Subject: Re: Validating check constraints without a table scan?

From: jian he <[email-masked]>
Date: Wed, 20 Nov 2024 17:45:39 +0800

On Fri, Nov 15, 2024 at 4:38 PM Philip Couling <[email-masked]> wrote:
>
> Is there a solid reason why adding a check constraint does not use existing indexes for validation.
>

can you give an sql example (except not-null)
where indexes can be used for check constraint validation?
i am not sure I understand it correctly.

---

From: Philip Couling <[email-masked]>
Date: Wed, 20 Nov 2024 11:16:20 +0000

Jian He

The context here is constraints for partitioning as suggested in
documentation
https://www.postgresql.org/docs/current/ddl-partitioning.html#DDL-PARTITIONING-DECLARATIVE-MAINTENANCE

An example constraint from the documentation:
ALTER TABLE measurement_y2008m02 ADD CONSTRAINT y2008m02
   CHECK ( logdate >= DATE '2008-02-01' AND logdate < DATE '2008-03-01' );

If logdate is indexed, then this constraint can be manually validated very
quickly using a SELECT that will take advantage of the index
SELECT 1 FROM measurement_y2008m02  WHERE logdate < DATE '2008-02-01' OR
logdate >= DATE '2008-03-01' LIMIT 1

If the constraint is valid the query will return quickly with no rows, if
any rows violate the constraint it will also return very quickly but return
with a single row with column value: 1.

I guess that validating constraints doesn't invoke the query planner, or
otherwise the conversion is too complex for the query planner. The
conversion being:

   - from:  NOT (logdate >= DATE '2008-02-01' AND logdate < DATE
   '2008-03-01')
   - to: logdate < DATE '2008-02-01' OR logdate >= DATE '2008-03-01'

Hope that clarifies it.