By large databases I’m roughly meaning databases with several hundred tables, and I usually see a lot of these tables with several hundred GB’s of data in them.
When I generally ask about the reason for no foreign key, I’m told
- they add overhead
- they give no benefit
- we can’t enter our data properly when we have them
The last one in the above list is generally down to poor modelling – an example being entering a later part of a financial transaction before the first part of the financial transaction has taken place. Once both parts of the financial transaction have taken place then the database is left in a consistent state – BUT, that generally being handled by the app NOT the database – OUCH!
There are times where key constraints are too much—often-updating fact tables might be one such scenario. And some of “too much” comes down to hardware quality. But for the most part, those key constraints are one of the clearest forms of database documentation available, not to mention their normal benefits.