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Re: Validating check constraints without a table scan?
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* Re: Validating check constraints without a table scan?
@ 2024-11-20 09:45 jian he <[email protected]>
  2024-11-20 11:16 ` Re: Validating check constraints without a table scan? Philip Couling <[email protected]>
  0 siblings, 1 reply; 2+ messages in thread

From: jian he @ 2024-11-20 09:45 UTC (permalink / raw)
  To: Philip Couling <[email protected]>; +Cc: [email protected]

On Fri, Nov 15, 2024 at 4:38 PM Philip Couling <[email protected]> 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.






^ permalink  raw  reply  [nested|flat] 2+ messages in thread

* Re: Validating check constraints without a table scan?
  2024-11-20 09:45 Re: Validating check constraints without a table scan? jian he <[email protected]>
@ 2024-11-20 11:16 ` Philip Couling <[email protected]>
  0 siblings, 0 replies; 2+ messages in thread

From: Philip Couling @ 2024-11-20 11:16 UTC (permalink / raw)
  To: jian he <[email protected]>; +Cc: [email protected]

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-MAINTENAN...

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 <[email protected]> wrote:

> On Fri, Nov 15, 2024 at 4:38 PM Philip Couling <[email protected]> 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.
>


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