Adding locks statistics

Hi hackers,

Please find attached a patch to add a new view (namely pg_stat_lock) that provides
lock statistics.  

It’s output is like the following:

postgres=# select * from pg_stat_lock;
     locktype     | requests | waits | timeouts | deadlock_timeouts | deadlocks | fastpath |          stats_reset

------------------+----------+-------+----------+-------------------+-----------+----------+-------------------------------
 relation         |   612775 |     1 |        0 |                 0 |         0 |   531115 | 2025-08-01
09:18:26.476275+00
 extend           |     3128 |     0 |        0 |                 0 |         0 |        0 | 2025-08-01
09:18:26.476275+00
 frozenid         |       11 |     0 |        0 |                 0 |         0 |        0 | 2025-08-01
09:18:26.476275+00
 page             |        1 |     0 |        0 |                 0 |         0 |        0 | 2025-08-01
09:18:26.476275+00
 tuple            |     3613 |     0 |        0 |                 0 |         0 |        0 | 2025-08-01
09:18:26.476275+00
 transactionid    |     6130 |     0 |        0 |                 0 |         0 |        0 | 2025-08-01
09:18:26.476275+00
 virtualxid       |    15390 |     0 |        0 |                 0 |         0 |    15390 | 2025-08-01
09:18:26.476275+00
 spectoken        |       12 |     0 |        0 |                 0 |         0 |        0 | 2025-08-01
09:18:26.476275+00
 object           |     8393 |     0 |        0 |                 0 |         0 |        0 | 2025-08-01
09:18:26.476275+00
 userlock         |        0 |     0 |        0 |                 0 |         0 |        0 | 2025-08-01
09:18:26.476275+00
 advisory         |       44 |     0 |        0 |                 0 |         0 |        0 | 2025-08-01
09:18:26.476275+00
 applytransaction |        0 |     0 |        0 |                 0 |         0 |        0 | 2025-08-01
09:18:26.476275+00

It means that it provides historical trends of locks usage per lock type.

It can be used for example for:

1. checking if "waits" is close to "requests". Then it means you usually have to
wait before acquiring the lock, which means you may have a concurrency issue.

2. lock_timeout and deadlock_timeout tuning (lock_timeout is visible only in the
logs if log_min_error_statement is set appropriately).

3. checking the "requests"/"fastpath" ratio to see if "max_locks_per_transaction"
needs tuning (see c4d5cb71d2).

If any points need more details, it might be a good idea to start sampling pg_locks.

The patch is made of 2 sub-patches:

0001 - It adds a new stat kind PGSTAT_KIND_LOCK for the lock statistics.

This new statistic kind is a fixed one because its key is the lock type
so that we know its size is LOCKTAG_LAST_TYPE + 1.

This statistic kind records the following counters:

- requests: Number of requests for this lock type.
- waits: Number of times requests for this lock type had to wait.
- timeouts: Number of times requests for this lock type had to wait longer than
lock_timeout.
- deadlock_timeouts: Number of times requests for this lock type had to wait longer
than deadlock_timeout.
- deadlocks: Number of times a deadlock occurred on this lock type.
- fastpath: Number of times this lock type was taken via fast path.

No extra details is added (like the ones, i.e relation oid, database oid, we
can find in pg_locks). The idea is to provide an idea on what the locking
behaviour looks like.

Those new counters are incremented outside of the wait events code path,
as suggested in [1].

There are no major design choices, it relies on the current statistics machinery.

0002 - It adds the pg_stat_lock view

It also adds documentation and some tests.

Remarks:

- maybe we could add some metrics related to the lock duration (we have some hints
thanks to the timeout ounters though)
- if this is merged, a next step could be to record those metrics per backend

[1]:
https://www.postgresql.org/message-id/CA%2BTgmobptuUWo7X5zcQrWKh22qeAn4eL%2B%3Dwtb8_ajCOR%2B7_tcw%40mail.gmail.com

Looking forward to your feedback,

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com