Re: index

From: Brian Selzer <>
Date: Mon, 28 Jul 2008 16:24:26 -0400
Message-ID: <%Vpjk.34162$>

"Bob Badour" <> wrote in message news:488de63f$0$4041$
> wrote:
>> Hi all,
>> I read the following in a book
>> 1) we can have several non clustered indexes on a table , but we can
>> have only one clustered index per table
>> 2) index on the primary key can be clustered or non clustered
>> now my question is
>> 1) to what extent these statements are true ?
> To the extent the statements accurately describe the physical
> implementation of a particular dbms, they are true.
> Theoretically, one can have multiple clustered indexes at the cost of
> duplicating the data, which of course incurs a cost. Vendors typically
> assUme nobody would ever want to incur that cost.

This is bunk. Badour is an idiot. Where there is a clustered index, that index /is/ the table--that is, the table is physically stored using whatever data structure is in use for indexes--probably b-trees. A table that does not have a clustered index, on the other hand, is physically stored as a heap. The leaf nodes of a clustered index /are/ the rows of the table. The leaf nodes of a non-clustered index are either pointers to rows on the heap (where there is no clustered index) or clustered index keys (along with a uniquifier [I didn't coin the term, so don't blame me] if the clustered index isn't also a unique index). Although it is possible to have multiple indexes that cover the entire heading, that is not the same thing as having multiple clustered indexes. Received on Mon Jul 28 2008 - 22:24:26 CEST

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