Thread

Re: [PATCH] Add pg_current_vxact_id() function to expose virtual transaction IDs

Henson Choi <assam258@gmail.com> — 2025-12-28T15:52:18Z

I looked into where VXID is actually used:

  SELECT c.relname, a.attname
  FROM pg_attribute a JOIN pg_class c ON a.attrelid = c.oid
  WHERE a.attname LIKE '%virtual%' AND a.attnum > 0;

   relname  |      attname
  ----------+--------------------
   pg_locks | virtualxid
   pg_locks | virtualtransaction

Only pg_locks has it. And you can already get your VXID from there:

  SELECT virtualtransaction FROM pg_locks
  WHERE pid = pg_backend_pid() LIMIT 1;

This always works since every transaction holds its own VXID lock.

For log correlation, PID works in most cases.

So I'm having trouble seeing a compelling use case. Could you share
a concrete scenario where this function would help?

The patch itself is clean, but I'm not sure about the justification.

2025년 12월 8일 (월) PM 9:10, Pavlo Golub <pavlo.golub@cybertec.at>님이 작성:

> Hi hackers,
>
> I'd like to propose a new function pg_current_vxact_id() that returns
> the
> current backend's virtual transaction ID (VXID).
>
> Virtual transaction IDs are fundamental to PostgreSQL's transaction
> tracking,
> appearing in pg_locks.virtualtransaction, log output via %v placeholder,
> and
> internal transaction management. However, there's currently no direct
> SQL
> function to retrieve the current VXID, forcing applications to query
> pg_locks
> or parse log files to obtain this information.
>
> The patch adds pg_current_vxact_id() which returns the VXID as text in
> the
> format "procNumber/lxid" (e.g., "3/42"), matching the format used
> throughout
> PostgreSQL for consistency.
>
> Use cases:
> 1. Application transaction tracking and correlation with logs
> 2. Monitoring read-only transactions (which never get regular XIDs)
> 3. Debugging transaction behavior without querying pg_locks
> 4. Building monitoring tools that need consistent transaction identity
>
> The function follows the same pattern as pg_current_xact_id() and
> pg_current_xact_id_if_assigned(), providing a clean API for a commonly
> needed piece of information.
>
> Changes:
> - Added function in xid8funcs.c (alongside related transaction ID
> functions)
> - OID 5101 (verified available with unused_oids script)
> - Comprehensive regression tests in xid.sql
> - Documentation in func-info.sgml and xact.sgml
> - Format kept in sync with existing VXID representations in elog.c and
> lockfuncs.c
>
> The v1 patch is attached. Tests pass cleanly with "meson test
> regress/regress".
>
> Best regards,
> Pavlo Golub
>