Re: [BUG / PATCH] PG19: VACUUM misestimates reltuples (often 0) after on-access VM setting

On Thu, Nov 12, 2026 at 8:51 AM Melanie Plageman
<melanieplageman(at)gmail(dot)com> wrote:
> Attached is what I propose to backpatch to 19. It is basically the
> same as what you proposed but split up a bit differently.

> + if (old_rel_tuples < 0 || old_rel_pages == 0)

Should this use old_rel_tuples <= 0?

After DELETE followed by VACUUM (TRUNCATE false), an empty table can
have reltuples = 0 and relpages > 0. After repopulating it, on-access
pruning can mark most pages all-visible, and VACUUM can preserve the
old zero estimate even though the table contains rows.

For example:

---------------------------------
SET vacuum_max_eager_freeze_failure_rate = 0;
CREATE TABLE repro (v text) WITH (autovacuum_enabled = false);

INSERT INTO repro SELECT repeat('x', 300) FROM generate_series(1, 100000);
DELETE FROM repro;
VACUUM (TRUNCATE false) repro;
SELECT relpages, reltuples FROM pg_class WHERE oid = 'repro'::regclass;

INSERT INTO repro SELECT repeat('x', 300) FROM generate_series(1, 100000);
SELECT count(*) FROM repro;
VACUUM repro;
SELECT relpages, reltuples FROM pg_class WHERE oid = 'repro'::regclass;
---------------------------------

Regards,

--
Fujii Masao