Re: Need help understanding pg_locks

On Jul11, 2011, at 17:31 , Bruce Momjian wrote:
> Tom Lane wrote:
>> Florian Pflug <fgp@phlo.org> writes:
>>> On Jul11, 2011, at 17:11 , Tom Lane wrote:
>>>> Yeah, I think this patch is going in the wrong direction altogether.
>>>> It would be better to modify the description of virtualtransaction
>>>> and pid to say that those are the "locking" entity.
>> 
>>> Hm, we already kinda of say that. Both descriptions include the phrase
>>> "... holding or awaiting this lock.". The column "mode" says
>>> "... held or desired by this process", which I guess is similar enough
>>> to make it clear that these are related.
>> 
>>> Its the columns which refer to the locked object which simply say 
>>> "object", and thus leave it open if that means locked or a locking.
>> 
>>> Could we split that table in two parts, one for the fields referring
>>> to the locked object and one for the locking entity, or does that depart
>>> too far from the way we document other system catalogs and views?
>> 
>> Then you'd have to join them, which would not be an improvement from
>> anybody's standpoint.
>> 
>> Maybe we could just add a paragraph above the "pg_locks Columns" table
>> that says explicitly that virtualtransaction and pid describe the entity
>> holding or awaiting the lock, and the others describe the object being
>> locked?  Any way you slice it, putting this information into the
>> per-column table is going to be repetitive.
> 
> Frankly, whenever anyone says "object", they might as well call it
> "thing".  It seems to be a content-less word.  Maybe just replace the
> word "object" with "lock".

I like that, as long as we make it ".. lock is/isn't *on* a ...", and not
just "... lock is/isn't a". After all, the lock very clearly isn't a
relation or xid or whatever - it's a, well, lock.

We'd then have OID of the database in which the lock exists, or zero if the lock is on a shared object, or null if the
lockis on a transaction ID.
 
 OID of the relation, or null if the lock is not on a relation or part of a relation.
 ...
 ID of a transaction, or null if the lock is not on a transaction ID
 ...

best regards,
Florian Pflug