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Re: Clusters physical representation

From: <markp7832_at_my-deja.com>
Date: Fri, 24 Dec 1999 20:02:11 GMT
Message-ID: <840jbe$li9$1@nnrp1.deja.com>


In article <OtL84.113$uG.4165_at_news.siol.net>,   "Miha Žnidaršič" <miha.znidarsic_at_ixtlan-team.si> wrote:
> Can someone please explain me physical representation of Clusters
(Cluster
> organized tables). How and where the PrimaryKey is stored, how are
other
> datas? etc
>

From the version 8 Concepts manual: "In general, clustered data blocks have an identical format to non-clustered data blocks with the addition of data in the table directory. However, Oracle stores all rows that share the same cluster key value in the same data block."

Just image a block with some fixed plus variable overhead areas for Oracle internal use followed by a series of rows which have the same key column(s) but the rows are for different tables. The flat file or sequential file equivilent is a series of records where the fields on each record are different. A record type indicator is added to each record that is used to identify the record layout to the reader. Oracle is basically doing the same thing.

There are two kinds of clusters, indexed and hash. The cluster index can be stored in any tablespace you want to assign it to. Preferably a tablespace devoted to indexes. You can not access the cluster unless the cluster index exists. The cluster index is like any other index.

A hash cluster has no index but instead substitutes a function that converts the key value into a storage location within the cluster. Hashing is a subject covered in most Algorithm manuals. Most hash clusters hold only one table where access is always on the key value.

--
Mark D. Powell -- The only advice that counts is the advice that  you follow so follow your own advice --

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Before you buy. Received on Fri Dec 24 1999 - 14:02:11 CST

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