Re: sequence locking

On Wednesday 21 Sep 2011 19:24:55 Tom Lane wrote:
> "Kevin Grittner" <Kevin.Grittner@wicourts.gov> writes:
> > Andres Freund <andres@anarazel.de> wrote:
> >> - Its impossible to emulate proper locking yourself because
> >> locking is not allowed for sequences
> >> 
> >> Any arguments against allowing it again? It seems to have been
> >> allowed in prehistoric times.
> 
> If you think that it used to be allowed, it'd be a good idea to see
> if you can find the archived discussions about changing it.
The message I was thinking about was
http://archives.postgresql.org/pgsql-hackers/2001-10/msg00930.php

> > It would be nice to allow it.  I've had to create a dummy table just
> > to use for locking a sequence (by convention).
> One question is what you think the lock means.  I believe for example
> that taking a non-exclusive regular table lock on a sequence would not
> prevent other sessions from doing nextval(); even an exclusive one would
> not prevent them from doing so if they had pre-cached values.
I don't see what a non-exclusive lock on a sequence should sensibly do so I 
don't see a problem with not supporting them.
That already cached values are not affected by the lock seems to be pretty 
logical to me - and not really problematic.
At least in my cases I would look at last_value from the sequence after 
locking it- which includes the cached values so its fine that they can be used. 
The case that somebody already acquired a sequence value that not visible to 
other sessions has to be taken into account anyway.


Greetings,

Andres