I'll take your vote! Especially since you have hard evidence that you
can't always "get there from here" and even with one value (we'll have
two at the beginning) get an index skip scan to occur
- Wolfgang Breitling <breitliw_at_centrexcc.com> wrote:
> 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
>
> --
> Please see the official ORACLE-L FAQ: http://www.orafaq.net
> --
> Author: Wolfgang Breitling
> INET: breitliw_at_centrexcc.com
>
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Received on Wed May 28 2003 - 09:59:57 CDT
