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Thread: Request for Implementation of Custom Error Messages for CHECK Constraints

Request for Implementation of Custom Error Messages for CHECK Constraints

From
"Miguel Ferreira"
Date:

PostgreSQL Project Leaders,

 

Currently, when a CHECK constraint is violated, PostgreSQL returns a generic error message that includes the constraint name. While informative for developers familiar with the database schema, this message can be less clear for other team members, end-users, or in application logs. This lack of specificity hinders the quick identification of the exact business rule that has been violated and can lead to more time-consuming debugging and less user-friendly error messages in applications.

 

Proposal:

 

I propose extending the syntax of the ALTER TABLE ADD CONSTRAINT statement (and potentially CREATE TABLE) to allow for the specification of a custom error message for each CHECK constraint. A possible syntax could be as follows:

 

ALTER TABLE table_name

ADD CONSTRAINT constraint_name

CHECK (condition)

MESSAGE 'Custom error message when the condition is not met.';

 

Benefits of Implementation:

 

    Improved User Experience: Applications could capture and display more contextual and helpful error messages to end-users, improving usability and reducing confusion.

    Enhanced Debugging: Developers could immediately identify the specific business rule that has been violated, speeding up the debugging and resolution of data integrity issues.

    Implicit Documentation: The custom message would serve as a way to document the intent of the constraint directly within the database schema, facilitating understanding and maintenance of the data model.

    Consistency: It would allow for a more consistent approach to providing informative feedback on business rule violations, complementing the existing capability in triggers.

 

 

 

Best regards,

 

 

Re: Request for Implementation of Custom Error Messages for CHECK Constraints

From
Marcos Pegoraro
Date:
Em sáb., 10 de mai. de 2025 às 21:57, David G. Johnston <david.g.johnston@gmail.com> escreveu:
Constraints can be targeted by “comment on”.

So, why not using that COMMENT ON message to raise when that constraint gets violated ?
A GUC like Constraint_Exception_Returns_MessageOn = {Never | Always | If_Exists}
with its default value set to Never, so it runs like today, but if changed to If_Exists it will try to 
get that message or always, to show that COMMENT ON, even empty.

alter table b add constraint fk_b_a foreign key(ID) references a(ID);
comment on constraint fk_b_a on b is 'There is a problem on Foreign Key on Table B related to Table A';

regards
Marcos
 

Re: Request for Implementation of Custom Error Messages for CHECK Constraints

From
Bruce Momjian
Date:
On Sat, May 10, 2025 at 07:58:47PM -0400, Tom Lane wrote:
> "Miguel Ferreira" <miguelmbferreira@gmail.com> writes:
> > I propose extending the syntax of the ALTER TABLE ADD CONSTRAINT
> > statement (and potentially CREATE TABLE) to allow for the
> > specification of a custom error message for each CHECK constraint.
> 
> Why don't you just choose better names for your constraints?
> 
> I'd argue that the proposed change might actually be a net loss
> for usability, if it entirely obscures the fact that what happened
> was a check-constraint violation.
> 
> It's also not very clear why we'd stop with check constraints,
> if the desire is to get rid of database-produced error messages
> in favor of something that somebody likes better.

Yeah, you could name the constraint
"Custom_error_message_when_the_condition_is_not_met." and then just
convert underscore to spaces and display that to the user.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  Do not let urgent matters crowd out time for investment in the future.

Re: Request for Implementation of Custom Error Messages for CHECK Constraints

From
"David G. Johnston"
Date:
On Monday, May 12, 2025, Bruce Momjian <bruce@momjian.us> wrote:

Yeah, you could name the constraint
"Custom_error_message_when_the_condition_is_not_met." and then just
convert underscore to spaces and display that to the user.


 The 63 byte limit seems much more likely to be a factor if the name has to serve the duty of a human-friendly error message.

David J.