Schema Evolution and Foreign Keys: Birth, Eviction, Change and Absence

Abstract

In this paper, we focus on the study of the evolution of foreign keys in the broader context of schema evolution for relational databases. Specifically, we study the schema histories of a six free, open-source databases that contained foreign keys. Our findings concerning the growth of tables verify previous results that schemata grow in the long run in terms of tables. Moreover, we have come to several surprising, new findings in terms of foreign keys. Foreign keys appear to be fairly scarce in the projects that we have studied and they do not necessarily grow in sync with table growth. In fact, we have observed different cultures for the handling of foreign keys, ranging from treating foreign keys as an indispensable part of the schema, in full sync with the growth of tables, to the unexpected extreme of treating foreign keys as an optional add-on that twice resulted in their full removal from the schema of the database.