[Zope] [Q] Design decisions: the use of ZCatalogs versus a backend DB?

[Kevin Dangoor]

----- Original Message -----
From: "Michel Pelletier" <michel@digicool.com>
To: "'Darran Edmundson'" <Darran.Edmundson@anu.edu.au>; "zope"
<zope@zope.org>
Sent: Monday, January 24, 2000 11:23 AM
Subject: RE: [Zope] [Q] Design decisions: the use of ZCatalogs versus a
backend DB?


> > -----Original Message-----
> > From: Darran Edmundson [mailto:Darran.Edmundson@anu.edu.au]
> >
> > It seems that ZCatalogs are meant to be used if (i) only
> > a fraction of the instance data needs to be indexed, or
> > (ii) the dataset is small enough that one can live with
> > the duplication.
>
> I'm not sure I understand, indexing requires duplicate data of some
> kind.  And it's not one to one duplication, the catalog goes to great
> lengths to store that information efficiently.

I think what Darran is asking here is actually with regards to the metadata
table.

Darran, you actually don't *have* to duplicate things in the catalog via the
metadata table if you don't want to. (As Michel points out, you will have
some duplication of data for the index itself, but an RDBMS would have the
same issue).

The metadata table allows very fast, efficient access to select pieces of
information about the cataloged object. This is good, because you don't have
to "wake up" all of the objects in the search (possibly filling up Zope's
cache). However, if you need to access all of the information in the object,
you may as well leave the metadata empty and just go ahead and get the
information directly from the object. (I don't remember off the top of my
head which method you use to get at the object that is pointed to by the
catalog).

Thus, the only duplication is the index itself, which is no more overhead
than you're likely to get out of an RDBMS.

Kevin