On Tue, 13 Sep 2022, 12:04 Laurenz Albe, <laurenz.a...@cybertec.at> wrote: > > On Tue, 2022-09-13 at 11:47 +0300, Nikita Malakhov wrote: >> Please correct me if I'm wrong, despite tuples being inserted and deleted by >> the same >> transaction - they are visible inside the transaction and usable by it, so >> considering them >> dead and cleaning up during execution is a bad idea until the transaction is >> ended. > > But once they are deleted or updated, even the transaction that created them > cannot > see them any more, right?
Not quite. The command that is deleting the tuple might still be running, and because deletions are only "visible" to statements at the end of the delete operation, that command may still need to see the deleted tuple (example: DELETE FROM tab t WHERE t.randnum > (select count(*) from tab)); that count(*) will not change during the delete operation. So in order to mark that tuple as all_dead, you need proof that the deleting statement finished executing. I can think of two ways to do that: either the commit/abort of that transaction (this would be similarly expensive as the normal commit lookup), or (e.g.) the existence of another tuple with the same XID but with a newer CID. That last one would not be impossible, but probably not worth the extra cost of command id tracking. Kind regards, Matthias van de Meent
