Horton

Re: SQL:2011 application time

Peter Eisentraut <peter@eisentraut.org>

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From: Peter Eisentraut <peter@eisentraut.org>
To: Paul Jungwirth <pj@illuminatedcomputing.com>, jian he <jian.universality@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2024-05-13T10:11:11Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Rename gist stratnum support function

  2. Remove support for temporal RESTRICT foreign keys

  3. Cache NO ACTION foreign keys separately from RESTRICT foreign keys

  4. Fix NO ACTION temporal foreign keys when the referenced endpoints change

  5. Improve whitespace in without_overlaps test

  6. Tests for logical replication with temporal keys

  7. Support for GiST in get_equal_strategy_number()

  8. Make the conditions in IsIndexUsableForReplicaIdentityFull() more explicit

  9. Replace get_equal_strategy_number_for_am() by get_equal_strategy_number()

  10. Improve internal logical replication error for missing equality strategy

  11. Simplify IsIndexUsableForReplicaIdentityFull()

  12. Fix ALTER TABLE / REPLICA IDENTITY for temporal tables

  13. doc: Update pg_constraint.conexclop docs for WITHOUT OVERLAPS

  14. doc: Add PERIOD to ALTER TABLE reference docs

  15. doc: Add WITHOUT OVERLAPS to ALTER TABLE reference docs

  16. Add temporal FOREIGN KEY contraints

  17. Add temporal PRIMARY KEY and UNIQUE constraints

  18. Add stratnum GiST support function

  19. Avoid crashing when a JIT-inlined backend function throws an error.

  20. Revert temporal primary keys and foreign keys

  21. Fix ON CONFLICT DO NOTHING/UPDATE for temporal indexes

  22. Add test for REPLICA IDENTITY with a temporal key

  23. Use half-open interval notation in without_overlaps tests

  24. Use daterange and YMD in without_overlaps tests instead of tsrange.

  25. Rename pg_constraint.conwithoutoverlaps to conperiod

  26. Fix comment on gist_stratnum_btree

  27. Add missing TAP test name

  28. Improve error handling of HMAC computations

  29. Rename functions to avoid future conflicts

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.