RE: skip scan index

You'd see it in the explain plan -- it should look something like this:

	TABLE ACCESS (BY INDEX ROWID) OF 'PHONEBOOK'
	INDEX (SKIP SCAN) OF 'I_PHONEBOOK_SKIP' (NON-UNIQUE)

--Brian

-----Original Message-----
Sent: Wednesday, May 28, 2003 9:45 AM
To: Multiple recipients of list ORACLE-L

List - If I wanted to know whether my query was taking advantage of index skip scans, how would I know? Is there something different in the EXPLAIN PLAN that I should look for? The discussion just made me curious.

Dennis Williams
DBA, 80%OCP, 100% DBA
Lifetouch, Inc.
dwilliams_at_lifetouch.com

-----Original Message-----
Sent: Wednesday, May 28, 2003 9:30 AM
To: Multiple recipients of list ORACLE-L

At 02:59 AM 5/28/2003 -0800, you wrote:
>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.

As others already said, it is a "index skip scan" access method, not a "skip scan" index. It is like an implicit OR where the optimizer looks up all distinct values for the missing prefix column(s) and augments the predicate (sort of) with these values and then does traditional index scans, ORing the results. It may not happen exactly that way, but conceptually that is what happens. From this you can deduce that it is an option only when there are relatively few distinct prefix values. In your case I doubt that the optimizer would ever choose a skip scan. Unless you have only a handfull (literally 5 or less) of fullfilment vendors. I don't have hard numbers as to the number of distinct prefix values beyond which a skip scan becomes too expensive compared to an FTS but during my tests in preparation for my IOUG presentation I had a hard time constructing an example where the optimizer would choose a skip scan - and I had tables with just 1 distinct prefix value.
My vote goes for your proposed two indices. Wolfgang Breitling
Oracle7, 8, 8i, 9i OCP DBA
Centrex Consulting Corporation
http://www.centrexcc.com

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