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Home -> Community -> Mailing Lists -> Oracle-L -> RE: skip scan index
I've had a couple kick in where full scans were happening before (badly
tuned SQL with out a proper index) and in one case I saw a three index
and-equal become a skip scan.
I'm about 70/30 against skip scans on performance improvements with hints.
RF
-----Original Message-----
To: Multiple recipients of list ORACLE-L
Sent: 5/28/2003 11:35 AM
I've done a little bit research and testing on this and one thing that
I've
found is that the optimizer will only choose a skip-scan route if the
leading column of the index is relatively non-selective. I haven't been
able to pin down how non-selective it has to be, but I've never had one
kick
in with a unique leading column, and it always seems to choose one if
the
leading column of the index has only a few distinct values in it.
--Brian
-----Original Message-----
Sent: Wednesday, May 28, 2003 5:00 AM
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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