ALTER TABLE ... ADD FOREIGN KEY ... NOT ENFORCED

The new SQL Standard (SQL:2011) contains this:
"Table constraints are either enforced or not enforced. Domain
constraints and assertions are always enforced.", 4.17.2

The SQL Standard allows you to turn the checking on and off for CHECK
constraints, UNIQUE constraints and FOREIGN KEYS.

Which of those make sense for us, if any? The ability to create FKs
without checking all the data has been frequently requested to me over
many years. OTOH, I can't really see any point in turning on/off all of
the other aspects mentioned by the SQL Standard, especially indexes.
It's lots of work and seems likely to end with poorer data quality. And
the obvious thing is if you don't want a CHECK constraint, just drop
it...

My proposal is that we add a short and simple clause NOT ENFORCED onto
the ADD constraint syntax. So we have

ALTER TABLE foo
    ADD FOREIGN KEY .... NOT ENFORCED;

The "enforced" state is not persisted - once added the FK is checked
every time. So there is no additional column on pg_constraint.

The benefit here is that we implement a capability that allows skipping
very long running SQL statements when required, and doesn't require too
much code. It has been discussed before on hackers, but that was before
it was part of the SQL Standard. Oracle has had this for years and it is
popular feature. We can expect other RDBMS to implement this feature,
now it is part of the standard.

If you want more than my good-bits-only proposal, it really isn't going
to happen for 9.1, and seems pretty pointless anyway.

Very short hack to implement this attached for discussion. No tests, not
even a compile - just showing how quick a patch this can be.

Thoughts? Alternative syntax?

--
 Simon Riggs           http://www.2ndQuadrant.com/books/
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