Re: How does Oracle use indexes?

From: Roy Crabtree <crabtree_at_zero.rtp.dg.com>
Date: Sat, 2 Oct 93 17:50:17 GMT
Message-ID: <1993Oct2.175017.23714_at_dg-rtp.dg.com>


In article <1993Sep30.163319.9322_at_spectrum.xerox.com> rdutc_at_ia.mc.xerox.com writes:
>Rick,
>
>A vendor has supplied an application with the following three indexes
>on a table:-
>Index1 uses columns col1, col2, col3, col4, col5, col6.
>Index2 uses columns col1, col2, col3.
>Index3 uses columns col1, col2.

	There is one instant where these might be useful;
	if you have col1/col2 heavily clustered on ascent, and
	col1/col2/col3 heavily clustered on descent, you may see performance
	differences.  otherwise, use only Index1.

>
>You are absolutely right, in your scenario, index2 and index 3 are reduntant. In a composite index, also known as a concatenated index, if you have a 5 column index the index will be used in V6 if the where clause references all or the leading portion of
> the columns in the composite index.
>---
>Ray Dutcher
>Oracle DBA
>Xerox Corp.
>1350 Jefferson Rd., 801-15A
>Rochester, NY 14623
>

royc Received on Sat Oct 02 1993 - 18:50:17 CET

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