On Sat, 29 Mar 2025 at 06:00, Evgeny Morozov <postgres...@realityexists.net> wrote: > > On 23/03/2025 2:35 pm, David Rowley wrote: > >> alter table entity_2 add column new_column text; > > Is this just an example command? You can't add a column to a > > partition directly. > > Yes, it was just the simplest way I could think of to take an exclusive > lock. But on this note: I guess it takes a lock before checking that the > table is partition because it might otherwise get attached/detached as a > partition between doing the check and taking a lock, but it would be > nice if it took a shared lock to do all checks, then upgraded to an > exclusive lock to actually do the work. Then the user would not have to > wait for an exclusive lock only for the command to fail.
Unfortunately, that's probably just swapping one problem for another. Once you have 2 sessions following such a pattern of locking, you're prone to unnecessary deadlocking. For example: -- session1 begin; lock table t in access share mode; -- gets lock -- session2 begin; lock table t in access share mode; -- gets lock lock table t in access exclusive mode; -- waits -- session1 lock table t in access exclusive mode; -- deadlock If you don't bother with the access share lock, there's no deadlock. David
