Re: Avoiding superfluous buffer locking during nbtree backwards scans - Mailing list pgsql-hackers

On Sun, Nov 10, 2024 at 9:53 PM Masahiro Ikeda <ikedamsh@oss.nttdata.com> wrote:
> I understand, thanks to your explanation.

Cool.

> Now, there is a case where _bt_readnextpage() calls
> _bt_parallel_seize(),
> _bt_readpage() sets so->needPrimScan=true, and _bt_parallel_done() is
> called
> with so->needPrimScan=true. Prior to this bugfix, _bt_parallel_seize()
> was
> called after _bt_readpage() sets so->needPrimScan=true, and it just
> returned
> false without calling _bt_parallel_done().

You influenced me to add something about this to my follow-up commit caca6d8d:

--- a/src/backend/access/nbtree/nbtsearch.c
+++ b/src/backend/access/nbtree/nbtsearch.c
@@ -2230,8 +2230,9 @@ _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno,
             !so->currPos.moreRight : !so->currPos.moreLeft))
        {
            /* most recent _bt_readpage call (for lastcurrblkno) ended scan */
+           Assert(so->currPos.currPage == lastcurrblkno && !seized);
            BTScanPosInvalidate(so->currPos);
-           _bt_parallel_done(scan);
+           _bt_parallel_done(scan);    /* iff !so->needPrimScan */
            return false;
        }

I added "iff !so->needPrimScan" to draw attention to the fact that we
don't necessarily really end the parallel scan when _bt_parallel_done
is called.

--
Peter Geoghegan