Re: pg_stat_reset_single_*_counters vs pg_stat_database.stats_reset - Mailing list pgsql-hackers
| From | David G. Johnston |
|---|---|
| Subject | Re: pg_stat_reset_single_*_counters vs pg_stat_database.stats_reset |
| Date | |
| Msg-id | CAKFQuwZDzxJ=SAHx0jF08ibZj_S6CG41Rq-RK_Xtnom2u6O0KA@mail.gmail.com Whole thread Raw |
| In response to | pg_stat_reset_single_*_counters vs pg_stat_database.stats_reset (Andres Freund <andres@anarazel.de>) |
| Responses |
Re: pg_stat_reset_single_*_counters vs pg_stat_database.stats_reset
|
| List | pgsql-hackers |
On Wed, Mar 23, 2022 at 5:55 PM Andres Freund <andres@anarazel.de> wrote:
Starting with the below commit, pg_stat_reset_single_function_counters,
pg_stat_reset_single_table_counters don't just reset the stats for the
individual function, but also set pg_stat_database.stats_reset.
commit 4c468b37a281941afd3bf61c782b20def8c17047
Author: Magnus Hagander <magnus@hagander.net>
Date: 2011-02-10 15:09:35 +0100
Track last time for statistics reset on databases and bgwriter
Tracks one counter for each database, which is reset whenever
the statistics for any individual object inside the database is
reset, and one counter for the background writer.
Tomas Vondra, reviewed by Greg Smith
/*
[...]
Maybe I just don't understand what these reset functions are intended for?
Their introduction [3] didn't explain much either. To me the behaviour of
resetting pg_stat_database.stats_reset but nothing else in pg_stat_database
makes them kind of dangerous.
*tl/dr;*
There seems to be three scopes here:
Cluster Stats - (add last_reset fields for consistency)
Database and Shared Object Stats (add last_reset to handle recording the resetting of just the records on this table, leaving stats_reset to be a flag meaning self-or-children)
Object Stats (add last_reset, don't need stats_reset since there are no children)
If we are OK with just changing pg_stat_database.stats_reset meanings to be less informative than what it is today (I strongly dislike such a silent behavioral change) we could simply standardize on the intended meaning of stats_reset on all three scopes, adding stats_reset as needed to track object-level resetting.
*Additional Exposition*
The description for the column declares that the field is reset when the statistics on the pg_stat_database are reset. That is also the expected behavior, knowing when any statistics in the whole database are reset is indeed not useful.
"Time at which these statistics were last reset"
The "these" clearly refers to the statistics columns in pg_stat_database.
In fact, pg_stat_archiver.stats_reset already exists (as does pg_bgwriter.stats_reset) with (I presume) this interpretation. This is a problem because pg_stat_database.stats_reset does not have the same meaning. So we have to either live with inconsistency or break something.
In the vein of living with inconsistency I would suggest changing the documentation of "pg_stat.database.stats_reset" to match the present behavior. Then add a new column (last_reset ?) to represent the existing description of "stats_reset".
I suppose we could document "stats_reset" as the itself-or-any-children reset timestamp, it's just that the archive and pgwriter don't have children in this sense while databases do. When the pg_stat_database.last_reset field changes the pg_stat_database.stats_reset would have to match anyway.
I don't have any issue with an indicator field saying "something regarding stats has changed" at the database level. It is much easier to monitor that and then inspect what may have changed rather than monitoring a last_reset column on every single catalog that has statistics that can be reset.
It also seems that each tracked object type needs to have its own last_reset field (we could choose to name it stats_reset too, leaving pg_stat_database.last_reset as the only anomaly) added as an implied behavior needed for such individualized resetting. I would go with *.last_reset though and leave the absence of pg_stat_archiver.last_reset as the anomaly (or just add it redundantly for consistency).
I don't see removing existing functionality as a good course to getting a consistent implementation; we should just push forward with figuring out what is missing and fill in those gaps. At worst if that isn't something we want to fix right now our new setup should at least leave the status quo behaviors in place.
I haven't looked into what kind of explicit resetting options are available but the above seems to cover tracking resetting regardless of how it is implemented. I've only spot checked some of the tables to identify the pattern.
David J.
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