Re: WIP partial replication patch

Josh Berkus <josh(at)agliodbs(dot)com> writes:
>> Another rather basic problem is that you've
>> got to pass system catalog updates downstream (in case they affect
>> the tables being replicated) but if you want partial replication then
>> many of those updates will be incorrect for the slave machine.

> Couldn't this be taken care of by replicating the objects but not any
> data for them? That is, the tables and indexes would exist, but be empty?

Seems a bit pointless. What exactly is the use-case for a slave whose
system catalogs match the master exactly (as they must) but whose data
does not?

Notice also that you have to shove the entire WAL downstream anyway ---
the proposed patch filters at the point of application, and would have a
hard time doing better because LSNs have to remain consistent.

It would also be rather tricky to identify which objects have to have
updates applied, eg, if you replicate a table you'd damn well better
replicate the data for each and every one of its indexes (which is a
non-constant set in general), because queries on the slave will expect
them all to be valid. Maybe it's possible to keep track of that, though
I bet things will be tricky when there are uncommitted DDL changes
(consider data changes associated with a CREATE INDEX on a replicated
table). In any case xlog replay functions are not the place to have
that kind of logic.

regards, tom lane