Hi,
On 2022-02-24 07:33:39 -0800, Andres Freund wrote:
> I added the SELECT relpages, reltuples, relallvisible FROM pg_class WHERE oid
> = 'tenk1'::regclass;
> just after the
> VACUUM ANALYZE tenk1;
>
> synchronous_commit=on
> + relpages | reltuples | relallvisible
> +----------+-----------+---------------
> + 345 | 10000 | 345
> +(1 row)
>
> synchronous_commit=off
> + relpages | reltuples | relallvisible
> +----------+-----------+---------------
> + 345 | 10000 | 0
> +(1 row)
>
> So it clearly is the explanation for the issue.
>
>
> Obviously we can locally work around it by adding a
> SET LOCAL synchronous_commit = local;
> to the COPY. But I'd like to fully understand what's going on.
It is the hint bit sets delayed by asynchronous commit. I traced execution and
we do end up not setting all visible due to reaching the
!HeapTupleHeaderXminCommitted() path in lazy_scan_prune()
case HEAPTUPLE_LIVE:
...
/*
* Is the tuple definitely visible to all
transactions?
*
* NB: Like with per-tuple hint bits, we can't
set the
* PD_ALL_VISIBLE flag if the inserter committed
* asynchronously. See SetHintBits for more
info. Check that
* the tuple is hinted xmin-committed because
of that.
*/
if (prunestate->all_visible)
{
TransactionId xmin;
if
(!HeapTupleHeaderXminCommitted(tuple.t_data))
So it might be reasonable to use synchronous_commit=on in test_setup.sql?
It's not super satisfying, but i don't immediately see what else could prevent
all-visible to be set in this case? There's no dead rows, so concurrent
snapshots shouldn't prevent cleanup.
I guess we could alternatively try doing something like flushing pending async
commits at the start of vacuum. But that probably isn't warranted.
Greetings,
Andres Freund
