Xref: utzoo comp.databases:7588 comp.object:1992
Path: utzoo!attcan!uunet!lll-winken!sol.ctr.columbia.edu!samsung!know!sdd.hp.com!ucsd!ucbvax!ucdavis!csusac!unify!dmg
From: dmg@Unify.Com (Dave Glende)
Newsgroups: comp.databases,comp.object
Subject: Re: Predicate Locks
Message-ID: <hd2z4bv@Unify.Com>
Date: 19 Oct 90 18:01:03 GMT
References: <90Oct18.180225edt.12896@neat.cs.toronto.edu>
Organization: Unify Corporation, Sacramento, CA, USA
Lines: 42

In article <90Oct18.180225edt.12896@neat.cs.toronto.edu> vinay@ai.toronto.edu ("Vinay K. Chaudhri") writes:
>
>
>The idea of using predicate locks for database concurrency control
>has been around for over ten years.  But still there are not many
>commercial systems which use predicate locking as a primary method
>for concurrency control.
>
>One reason for this lack of applicability is because it is not 
>computationally inefficient to check overlap between two predicate
>locks.  People have given suggestions for developing some heuristics
>but there does not seem to be any comprehensive approach available.
>
>Another difficulty seems to be in devising an efficient implementation
>technique for predicate lock table.  Maintaining a predicate lock
>table of some reasonable size (say 100) and comparing a new predicate
>against these 100 expressions is prohibitive, leave aside the 
>computational reasons.

Using predicate locks as the sole concurrency control mechanism is, as
you've said, quite expensive.  It would be possible to use them in
small applications, but in a typical commercial situation the cost
becomes too high especially in worse-case scenarios.

However, certain types of predicate locks can be quite useful and still
cost effective.

>
>My question is the following:  Are there really no commercial systems
>that use predicate locking ?  Are the reasons for their in-applicability
>as cited above are correct ?  Are predicate locks dead as a research
>or application area ?
>

UNIFY 2000 uses a combination of predicate locks and row locking
for concurrency control.

-- 

David Glende   Work:(916) 920-9092  | Unify Corporation
dmg@unify.com  FAX :(916) 921-5340  | 3870 Rosin Court 
                                    | Sacto, CA 95834