Re: number and type of locks for an alter sequence

On 1/28/19 6:07 AM, Thomas Poty wrote:
> Hello ,
>
> In PG10, when I am altering a sequence, i see 2 locks in pg_locks
> matching my sequence:
> a RowExclusiveLock
> and a ShareRowExclusiveLock
>
> My question is Why 2 locks?
> Someone on irc said it was for catalog but i am no sure because
> pg_locks.relationid is the same for the 2 locks.
>
> At the same time, If I am trying to call nextval with an othersession,
> the session wants to get a RowExclusiveLock on the sequence.
>
> So, to me, the lock ShareRowExclusiveLock is when the sequence is beeing
> altered.
> And the RowExclusiveLock is for concurent access between alter/drop
> sequence and nextval/setval/curval/lastval.
>
> Documentation seems not talking about this...
> (https://www.postgresql.org/docs/10/explicit-locking.html) Maybe should
> I open a ticket?

A sequence, for practical purposes, is a table. So in the above
substitute TABLE with SEQUENCE:

>
> Thanks

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com