On Wed, Aug 12, 2020 at 1:08 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Amit Langote <amitlangote09@gmail.com> writes:
> > On Wed, Aug 12, 2020 at 3:02 AM Andres Freund <andres@anarazel.de> wrote:
> >> Medium term I think we should just plain out forbid references to system
> >> columns in partioned tables Or at least insist that all partitions have
> >> that column.
>
> > Performance-wise I would prefer the former, because the latter would
> > involve checking *all* partitions statically in the INSERT case,
> > something that we've avoided doing so far.
>
> It's not like we don't have a technology for doing that. The way this
> ideally would work, IMV, is that the parent partitioned table either
> has or doesn't have a given system column. If it does, then every
> child must too, just like the way things work for user columns.
Ah, I may have misread "insisting that all partitions have a given
system column" as doing that on every query, but maybe Andres meant
what you are describing here.
> This'd require (a) some sort of consensus about which kinds of system
> columns can make sense --- as Andres noted, 32-bit xmin might not be
> the best choice here --- and (b) some notation for users to declare
> which of these columns they want in a partitioned table. Once upon
> a time we had WITH OIDS, maybe that idea could be extended.
For (a), isn't there already a consensus that all table AMs support at
least the set of system columns described in 5.5 System Columns [1]
even if the individual members of that set are no longer the best
choice at this point? I do agree that we'd need (b) in some form to
require AMs to fill those columns which it seems is not the case
currently.
--
Amit Langote
EnterpriseDB: http://www.enterprisedb.com
[1] https://www.postgresql.org/docs/current/ddl-system-columns.html
