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.klaver@aklaver.com