Horton

Re: Sync scan & regression tests

Thomas Munro <thomas.munro@gmail.com>

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From: Thomas Munro <thomas.munro@gmail.com>
To: Konstantin Knizhnik <knizhnik@garret.ru>
Cc: pgsql-hackers@lists.postgresql.org
Date: 2023-08-06T23:29:58Z
Lists: pgsql-hackers 
On Mon, Aug 7, 2023 at 7:21 AM Konstantin Knizhnik <knizhnik@garret.ru> wrote:
> Two tests are failed because of sync scan - this tests cluster.sql and
> portals.sql perform seqscan without explicit order by and expect that
> data will be returned in particular order. But because of sync scan it
> doesn't happen. Small shared buffers are needed to satisfy seqscan
> criteria in heapam.c: `scan->rs_nblocks > NBuffers / 4` for tenk1 table.

I wondered the same thing while working on the tests in commit
8ab0ebb9a84, which explicitly care about physical order, so they *say
so* with ORDER BY ctid.  But the problem seems quite widespread, so I
didn't volunteer to try to do something like that everywhere, when Tom
committed cbf4177f for 027_stream_regress.pl.

FWIW here's another discussion of that cluster test, in which I was
still figuring out some surprising ways this feature can introduce
non-determinism even without concurrent access to the same table.

https://www.postgresql.org/message-id/flat/CA%2BhUKGLTK6ZuEkpeJ05-MEmvmgZveCh%2B_w013m7%2ByKWFSmRcDA%40mail.gmail.com

Commits

  1. Avoid edge case in pg_visibility test with small shared_buffers

  2. Report syncscan position at end of scan.

  3. hio: Take number of prior relation extensions into account

  4. Reduce code duplication between heapgettup and heapgettup_pagemode

  5. Fix CLUSTER tuplesorts on abbreviated expressions.

  6. Set PD_ALL_VISIBLE and visibility map bits in COPY FREEZE