On Tue, Jun 17, 2025 at 1:22 PM Andres Freund <andres@anarazel.de> wrote:
Hi,
On 2025-06-17 10:30:34 +0200, Mats Kindahl wrote: > Hi PostgreSQL Hackers, > > If you try to create a logical replication slot inside a transaction where > you have issued writes, it will abort with an error message: > > mats=# start transaction; > START TRANSACTION > mats=*# select txid_current(); > txid_current > -------------- > 406252 > (1 row) > > mats=*# insert into foo values (txid_current()); > INSERT 0 1 > mats=*# select txid_current(); > txid_current > -------------- > 406252 > (1 row) > > mats=*# select * from pg_create_logical_replication_slot('my_slot', > 'test_decoding', false, true); > ERROR: cannot create logical replication slot in transaction that has > performed writes > mats=!# rollback; > ROLLBACK > > > I'm slightly unsure why, but I assume that it is because if the transaction > is rolled back, it will have the XID of a transaction that does not "exist" > (quotes because the XID is still assigned and incremented, even if the > transaction is rolled back).
Logical slot creation basically waits for concurrent transactions to end, that wouldn't work if itself had a transaction open (it'd be a self deadlock).
Thanks Andres, that explains it.
> If that is the case, then having the call first in the transaction will > still work fine, since no writes are done yet, but if that transaction is > rolled back, the slot will point to a "non-existing" XID similar to the > first case.
