bdrouvot / pg_subxact_counters

pg_subxact_counters

Features

Get global subtransactions counters.

Why?

Subtransactions can lead to performance issue, as mentioned in some posts, see for example:

• Subtransactions and performance in PostgreSQL
• PostgreSQL Subtransactions Considered Harmful
• Why we spent the last month eliminating PostgreSQL subtransactions

The purpose of this extension is to provide counters to monitor the subtransactions (generation rate, overflow, state).
One idea could be to sample the pg_subxact_counters view on a regular basis.

Installation

This repository provides C and Rust code, please refer to the one of interest.

Example

postgres=# \dx+ pg_subxact_counters
Objects in extension "pg_subxact_counters"
       Object description
--------------------------------
 function pg_subxact_counters()
 view pg_subxact_counters
(2 rows)

postgres=# select * from pg_subxact_counters;
 subxact_start | subxact_commit | subxact_abort | subxact_overflow
---------------+----------------+---------------+------------------
             0 |              0 |             0 |                0
(1 row)

postgres=# begin;
BEGIN
postgres=*# savepoint a;
SAVEPOINT
postgres=*# savepoint b;
SAVEPOINT
postgres=*# commit;
COMMIT
postgres=# select * from pg_subxact_counters;
 subxact_start | subxact_commit | subxact_abort | subxact_overflow
---------------+----------------+---------------+------------------
             2 |              2 |             0 |                0
(1 row)

Fields meaning

• subxact_start: number of substransactions that started
• subxact_commit: number of substransactions that committed
• subxact_abort: number of substransactions that aborted (rolled back)
• subxact_overflow: number of times a top level XID have had substransactions overflowed

Remarks

• subxact_start - subxact_commit - subxact_abort: number of subtransactions that have started and not yet committed/rolled back.
• subxact_overflow does not represent the number of substransactions that exceeded PGPROC_MAX_CACHED_SUBXIDS.
• the counters are global, means they record the activity for all the databases in the instance.