On 03.04.24 07:30, Paul Jungwirth wrote:
But is it *literally* unique? Well two identical keys, e.g. (5,
'[Jan24,Mar24)') and (5, '[Jan24,Mar24)'), do have overlapping ranges,
so the second is excluded. Normally a temporal unique index is *more*
restrictive than a standard one, since it forbids other values too (e.g.
(5, '[Jan24,Feb24)')). But sadly there is one exception: the ranges in
these keys do not overlap: (5, 'empty'), (5, 'empty'). With
ranges/multiranges, `'empty' && x` is false for all x. You can add that
key as many times as you like, despite a PK/UQ constraint:
postgres=# insert into t values
('[1,2)', 'empty', 'foo'),
('[1,2)', 'empty', 'bar');
INSERT 0 2
postgres=# select * from t;
id | valid_at | name
-------+----------+------
[1,2) | empty | foo
[1,2) | empty | bar
(2 rows)
Cases like this shouldn't actually happen for temporal tables, since
empty is not a meaningful value. An UPDATE/DELETE FOR PORTION OF would
never cause an empty. But we should still make sure they don't cause
problems.
We should give temporal primary keys an internal CHECK constraint saying
`NOT isempty(valid_at)`. The problem is analogous to NULLs in parts of a
primary key. NULLs prevent two identical keys from ever comparing as
equal. And just as a regular primary key cannot contain NULLs, so a
temporal primary key should not contain empties.
The standard effectively prevents this with PERIODs, because a PERIOD
adds a constraint saying start < end. But our ranges enforce only start
<= end. If you say `int4range(4,4)` you get `empty`. If we constrain
primary keys as I'm suggesting, then they are literally unique, and
indisunique seems safer.
Should we add the same CHECK constraint to temporal UNIQUE indexes? I'm
inclined toward no, just as we don't forbid NULLs in parts of a UNIQUE
key. We should try to pick what gives users more options, when possible.
Even if it is questionably meaningful, I can see use cases for allowing
empty ranges in a temporal table. For example it lets you "disable" a
row, preserving its values but marking it as never true.
It looks like we missed some of these fundamental design questions early
on, and it might be too late now to fix them for PG17.
For example, the discussion on unique constraints misses that the
question of null values in unique constraints itself is controversial
and that there is now a way to change the behavior. So I imagine there
is also a selection of possible behaviors you might want for empty
ranges. Intuitively, I don't think empty ranges are sensible for
temporal unique constraints. But anyway, it's a bit late now to be
discussing this.
I'm also concerned that if ranges have this fundamental incompatibility
with periods, then the plan to eventually evolve this patch set to
support standard periods will also have as-yet-unknown problems.
Some of these issues might be design flaws in the underlying mechanisms,
like range types and exclusion constraints. Like, if you're supposed to
use this for scheduling but you can use empty ranges to bypass exclusion
constraints, how is one supposed to use this? Yes, a check constraint
using isempty() might be the right answer. But I don't see this
documented anywhere.
On the technical side, adding an implicit check constraint as part of a
primary key constraint is quite a difficult implementation task, as I
think you are discovering. I'm just reminded about how the patch for
catalogued not-null constraints struggled with linking these not-null
constraints to primary keys correctly. This sounds a bit similar.
I'm afraid that these issues cannot be resolved in good time for this
release, so we should revert this patch set for now.
