> Hi Peter, > > I understand what you're pointing at and I agree that it could be a good > thing to be able to dump/restore a table without problem. > > My point was that check constraints weren't supposed to be used that way > theorically (or maybe i'm mistaken ?) so I thought maybe we should just > inform the user that this kind of use of a check constraint is a misuse of > that feature.
Tom Lane pointed out in another thread that the SQL standard lists feature F673 "Reads SQL-data routine invocations in CHECK constraints" which permits CHECK constraints to examine tables, so saying "you're not supposed to do this", while correct from a Postgres perspective, would be short-sighted ISTM, because we will make ourselves liars as soon as we implement the feature.
I agree that we should point this out in *some* way, just not sure how. Maybe something like "Postgres does not currently support CHECK constraints containing queries, therefore we recommend to avoid them." I would not mention pg_dump by name, just say dumps may not restore depending on phase of moon.
(BTW I'm not sure of the term "other tables". You could have a query that references the same table ...) -- Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
I like this:
> "Postgres does not currently support CHECK constraints containing queries, therefore we recommend to avoid them."
Perhaps adding:
> CHECK constraints are currently meant to be used as row constraints only.
> Use - if possible - UNIQUE or EXCLUDE constraints. for constraints that involve many or all rows of a table,
> and FOREIGN KEY constraints for cross table constraints.
> More complex constraints will be available when ASSERTION are implemented.
And then adding some warning about using functions in CHECK constraints to bypass current limitations.