Horton

Re: shared-memory based stats collector - v70

Greg Stark <stark@mit.edu>

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From: Greg Stark <stark@mit.edu>
To: Andres Freund <andres@anarazel.de>
Cc: "Drouvot, Bertrand" <bdrouvot@amazon.com>, Tom Lane <tgl@sss.pgh.pa.us>, Melanie Plageman <melanieplageman@gmail.com>, Kyotaro Horiguchi <horikyota.ntt@gmail.com>, Justin Pryzby <pryzby@telsasoft.com>, Thomas Munro <thomas.munro@gmail.com>, "David G. Johnston" <david.g.johnston@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-08-10T19:48:15Z
Lists: pgsql-hackers 
One thing that's bugging me is that the names we use for these stats
are *all* over the place.

The names go through three different stages

pgstat structs  ->  pgstatfunc tupledescs  ->  pg_stat_* views

(Followed by a fourth stage where pg_exporter or whatever names for
the monitoring software)

And for some reason both transitions (plus the exporter) felt the need
to fiddle with the names or values. And not in any sort of even
vaguely consistent way. So there are three (or four) different sets of
names for the same metrics :(

e.g.

* Some of the struct elements have abbreviated words which are
expanded in the tupledesc names or the view columns -- some have long
names which get abbreviated.

* Some struct members have n_ prefixes (presumably to avoid C keywords
or other namespace issues?) and then lose them at one of the other
stages. But then the relation stats do not have n_ prefixes and then
the pg_stat view *adds* n_ prefixes in the SQL view!

* Some columns are added together in the SQL view which seems like
gratuitously hiding information from the user. The pg_stat_*_tables
view actually looks up information from the indexes stats and combines
them to get idx_scan and idx_tup_fetch.

* The pg_stat_bgwriter view returns data from two different fixed
entries, the checkpointer and the bgwriter, is there a reason those
are kept separately but then reported as if they're one thing?


Some of the simpler renaming could be transparently fixed by making
the internal stats match the public facing names. But for many of them
I think the internal names are better. And the cases where the views
aggregate data in a way that loses information are not something I
want to reproduce.

I had intended to use the internal names directly, reasoning that
transparency and consistency are the direction to be headed. But in
some cases I think the current public names are the better choice -- I
certainly don't want to remove n_* prefixes from some names but then
add them to different names! And some of the cases where the data is
combined or modified do seem like they would be missed.

-- 
greg

Commits

  1. Improve comment about dropped entries in pgstat.c

  2. Fix temporary memory leak in system table index scans

  3. pgstat: set timestamps of fixed-numbered stats after a crash.

  4. pgstat: Update docs to match the shared memory stats reality.

  5. pgstat: Hide instability in stats.spec with -DCATCACHE_FORCE_RELEASE.

  6. pgstat: add/extend tests for resetting various kinds of stats.

  7. Add minimal tests for recovery conflict handling.

  8. pgstat: test stats interactions with physical replication.

  9. pgstat: add tests for handling of restarts, including crashes.

  10. pgstat: add tests for transaction behaviour, 2PC, function stats.

  11. pgstat: add pg_stat_have_stats() test helper.

  12. pgstat: add pg_stat_force_next_flush(), use it to simplify tests.

  13. pgstat: move pgstat.c to utils/activity.

  14. pgstat: store statistics in shared memory.

  15. pgstat: remove stats_temp_directory.

  16. pgstat: rename STATS_COLLECTOR GUC group to STATS_CUMULATIVE.

  17. pgstat: revise replication slot API in preparation for shared memory stats.

  18. pgstat: scaffolding for transactional stats creation / drop.

  19. pgstat: introduce PgStat_Kind enum.

  20. pgstat: prepare APIs used by pgstatfuncs for shared memory stats.

  21. pgstat: add pgstat_copy_relation_stats().

  22. pgstat: rename some pgstat_send_* functions to pgstat_report_*.

  23. pgstat: stats collector references in comments.

  24. pgstat: move transactional code into pgstat_xact.c.

  25. dsm: allow use in single user mode.

  26. dshash: revise sequential scan support.

  27. pgstat: remove some superflous comments from pgstat.h.

  28. pgstat: reorder pgstat.[ch] contents.

  29. pgstat: split different types of stats into separate files.

  30. pgstat: introduce pgstat_relation_should_count().

  31. pgstat: separate "xact level" handling out of relation specific functions.

  32. pgstat: rename pgstat_initstats() to pgstat_relation_init().

  33. pgstat: split out WAL handling from pgstat_{initialize,report_stat}.

  34. pgstat: run pgindent on pgstat.c/h.

  35. pgstat: split relation, database handling out of pgstat_report_stat().

  36. Move code around in StartupXLOG().

  37. pgstat: Prepare to use mechanism for truncated rels also for droppped rels.

  38. pgstat: Split out relation stats handling from AtEO[Sub]Xact_PgStat() etc.

  39. pgstat: Schedule per-backend pgstat shutdown via before_shmem_exit().

  40. Schedule ShutdownXLOG() in single user mode using before_shmem_exit().

  41. Make parallel worker shutdown complete entirely via before_shmem_exit().

  42. pgstat: Bring up pgstat in BaseInit() to fix uninitialized use of pgstat by AV.

  43. pgstat: split reporting/fetching of bgwriter and checkpointer stats.

  44. Split backend status and progress related functionality out of pgstat.c.

  45. Split wait event related code from pgstat.[ch] into wait_event.[ch].

  46. Make archiver process an auxiliary process.

  47. Force to send remaining WAL stats to the stats collector at walwriter exit.

  48. Add pg_stat_database counters for sessions and session time

  49. Collect statistics about SLRU caches

  50. Don't run atexit callbacks in quickdie signal handlers.

  51. Create a "fast path" for acquiring weak relation locks.