"Qingqing Zhou" <zhouqq(at)cs(dot)toronto(dot)edu> writes:
> Pg_xactlock is always there as a special relation.
pg_xactlock isn't really a relation. The way I think about it is that
it's a dummy entry in pg_class that exists to reserve a relation OID
for a specific purpose --- namely, we can lock transaction IDs by
locking what would otherwise be a page of that relation.
There was some talk recently about reorganizing the locktag design
so that transaction lock tags would be clearly distinguishable from
any lock associated with a relation. If we got that done, there'd
be no need for the pg_xactlock entry at all.
> I am not sure about 'c'.
'c' entries in pg_class are for composite types. They don't have
any associated disk storage either.
regards, tom lane