The Free On-line Dictionary of Computing (30 December 2018):

functional database

    A database which uses a functional
   language as its query language.

   Databases would seem to be an inappropriate application for
   functional languages since, a purely functional language
   would have to return a new copy of the entire database every
   time (part of) it was updated.  To be practically scalable,
   the update mechanism must clearly be destructive rather than
   functional; however it is quite feasible for the query
   language to be purely functional so long as the database is
   considered as an argument.

   One approach to the update problem would use a monad to
   encapsulate database access and ensure it was single
   threaded.  Alternative approaches have been suggested by
   Trinder, who suggests non-destructive updating with shared
   data structures, and Sutton who uses a variant of a Phil
   Wadler's linear type system.

   There are two main classes of functional database languages.
   The first is based upon Backus' FP language, of which
   FQL is probably the best known example.  Adaplan is a more
   recent language which falls into this category.

   More recently, people have been working on languages which are
   syntactically very similar to modern functional programming
   languages, but which also provide all of the features of a
   database language, e.g. bulk data structures which can be
   incrementally updated, type systems which can be incrementally
   updated, and all data persisting in a database.  Examples are
   PFL [Poulovassilis&Small, VLDB-91], and Machiavelli [Ohori
   et al, ACM SIGMOD Conference, 1998].

   Query optimisation is very important for database languages
   in general and the referential transparency of functional
   languages allows optimisations which would be harder to verify
   in presence of side-effects.

   [Trinder, P., "Referentially transparent database languages",
   1989 Glasgow Workshop on Functional programming]

   [Breazu-Tannen et al., DBPL-91].

   [Poulovassilis, VLDB-94].

   (1995-05-09)