[laserDinosaur]

>And they managed it just fine despite working with auto-incrementing big ints.

I wonder how. I've had to do several big merges in my career, and it was always a nightmare because of all the external systems which were already referencing and storing those pre-existing ints. Sure, merging the databases is easy if you don't mind regenerating all the Id's, but it's not usually that simple.

[grey-area]

Simplest way is to keep the identifiers from DB A and increment all the identifiers from DB B by an offset. Third parties complicates things of course but internally it can be pretty simple, so maybe they just didn't have too many third parties using the IDs.

[hu3]

That was it if I recall.

They wrote a small script with the logic involved in the merging. PKs and FKs of only one database had to be incremented by an offset of max(table.pk) + safe margin.

They did this for each table.

Once this script was tested multiple times with subsets of each database, they stopped production and ran the script against it (with backup fallbacks). A small downtime window in a Sunday.

And that was it. The databases never had to pay the UUID tax, before or after.