which an index AM could determine the planner's intended parallel worker count during scan estimation?
   
   No.
   
   -- 
   Peter Geoghegan