On Tue, Oct 15, 2024 at 10:06 PM Yushi Ogiwara <btogiwarayuu...@oss.nttdata.com> wrote: > > Hi, > > Thank you for your comment. > > Regarding the first patch, I believe it works correctly when > consume_xids(1) is called. This is because the lastxid variable in the > consume_xids_common function is initialized as lastxid = > ReadNextFullTransactionId(), where the ReadNextFullTransactionId > function returns the (current XID) + 1.
But it's possible that concurrent transactions consume XIDs in meanwhile, no? > > Separately, I found that consume_xids(0) does not behave as expected. > Below is an example: > > postgres=# select txid_current(); > txid_current > -------------- > 45496 > (1 row) > > postgres=# select consume_xids(0); > consume_xids > -------------- > 45497 > (1 row) > > postgres=# select consume_xids(0); > consume_xids > -------------- > 45497 > (1 row) > > In the example, the argument to consume_xids is 0, meaning it should not > consume any XIDs. However, the first invocation of consume_xids(0) looks > like unexpectedly consuming 1 XID though it's not consuming actually. > This happens because consume_xids(0) returns the value from > ReadNextFullTransactionId. > > I have updated the patch (skip.diff, attached to this e-mail) to address > this issue. Now, when consume_xids(0) is called, it returns > ReadNextFullTransactionId().value - 1, ensuring no XID is consumed as > shown below: > > postgres=# select txid_current(); > txid_current > -------------- > 45498 > (1 row) > > postgres=# select consume_xids(0); > consume_xids > -------------- > 45498 > (1 row) Hmm, I think if we expect this function to return the last XID that the function actually consumed, calling consume_xids with 0 should raise an error instead. Even if it returns ReadNextFullTransactionId().value - 1 as you proposed, other concurrent transactions might consume XIDs between txid_current() and consume_xids(0), resulting in consume_xids() appearing to have consumed XIDs. Regards, -- Masahiko Sawada Amazon Web Services: https://aws.amazon.com
