[hu3]

Correct me if I'm wrong but, FKs are rarely created for existing columns.

You usually create the column and the FK in the same script. And usually starting with a NULL value for existing rows.

And if it's a new table then there's no rows anyway.

So the most common operations when creating FK's aren't expensive as far as I know.

You know what's expensive? Creating an index on a large table because you or your RDMS forgot to create the index when the FK was created and now JOINS are crawling to halt.

[stouset]

FK indexes by necessity need to be placed on the foreign table, which is just as likely to be a preexisting table that already contains data.

To be clear I 100% agree that adding indexes later is extremely painful. A little care when first creating tables goes a long way, and I’ve never seen a database fall over due to preemptive over-indexing but I’ve seen countless do so thanks to being underindexed.

Still, taking a DDL operation which is presumed to be essentially instantaneous and adding a default behavior that requires locking completely separate tables for a potentially-lengthy update does give me pause.