On Tue, 22 Jan 2008, Tom Lane wrote: > Stephan Szabo <[EMAIL PROTECTED]> writes: > > On Tue, 22 Jan 2008, Tom Lane wrote: > >> Hmm. I wonder why we are bothering with FOR SHARE locks on the > >> referencing table, when we don't have any intention to change > >> those rows. Is there some race condition that's needed to prevent? > > > I think it may be if you've done something like updated the row in another > > transaction it waits for the final state of that transaction rather than > > erroring immediately. > > > Given something like: > > create table t1(a int primary key); > > create table t2(b int references t1); > > insert into t1 values (1); > > insert into t1 values (2); > > insert into t2 values (1); > > T1: begin; > > T2: begin; > > T1: update t2 set b=2; > > T2: delete from t1 where a=1; > > -- I think here, if we don't use something that tries to get a row lock > > -- the delete will fail because it still sees the t2 row having b=1 > > -- while with the lock, it'll succeed if T1 commits and fail if T1 > > -- aborts? > > But how much do we care about that? The case that's actually necessary > for correctness, I think, is to block if we are trying to delete a=2 > --- but that happens because T1 took a shared row lock on that row. > Doing it in the other direction too seems like it'll introduce > performance and deadlock issues.
Well, from an end user standpoint, I think it's basically similar to the case with unique where if you delete a row in T1 and try inserting a row that would conflict in T2 before T1 commits, T2 waits rather than immediately erroring. ---------------------------(end of broadcast)--------------------------- TIP 5: don't forget to increase your free space map settings
