| From: | Thomas Munro <thomas(dot)munro(at)gmail(dot)com> |
|---|---|
| To: | Konstantin Knizhnik <knizhnik(at)garret(dot)ru> |
| Cc: | pgsql-hackers(at)lists(dot)postgresql(dot)org |
| Subject: | Re: Sync scan & regression tests |
| Date: | 2023-08-06 23:29:58 |
| Message-ID: | CA+hUKGJeerdJ8EzqB8r3CdPx4NaU1v5ZE7D+UBmr_n+Z5PDD_Q@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Mon, Aug 7, 2023 at 7:21 AM Konstantin Knizhnik <knizhnik(at)garret(dot)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.
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