Databases on CS Theorems

Codd's Theorem

Fri, 05 Nov 2027 12:00:00 +0100

Symptom

You write:

SELECT name FROM users WHERE age > 30 AND city = 'Berlin';

You did not say whether to scan the table or use an index. You did not say which of the two predicates to apply first. You did not say anything about join order, because there is no join, but if there were you would not have said that either.

Paxos, Raft, and the Shape of Consensus

Fri, 20 Aug 2027 12:00:00 +0100

Symptom

Your database has two nodes that both believe they are the primary.

Each has been accepting writes. Each has a coherent log. Neither has crashed. The old primary was declared dead by a health check that timed out during a GC pause, a new one was elected, and then the old one woke up still holding what it thinks is a valid lease. You now have two divergent histories of the same data and no principled way to merge them, because both contain acknowledged writes that customers were told had succeeded.

Quorum Intersection (R + W > N)

Fri, 13 Aug 2027 12:00:00 +0100

Symptom

You have three replicas of a value. A client writes v2 and gets an acknowledgement. Another client reads and gets v1.

Nothing crashed. No network partition healed badly. Both operations completed successfully and reported success. The write went to one replica, the read came from another, and the two never spoke.