Re: shared-memory based stats collector - v70

Hi,

On 2022-04-07 00:28:45 -0700, Andres Freund wrote:
> I've gotten through the main commits (and then a fix for the apparently
> inevitable bug that's immediately highlighted by the buildfarm), and the first
> test. I'll call it a night now, and work on the other tests & docs tomorrow.

I've gotten through the tests now. There's one known, not yet addressed, issue
with the stats isolation test, see [1].

Working on the docs. Found a few things worth raising:

1)
Existing text:
When the server shuts down cleanly, a permanent copy of the statistics
data is stored in the <filename>pg_stat</filename> subdirectory, so that
statistics can be retained across server restarts. When recovery is
performed at server start (e.g., after immediate shutdown, server crash,
and point-in-time recovery), all statistics counters are reset.

The existing docs patch hadn't updated yet. My current edit is

When the server shuts down cleanly, a permanent copy of the statistics
data is stored in the <filename>pg_stat</filename> subdirectory, so that
statistics can be retained across server restarts. When crash recovery is
performed at server start (e.g., after immediate shutdown, server crash,
and point-in-time recovery, but not when starting a standby that was shut
down normally), all statistics counters are reset.

but I'm not sure the parenthetical is easy enough to understand?

2)
The edit is not a problem, but it's hard to understand what the existing
paragraph actually means?

diff --git a/doc/src/sgml/high-availability.sgml b/doc/src/sgml/high-availability.sgml
index 3247e056663..8bfb584b752 100644
--- a/doc/src/sgml/high-availability.sgml
+++ b/doc/src/sgml/high-availability.sgml
@@ -2222,17 +2222,17 @@ HINT: You can then restart the server after making the necessary configuration
...
<para>
- The statistics collector is active during recovery. All scans, reads, blocks,
+ The cumulative statistics system is active during recovery. All scans, reads, blocks,
index usage, etc., will be recorded normally on the standby. Replayed
actions will not duplicate their effects on primary, so replaying an
insert will not increment the Inserts column of pg_stat_user_tables.
The stats file is deleted at the start of recovery, so stats from primary
and standby will differ; this is considered a feature, not a bug.
</para>

<para>

I'll just commit the necessary bit, but we really ought to rephrase this.

Greetings,

Andres Freund

[1] https://www.postgresql.org/message-id/20220407165709.jgdkrzqlkcwue6ko%40alap3.anarazel.de