Virtual tx id

On Wed, 21 Sep 2022 at 11:58, James Sewell <james.sewell@gmail.com> wrote:

Hello Hackers!

Is it possible to get the current virtual txid from C somehow?

The virtual txid is consisted of MyProc->backendId and MyProc->lxid. Do you
mean a C function that returns virtual txid?

I've looked through the code, but can't seem to find anything other than
getting a NULL when there is no (real) xid assigned. Maybe I'm missing
something?

Do you mean use SQL function to check the virtual? IIRC, there is no such
functions. Maybe you can use pg_export_snapshot() to get virtual txid (v10
and later), the filename of exported snapshot consists MyProc->backendId
and MyProc->lxid.

--
Regrads,
Japin Li.
ChengDu WenWu Information Technology Co.,Ltd.

HI,
On Sep 21, 2022, 11:59 +0800, James Sewell <james.sewell@gmail.com>, wrote:

Hello Hackers!

Is it possible to get the current virtual txid from C somehow?

I've looked through the code, but can't seem to find anything other than getting a NULL when there is no (real) xid assigned. Maybe I'm missing something?

Cheers,
James

Virtual xid is meaningful only inside a read-only transaction.

It’s made up of MyProc->BackendId and MyProc->LocalTransactionId.

To catch it, you can begin a transaction, but don’t exec sqls that could change the db.

And gdb on process to see( must exec a read only sql to call StartTransaction, else MyProc->lxid is not assigned).

```
Begin;
// do nothing

```
gdb on it and see

```
p MyProc->lxid
p MyProc->backendId

```

Regards,
Zhang Mingli

Hi,

On Wed, Sep 21, 2022 at 01:58:47PM +1000, James Sewell wrote:

Hello Hackers!

Is it possible to get the current virtual txid from C somehow?

I've looked through the code, but can't seem to find anything other than
getting a NULL when there is no (real) xid assigned. Maybe I'm missing
something?

It should be MyProc->lxid, and prepend it with MyBackendId if you want to
compare something like pg_locks.virtualtransaction.