Re: On what OLAP can and what OLAP can't - A little problem

From: Jan Hidders <hidders_at_gmail.com>
Date: 13 Sep 2006 00:55:42 -0700
Message-ID: <1158134142.648839.243660_at_e3g2000cwe.googlegroups.com>


pamelafluente_at_libero.it schreef:

>
> Make it simple. Assume you have a few tables. Assume that there are a
> few relationships and that for instance if you have 2 table A, B in
> 1-N relationship, you
> have on table A defined some function that is, say, "replication
> sensitive", such as count (not count distinct) or sum. When whe make
> some join of these tables and compute such functions we obtain an
> incorrect calculation of the functions due to record replication.
> Leader softwares such as Business Objects are able to devise some union
> of subquery to avoid that replication problem. Experiment show that
> such software are usually able to deal with that if functions are on
> "fact" table (say on the N side of the relationship), but they seem to
> have big problems when the functions are applied on the dimension table
> (say on side 1 of relationship).

What do you mean by "algorithm"? Do you want the SQL expression or a program that would implement that expression?

For now, if I understand your question correctly, it seems the magic word you are looking for is "semijoin". There are several algorithms for that operator and the choice depends on several factors such as the presence of indexes, selectivity of join conditions,sizes of buffers, sizes of the involved tables, et cetera.

  • Jan Hidders
Received on Wed Sep 13 2006 - 09:55:42 CEST

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