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I have an already existing database with tables. I added foreign key relationships (because they were referring data from another table, just that relationship was not explicit in the way tables were created) for one of the tables.
How does this change impact the existing database? Does the database engine have to do some extra work on existing data in the database? Can this change be a “breaking change” if you already have an application that uses the current database schema?
If you added a referential constraint, then the database stores that constraint and ensures it is maintained. For example, if table A has a foreign key referring to table B, then you cannot insert a row into table A that refers to a key that does not exist in table B.