Postgres backend statistics (Part 1): I/O statistics

commit 9aea73fc61d4e77e000724ce0b2f896590a10e03
Author: Michael Paquier <michael@paquier.xyz>
Date:   Thu Dec 19 13:19:22 2024 +0900

Add backend-level statistics to pgstats

This adds a new variable-numbered statistics kind in pgstats, where the
object ID key of the stats entries is based on the proc number of the
backends.  This acts as an upper-bound for the number of stats entries
that can exist at once.  The entries are created when a backend starts
after authentication succeeds, and are removed when the backend exits,
making the stats entry exist for as long as their backend is up and
running.  These are not written to the pgstats file at shutdown (note
that write_to_file is disabled, as a safety measure).

Currently, these stats include only information about the I/O generated
by a backend, using the same layer as pg_stat_io, except that it is now
possible to know how much activity is happening in each backend rather
than an overall aggregate of all the activity.  A function called
pg_stat_get_backend_io() is added to access this data depending on the
PID of a backend.  The existing structure could be expanded in the
future to add more information about other statistics related to
backends, depending on requirements or ideas.

Auxiliary processes are not included in this set of statistics.  These
are less interesting to have than normal backends as they have dedicated
entries in pg_stat_io, and stats kinds of their own.

This commit includes also pg_stat_reset_backend_stats(), function able
to reset all the stats associated to a single backend.

db1=# SELECT * FROM pg_stat_get_backend_io(pg_backend_pid());
  backend_type  |    object     |  context  | reads | read_time | writes | write_time | writebacks | writeback_time | extends | extend_time | op_bytes |  hits   | evictions | reuses | fsyncs | fsync_time | stats_reset
----------------+---------------+-----------+-------+-----------+--------+------------+------------+----------------+---------+-------------+----------+---------+-----------+--------+--------+------------+-------------
 client backend | relation      | bulkread  |     0 |         0 |      0 |          0 |          0 |              0 |         |             |     8192 |       0 |         0 |      0 |        |            |
 client backend | relation      | bulkwrite |     0 |         0 |      0 |          0 |          0 |              0 |       0 |           0 |     8192 |       0 |         0 |      0 |        |            |
 client backend | relation      | normal    |     8 |         0 |    832 |          0 |          0 |              0 |    7967 |           0 |     8192 | 1816551 |      2433 |        |      0 |          0 |
 client backend | relation      | vacuum    |     0 |         0 |      0 |          0 |          0 |              0 |       0 |           0 |     8192 |       0 |         0 |      0 |        |            |
 client backend | temp relation | normal    |     0 |         0 |      0 |          0 |            |                |       0 |           0 |     8192 |       0 |         0 |        |        |            |
(5 rows)

db1=# SELECT datname, application_name, pid, sum(hits)
db1-# FROM pg_stat_activity, pg_stat_get_backend_io(pid)
db1-# WHERE hits != 0
db1-# GROUP BY datname, application_name, pid
db1-# ORDER BY 4 desc
db1-# LIMIT 1;

 datname | application_name |  pid   |   sum
---------+------------------+--------+---------
 db1     | app2             | 502479 | 1816958
(1 row)

db1=# SELECT application_name, writes, round(100 * writes/sum(writes) over(),2) AS "%"
FROM  (SELECT application_name, sum(writes) AS writes
       FROM pg_stat_activity, pg_stat_get_backend_io(pid)
       WHERE writes != 0 GROUP BY application_name);
 application_name | writes |   %
------------------+--------+-------
 app1             |   2374 | 34.11
 app2             |   4586 | 65.89
(2 rows)