One of the things that I’ve been seeing more often than I would like to lately is large databases with no foreign keys – or minimal foreign keys (and often with those minimal set of keys disabled)
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!
My first though here is “I’m glad I don’t use this company” – and I also wonder why auditing or testing has not picked it up. Quite often it has but somebody, somewhere has decided that there’s a reasonably low chance of that occurring and so no action has been taken – until the ‘bug’ is discovered sometime later and at that point takes a long time – and a lot of money – to fix.
Points 1 and 2 are quoted more often than I would like also – usually stemming from statements that were made by somebody who worked with a long dead version of the database and didn’t quite take the time to understand fully how relational databases work – but who had tremendous influence in the company and a reputation that meant they were not to be crossed. (they have since been proved wrong but have failed to accept it)
These are simply examples of now bad culture can lead to problems years down the road.
Looking at a lot of database that lacked foreign keys, it’s usually pretty simply to find examples of inconsistent data in the tables. – caused by the three issues above.
The moral of the story here is
Always question, always learn and really try to avoid issues that are bound to come back and haunt you.