Re: Tracking last scan time

Hi,

On 2022-10-12 12:50:31 -0700, Andres Freund wrote:
> I think this should have at a basic test in src/test/regress/sql/stats.sql. If
> I can write one in a few minutes I'll go for that, otherwise will reply
> detailing difficulties.

Took a bit longer (+lunch). Attached.


In the attached 0001, the patch to make GetCurrentTransactionStopTimestamp()
set xactStopTimestamp, I added a few comment updates and an Assert() to ensure
that CurrentTransactionState->state is TRANS_(DEFAULT|COMMIT|ABORT|PREPARE). I
am worried that otherwise we might end up with someone ending up using it in a
place before the end of the transaction, which'd then end up recording the
wrong timestamp in the commit/abort record.


For 0002, the commit adding lastscan, I added catversion/stats version bumps
(because I was planning to commit it already...), a commit message, and that
minor docs change mentioned earlier.


0003 adds the tests mentioned above. I plan to merge them with 0002, but left
them separate for easier review for now.

To be able to compare timestamps for > not just >= we need to make sure that
two subsequent timestamps differ. The attached achieves this by sleeping for
100ms between those points - we do that in other places already. I'd started
out with 10ms, which I am fairly sure would suffice, but then deciced to copy
the existing 100ms sleeps.

I verified tests pass under valgrind, debug_discard_caches and after I make
pgstat_report_stat() only flush when force is passed in.

Greetings,

Andres Freund