Stephan Szabo <sszabo@megazone.bigpanda.com> 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.
regards, tom lane
