Re: pg_stat_bgwriter.buffers_backend is pretty meaningless (and more?)

Hi,

On 2022-09-27 14:20:44 -0400, Melanie Plageman wrote:
> v30 attached
> rebased and pgstat_io_ops.c builds with meson now
> also, I tested with pgstat_report_stat() only flushing when forced and
> tests still pass

Unfortunately tests fail in CI / cfbot. E.g.,
https://cirrus-ci.com/task/5816109319323648

https://api.cirrus-ci.com/v1/artifact/task/5816109319323648/testrun/build/testrun/main/regress/regression.diffs
diff -U3 /tmp/cirrus-ci-build/src/test/regress/expected/stats.out /tmp/cirrus-ci-build/build/testrun/main/regress/results/stats.out
--- /tmp/cirrus-ci-build/src/test/regress/expected/stats.out 2022-10-01 12:07:47.779183501 +0000
+++ /tmp/cirrus-ci-build/build/testrun/main/regress/results/stats.out 2022-10-01 12:11:38.686433303 +0000
@@ -997,6 +997,8 @@
-- Set temp_buffers to a low value so that we can trigger writes with fewer
-- inserted tuples.
SET temp_buffers TO '1MB';
+ERROR: invalid value for parameter "temp_buffers": 128
+DETAIL: "temp_buffers" cannot be changed after any temporary tables have been accessed in the session.
CREATE TEMPORARY TABLE test_io_local(a int, b TEXT);
SELECT sum(alloc) AS io_sum_local_allocs_before FROM pg_stat_io WHERE io_context = 'local' \gset
SELECT sum(read) AS io_sum_local_reads_before FROM pg_stat_io WHERE io_context = 'local' \gset
@@ -1037,7 +1039,7 @@
SELECT :io_sum_local_writes_after > :io_sum_local_writes_before;
?column?
----------
- t
+ f
(1 row)

SELECT :io_sum_local_extends_after > :io_sum_local_extends_before;

So the problem is just that something else accesses temp buffers earlier in
the same test.

That's likely because since you sent your email

commit d7e39d72ca1c6f188b400d7d58813ff5b5b79064
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Date: 2022-09-29 12:14:39 -0400

Use actual backend IDs in pg_stat_get_backend_idset() and friends.

was applied, which adds a temp table earlier in the same session.

I think the easiest way to make this robust would be to just add a reconnect
before the place you need to set temp_buffers, that way additional temp tables
won't cause a problem.

Setting the patch to waiting-for-author for now.

Greetings,

Andres Freund