Hello all,
I have some ideas how to increase expressive power of the PostgreSQL query language. It is not a secret that SQL is
verypoor to express many important queries, and we have to use means of procedural extensions of SQL to realize them.
Howeverthis is not good idea to split query language into two parts (declarative and procedural) at least because query
languagemust finally operate with the objects of data domain, not with the bits the objects consist of. Thus another
alternativeto increase expressive power of query language is to develop its declarative (i.e. nonprocedural) part. And
herewe come to deductive database (DDB) with its logic language Datalog. Every logic query is a set of inverse
implications,which describe what to find (i.e. exactly declarative approach) and not how to find (i.e. exactly
proceduralapproach). We can translate logic query into set of SQL commands and then run them to get result. Some
sampleswith the DLQ compiler can be downloaded from www.datalab.kharkov.ua (DLQ is our original version of Datalog
whichwas developed with the purpose to tie closely RDB and DDB). Now some words about what must be done to realize
describedfeature. The simple quickest way but the way without future is to write language handler. Other more correct
wayis to slightly extend DML part of SQL and more essentially extend DDL. For example, we have relation Inheritance
withtwo attributes ClassID and ParentID. Now we want to define all descendants or all ancestors. For this goal we
definepredicate inheritance_all with the next two rules (i.e. inverse implications):
inheritance_all(ClassID, ParentID) :- inheritance(ClassID, ParentID); inheritance_all(ClassID, ParentID) :-
inheritance(ClassID,X), inheritance_all(X, ParentID).
We put this rules into database and call, for example, the next SQL commands:
-- find all descendents SELECT * FROM ddb_name.inheritance_all(_, _)
-- find all descendents from ParentID = 1 SELECT * FROM ddb_name.inheritance_all(_, 1)
where ddb_name is the name of deductive database where our rules are kept, _" designates anonymous variable (see
Prolognotation for details).
Regards, Dmitriy
