Re: Validating check constraints without a table scan?

[Philip Couling <couling@xxxxxxxxx>]

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.

On Wed, 20 Nov 2024 at 09:45, jian he <jian.universality@xxxxxxxxx> wrote:

On Fri, Nov 15, 2024 at 4:38 PM Philip Couling <couling@xxxxxxxxx> 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.