From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Stephan Szabo <sszabo(at)megazone(dot)bigpanda(dot)com> |
Cc: | depesz(at)depesz(dot)com, pgsql-bugs(at)postgresql(dot)org |
Subject: | Re: bad message or bad privilege check in foreign key constraint |
Date: | 2008-01-23 00:48:31 |
Message-ID: | 10372.1201049311@sss.pgh.pa.us |
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Lists: | pgsql-bugs |
Stephan Szabo <sszabo(at)megazone(dot)bigpanda(dot)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
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