This is a really interesting message found on the Byte magazine discussions for anyone who ever thought about implimenting a Postgres storage for Zope. -Michel robin <robin@nospam.org> wrote in message news:<7u0kr7$st4$1@cmpweb-media0.web.cerf.net>...

Jon Udell <udell@monad.net> wrote in message news:380367CF.7AAB0D90@monad.net...

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What features does DB2 have which make it an ODBMS?

I'm not sure.

But...can you tell us more about the object features of PostgreSQL,

and

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how you're using them?

Jon Udell

PostgreSQL adds an "inherits" clause to the CREATE TABLE statement, so an unimaginative example would be:

create table people ( forename text, surname text, initials text, email text );

create table staff ( office text, extension text ) inherits (people);

create table clients ( organisation oid ) inherits (people);

This would create three tables, people, with fields; forename, surname, initials, email, staff; with fields forename, surname, initials, email, office and extension, and clients; with fields forename, surname, initials, email and organisation.

Queries can then be made on any of the tables individually in the normal way, but also using an extension syntax:

select * from table people*;

Which would query table people, and all tables which inherit from it. In fact the Postgres documentation makes reference to "classes" instead of tables. Clearly a query such as this can only return the fields which are common to all of the tables in the hierarchy, i.e. the fields of the base class.

I have used this feature with Perl to implement a simple object persistence veneer above DBI. I create a base class called object to start the hierarchy;

create table object (class text default 'object');

and then further classes are like this;

create table people (class text default 'people', forename text, surname text, initials text, email text );

etc.

The "class text default '<class name>'" field results in every row having the name of the table it was entered into associated with it. PostgreSQL also assigns a unique OID to every row in the database, so using the people example, I can do "select *,oid from people*", retrieve the results using fetchrow_hashref and then bless the rows with the contents of their own class field.

The result is a list of perl objects, each of the appropriate type. The perl classes can in turn inherit from a base dbobject class which can generate the correct insert code automatically.

This is how I handled object persistence in a number of web based applications for written for clients of my previous employer. I've changed jobs, and expect to be working in Java, probably with DB2 and Websphere. I know little about DB2, except that it's been called an "ORDBMS", and I wondered what the term is referring to.

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-Robin Barooah robin(at)sublime.org