Re: SQL:2011 application time - Mailing list pgsql-hackers

On Wed, Jun 5, 2024 at 4:56 PM Paul Jungwirth
<pj@illuminatedcomputing.com> wrote:
> **Option 2**: Add a new operator, called &&&, that works like && except an empty range *does*
> overlap another empty range. Empty ranges should still not overlap anything else. This would fix the
> exclusion constraint. You could add `(5, 'empty')` once but not twice. This would allow empties to
> people who want to use them. (We would still forbid them if you define a PERIOD, because those come
> with the CHECK constraint mentioned above.)
> And there is almost nothing to code. But it is mathematically suspect to say an empty range overlaps
> something small (something with zero width) but not something big. Surely if a && b and b <@ c, then
> a && c? So this feels like the kind of elegant hack that you eventually regret.

I think this might be fine.

> **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.

> **Option 4**: Teach GiST indexes to enforce uniqueness. We didn't discuss this at pgconf, at least
> not in reference to the empties problem. But I was thinking about this request from Matthias for
> temporal PKs & UQs to support `USING INDEX idx`.[2] It is confusing that a temporal index has
> indisunique, but if you try to create a unique GiST index directly we say they don't support UNIQUE
> indexes! Similarly `pg_indexam_has_property(783, 'can_unique')` returns false. There is something
> muddled about all that. So how about we give the GiST AM handler amcanunique?
>
> As I understand it, GiST indexes are capable of uniqueness,[3] and indeed today you can create an
> exclusion constraint with the same effect, but in the past the core had no way of asking an opclass
> which operator gave equality. With the stratnum support proc from 6db4598fcb (part of this patch
> series, but reverted from v17), we could get a known operator for "equals". If the index's opclasses
> had that sproc and it gave non-zero for RTEqualStrategyNumber, then CREATE UNIQUE INDEX would
> succeed. We would just ("just") need to make GiST raise an error if it found a duplicate. And if
> *that* was happening, the empty ranges wouldn't cause a problem.

Isn't this just a more hacky version of option (2)?

--
Robert Haas
EDB: http://www.enterprisedb.com