Re: AS OF queries

On 24 December 2017 at 04:53, konstantin knizhnik <k(dot)knizhnik(at)postgrespro(dot)ru
> wrote:

>
>
> But what if I just forbid to change recent_global_xmin?
> If it is stalled at FirstNormalTransactionId and never changed?
> Will it protect all versions from been deleted?
>

That's totally impractical, you'd have unbounded bloat and a nonfunctional
system in no time.

You'd need a mechanism - akin to what we have with replication slots - to
set a threshold for age.

> Then there's another issue that logical replication has had to deal
> > with -- catalog changes. You can't start looking at tuples that have a
> > different structure than the current catalog unless you can figure out
> > how to use the logical replication infrastructure to use the old
> > catalogs. That's a huge problem to bite off and probably can just be
> > left for another day if you can find a way to reliably detect the
> > problem and raise an error if the schema is inconsistent.
>
>
> Yes, catalog changes this is another problem of time travel.
> I do not know any suitable way to handle several different catalog
> snapshots in one query.
>

I doubt it's practical unless you can extract it to subplans that can be
materialized separately. Even then, UDTs, rowtype results, etc...

> The question is how we should handle such catalog changes if them are
> happen. Ideally we should not allow to move back beyond this point.
> Unfortunately it is not so easy to implement.
>

I think you can learn a lot from studying logical decoding here.

--
Craig Ringer http://www.2ndQuadrant.com/
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