James,
Are you sure that a new type of constraint is the way to go for this?
It doesn't solve our issues in the data warehousing space. The spec we
started with for "Error-tolerant COPY" is:
1) It must be able to handle parsing errors (i.e. bad char set);
2) It must be able to handle constraint violations;
3) It must output all row errors to a log or "errors" table which makes
it possible to determine which input row failed and why;
4) It must not slow significantly (like, not more than 15%) the speed of
bulk loading.
On that basis, Alon started working on a low-level error trapper for
COPY. It seems like your idea, which would involve a second constraint
check, would achieve neigher #1 nor #4.
--Josh Berkus