functional database
(foldoc) | 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)
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