Обсуждение: postgreSQL as deductive DBMS
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
Having written my thesis about deductive DBS I cannot resist giving my 2 cent. On Mon, May 16, 2005 at 01:42:24PM +0400, Dmitriy Letuchy wrote: > Now some words about what must be done to realize described feature. The simple quickest way but the way without futureis to write language handler. Other more correct way is to slightly extend DML part of SQL and more essentially extendDDL. For example, we have relation Inheritance with two attributes ClassID and ParentID. Now we want to define alldescendants or all ancestors. For this goal we define predicate inheritance_all with the next two rules (i.e. inverseimplications): > > 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(_, _) How do you plan to execute this statement. As you mentioned above all logic queries can be rewritten into SQL ones, but you have to find a way to handle recursion. I would think the best way is to add recursion to SQL and then completely rewrite the statements. Also, and that's where it starts to become interesting, how do you plan to handle negation inside recursion? Michael -- Michael Meskes Email: Michael at Fam-Meskes dot De, Michael at Meskes dot (De|Com|Net|Org) ICQ: 179140304, AIM/Yahoo: michaelmeskes, Jabber: meskes@jabber.org Go SF 49ers! Go Rhein Fire! Use Debian GNU/Linux! Use PostgreSQL!
Dimitry, > Thus another alternative to increase expressive power of query language is > to develop its declarative (i.e. nonprocedural) part. And here we come to > deductive database (DDB) with its logic language Datalog. You may want to look at the work of Rada Chirkova, who has already written a PostgreSQL-parse-tree-to-DataLog converter: http://research.csc.ncsu.edu/selftune/Report_031005.pdf -- Josh Berkus Aglio Database Solutions San Francisco