Re: [Proposal] Fully WAL logged CREATE DATABASE - No Checkpoints

Hi,

On 2021-06-17 14:22:52 -0400, Robert Haas wrote:
> On Thu, Jun 17, 2021 at 2:17 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > Adding a hacky special case implementation for cross-database relation
> > accesses that violates all kinds of assumptions (like holding a lock on
> > a relation when accessing it / pinning pages, processing relcache
> > invals, ...) doesn't seem like a good plan.
>
> I agree that we don't want hacky code that violates assumptions, but
> bypassing shared_buffers is a bit hacky, too. Can't we lock the
> relations as we're copying them? We know pg_class's OID a fortiori,
> and we can find out all the other OIDs as we go.

We possibly can - but I'm not sure that won't end up violating some
other assumptions.

> I'm just thinking that the hackiness of going around shared_buffers
> feels irreducible, but maybe the hackiness in the patch is something
> that can be solved with more engineering.

Which bypassing of shared buffers are you talking about here? We'd still
have to solve a subset of the issues around locking (at least on the
source side), but I don't think we need to read pg_class contents to be
able to go through shared_buffers? As I suggested, we can use the init
fork presence to infer relpersistence?

Or do you mean that looking at the filesystem at all is bypassing shared
buffers?

Greetings,

Andres Freund