Foreign Key Constraint into an different database An SQL server hosts all the data for multiple entities (e.g sub-companies or organizational units OU).
An &quot;enterprise&quot; database holds the &quot;enterprise&quot; data such as configuration data, data about processes that are enterprise-wide ones, and at least one lookup table which stores the data about all the OUs.
The initial design called for the option to be able to split the database of some or all OUs into their own database server that can even physically reside in a different location. So far, both databases reside on the same SQL server.
Every table has a column identifying the id of the OU (OUId) as an FK for OU information and will enable to make the split in the future.
Would you replicate the OU table instead of setting an FK to an external database? How much performance hit an FK to an external database can cause?
Thanks
Would you replicate the OU table ...?

Yes.  Because local lookups of master/reference data are important for application performance, reliability, and to enforce referential integrity because this:

an FK to an external database

is not possible.