如何向表添加“CHECK EXISTS”约束？

Question

I'm fairly new to SQL/PSQL, and am familiar so far with adding a constraint to a table that checks that there NOT EXISTS a certain condition. This makes sense to me, as each time a new row is added, it can check whether that row breaks the condition, and continue if it doesn't.

But how would I add a constraint to a table to check that there EXISTS a row that satisfies a condition? At the moment, when the first row is inserted (which doesn't satisfy the condition, but a later one may do) it fails as the condition is not met.

How do I implement this correctly?

I tried writing it exactly like a CHECK NOT EXISTS constraint, but without the NOT. This resulted in the above issue, and I understand why.

A basic idea of what I've tried so far, and is conceptually what I want to work:

CREATE OR REPLACE FUNCTION CheckSomeLargeAttendance()
RETURNS BOOLEAN AS $$
BEGIN
    RETURN(
      EXISTS(
      SELECT 1
      FROM event e
      WHERE e.attending > 100
      )
    );
END;
$$ language plpgsql;

ALTER TABLE event
ADD CONSTRAINT SomeLargeAttendance CHECK(
  CheckSomeLargeAttendance()
);

Ie I want to check that, when all data is added to the table, there is at least one event with an attendance of more than 100.

Answer 1

What you are doing is unsupported, and it will lead to trouble. The documentation says:

PostgreSQL does not support CHECK constraints that reference table data other than the new or updated row being checked. While a CHECK constraint that violates this rule may appear to work in simple tests, it cannot guarantee that the database will not reach a state in which the constraint condition is false (due to subsequent changes of the other row(s) involved). This would cause a database dump and restore to fail. The restore could fail even when the complete database state is consistent with the constraint, due to rows not being loaded in an order that will satisfy the constraint. If possible, use UNIQUE , EXCLUDE , or FOREIGN KEY constraints to express cross-row and cross-table restrictions.

If you dig around in the mailing list archives, you will find reports of such failures, for example here and here .

You could have such a check with a statement level trigger (but don't add the trigger before you insert the first row):

CREATE FUNCTION has_big_event() RETURNS trigger
   LANGUAGE plpgsql AS
$$BEGIN
   IF NOT EXISTS (SELECT FROM event WHERE attending > 100)
   THEN
      RAISE EXCEPTION 'the table does not contain a big enough event';
   END IF;

   RETURN NULL;
END;$$;

CREATE TRIGGER has_big_event BEFORE INSERT OR UPDATE ON event
   FOR EACH STATEMENT EXECUTE FUNCTION has_big_event();