Re: [PATCH v4] Add pg_current_vxact_id() function

>
>
> On a standby the function actually returns the VXID just fine -- the
> code only guards on MyProc/procNumber, which is the same condition %v
> uses, and StartTransaction assigns the vxid unconditionally during
> recovery (no RecoveryInProgress guard anywhere). We should change
> the commit message.
>
>
You are absolutely right, thank you for catching that. The guard is:

if (MyProc == NULL || MyProc->vxid.procNumber == INVALID_PROC_NUMBER)

That condition has nothing to do with recovery. It only covers very early
backend startup before MyProc is initialized. The function works correctly
on standbys, exactly as %v does in log output.

The C comment and commit message have been corrected in v5.

Patch v5 attached (only change from v4: corrected the recovery claim in
the commit message and C comment).

> The only thing a VXID gives you over a PID is telling
> multiple transactions within the same backend/session, and I haven't
> been able to come up with a concrete case where you'd actually need that.
>
> Does anyone have one?
>
> The other cases I have in mind involve correlating log entries that use %v
in
log_line_prefix. With %p you can identify the backend and with with %v you
can
additionally identify *which transaction within that backend* produced a
log line. I see this as useful when a long-lived session issues many
transactions
and you want to match a specific ERROR or lock-wait line to the exact
transaction
in yourapplication log, not just to the session.

I believe monitoring connection poolers could be improved with this
function.
When connection poolers run in session mode a pooler backend is reused
across
multiple client-level "sessions". The PID is the same for all of them, but
the
localXID monotonically increases, so pg_current_vxact_id() lets the
application
record a correlation token that uniquely identifies each pooled client
transaction
in the server log.

And, of course, monitoring tools that join against pg_locks to detect
blocking already
work with VXIDs. pg_current_vxact_id() lets an application embed its own
VXID
in application-level logging without a round-trip through pg_locks.

Regards,
Pavlo Golub
CYBERTEC PostgreSQL International GmbH