On 6/12/24 07:31, Robert Haas wrote:
> On Wed, Jun 5, 2024 at 4:56 PM Paul Jungwirth
>> **Option 3**: Forbid empties, not as a reified CHECK constraint, but just with some code in the
>> executor. Again we could do just PKs or PKs and UNIQUEs. Let's do both, for all the reasons above.
>> Not creating a CHECK constraint is much less clunky. There is no catalog entry to create/drop. Users
>> don't wonder where it came from when they say `\d t`. It can't conflict with constraints of their
>> own. We would enforce this in ExecConstraints, where we enforce NOT NULL and CHECK constraints, for
>> any table with constraints where conperiod is true. We'd also need to do this check on existing rows
>> when you create a temporal PK/UQ. This option also requires a new field in pg_class: just as we have
>> relchecks, relhasrules, relhastriggers, etc. to let us skip work in the relcache, I assume we'd want
>> relperiods.
>
> I don't really like the existing relhasWHATEVER fields and am not very
> keen about adding more of them. Maybe it will turn out to be the best
> way, but finding the right times to set and unset such fields has been
> challenging over the years, and we've had to fix some bugs. So, if you
> go this route, I recommend looking carefully at whether there's a
> reasonable way to avoid the need for such a field. Other than that,
> this idea seems reasonable.
Here is a reworked patch series following Option 3: rather than using a cataloged CHECK constraint,
we just do the check in the executor (but in the same place we do CHECK constraints). We also make
sure existing rows are empty-free when you add the index.
I took the reverted commits from v17, squashed the minor fixes, rebased everything, and added a new
patch to forbid empty ranges/multiranges wherever there is a WITHOUT OVERLAPS constraint. It comes
right after the PK patch in the series. I don't intend it to be committed separately, but I thought
it would make review easier, since the other code has been reviewed a lot already.
I did add a relperiods column, but I have a mostly-complete branch here (not included in the
patches) that does without. Not maintaining that new column is simpler for sure. The consequence is
that the relcache must scan for WITHOUT OVERLAPS constraints on every table. That seems like a high
performance cost for a feature most databases won't use. Since we try hard to avoid that kind of
thing (e.g. [1]), I thought adding relperiods would be preferred. If that's the wrong tradeoff I can
change it.
One idea I considered was to include WITHOUT OVERLAPS constraints in the relchecks count. But that
feels pretty hacky, and it is harder than it sounds, since index constraints are handled pretty far
from where we update relchecks now. It doesn't save any complexity (but rather makes it worse), so
the only reason to do it would be to avoid expanding pg_class records.
These patches still add some if-clauses to psql and pg_dump that say `if (fout->remoteVersion >=
170000)`. But if I change them to 180000 I get failures in e.g. the pg_dump tests. What do other
people do here before a release is cut?
Rebased on 3e53492aa7.
[1]
https://github.com/postgres/postgres/blob/5d6c64d290978dab76c00460ba809156874be035/src/backend/utils/cache/relcache.c#L688-L713
Yours,
--
Paul ~{:-)
pj@illuminatedcomputing.com
