RE: skip scan index

I have doubt about the real advantages of index-skip-scan operation.

Someone please clarify my doubts.

1. Its more like multiple indexes for unique values of the leading column, won't it be logically equivalent to scanning as many indexes as the number of distinct values of the leading column? If this is the case, until the row size is large, won't it be advisable to use a full-table scan rather than multiple index scans?
2. The real advantage will be in case the leading column has less number of unique values. If that is the case would it not be advisable not to have a index on that column at all?

Regards
Naveen

> -----Original Message-----
> From: Mark Leith [mailto:mark_at_cool-tools.co.uk]
> Sent: Wednesday, May 28, 2003 5:25 PM
> To: Multiple recipients of list ORACLE-L
> Subject: RE: skip scan index
>
>
> Rachel,
>
> http://technet.oracle.com/oramag/webcolumns/2003/techarticles/
> schumacher_ski
> pscan.html
> http://www.oracle-base.com/Articles/9i/IndexSkipScanning.asp
>
> I don't have any personal experience with them myself :( The
> first link
> gives a pretty good overview though..
>
> Mark
>
> -----Original Message-----
> Carmichael
> Sent: 28 May 2003 12:00
> To: Multiple recipients of list ORACLE-L
>
>
> Okay, I have a developer here who has been reading the docs (this can
> be dangerous!)
>
> we are adding functionality to one of our applications, this will
> involve using multiple fulfillment houses, so we'll be adding the
> fulfillment vendor id to the order table. Easy, this is not a problem.
> We want to be able to search by order date and by fulfillment vendor
> id/order date
>
> Traditional design would be to add two indexes: one on order date, and
> a concatenated one on fulfillment vendor id/order date.
>
> The developer is telling me to create a "skip scan index" instead of
> two different ones. MY reading in the FM tells me that skip scan index
> is not a type of index, but rather a way Oracle uses to use an index
> even if the leftmost column is not in the query.
>
> Is there any benefit in my building only the one index? Our order
> volume is not so high (and never will be) that there is a visible
> performance impact if I have the two indices.
>
> This is 9i, 9.2.0.1, will be upgrading to 9.2.0.2 in the near future.
> Solaris
>
> Any suggestions/comments/war stories would be appreciated. I know I've
> seen Jonathan post on skip scan indexes before but I can't find the
> specific reference at the moment.
>
> Rachel
>
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  INET: naveen_nahata_at_mindtree.com

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