RE: use of reverse key index,cost based optimizer

Jared

   Like you, I have always understood that b+ tree indexes maintain their balance automatically. The statement I was referring to is in the Oracle9i Database Performance Planning manual, Chapter 1:

"Use of sequences, or timestamps, to generate key values that are indexed themselves can lead to database hotspot problems, which affect response time and throughput. This is usually the result of a monotonically growing key that results in a right-growing index. To avoid this problem, try to generate keys that insert over the full range of the index. This results in a well-balanced index that is more scalable and space efficient. You can achieve this by using a reverse key index or using a cycling sequence to prefix and sequence values."

I hate to admit it, but I ran across this tidbit while I was studying for the OCP. I have no idea what a well-balanced index means. Had good supportive parents?

But just when you think the reverse key index must be great, the Concepts manual points out:
"Using the reverse key arrangement eliminates the ability to run an index range scanning query on the index. Because lexically adjacent keys are not stored next to each other in a reverse-key index, only fetch-by-key or full-index (table) scans can be performed."

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

-----Original Message-----
Sent: Sunday, June 01, 2003 2:25 AM
To: Multiple recipients of list ORACLE-L

Skewed, yes.

Unbalanced, no.

Jared

On Saturday 31 May 2003 00:34, Rajesh.Rao_at_jpmchase.com wrote:
> Assume an index on employee number. The number is assigned sequentially,
> and as such, the rightmost index leaf block would always be used. A
> possible hot block. A reverse key index can avoid this. Also, assume when
> an employee retires or quits, the record is deleted. But the space freed
> within the index leaf block will never be used (unless of course, all
> entries from that leaf block are deleted). A reverse key index can help
> you avoid these "holes" or otherwise skewed indexes, and help the index
> become more "balanced", but has the pitfall that is mentioned.
>
> Raj
>
>
>
> Jared.Still_at_ra
> disys.com To: Multiple recipients of
> list ORACLE-L <ORACLE-L_at_fatcity.com> Sent by: cc:
> root_at_fatcity.c Subject: RE: use of reverse
> key index,cost based optimizer om
>
>
> 05/30/2003
> 10:44 PM
> Please respond
> to ORACLE-L
>
>
>
>
>
>
> Dennis,
>
> My understanding of B*tree is that it is always balanced. Monotonically
> increasing
> keys will create a right hand index, but nonetheless balanced.
>
> If wrong, I'm sure to be corrected. :)
>
> Also, I don't believe the reverse key index will help queries any. I'm
> guessing that under
> normal circumstances it would increase the number of index blocks that
> needed to be
> cached.
>
> In the case of a range scan, it would definitely not perform as well, and
> increase the likelihood
> of a FFS or FTS, depending on the queries normally used in a system.
>
> The primary purpose of these was to reduce block pings on OPS IIRC, which
> would also reduce
> block contention on inserts as you said.
>
>
> Jared
>
>
>
>
>
>
>
>
> DENNIS WILLIAMS <DWILLIAMS_at_LIFETOUCH.COM>
> Sent by: root_at_fatcity.com
> 05/30/2003 12:09 PM
> Please respond to ORACLE-L
>
>
> To: Multiple recipients of list ORACLE-L
<ORACLE-L_at_fatcity.com>
> cc:
> Subject: RE: use of reverse key index,cost based optimizer
>
>
> helpdesk
> I don't see where anyone responded. If you look up reverse key index in
> the documentation, it says something about if you have a column where most
> of the values have leading values that are close. Reverse key will help
> the
> btree of the index be more balanced. That helps on queries. And on inserts
> you aren't continually hitting the same block, but spreading the inserts.
> Oracle has two SQL optimizers, rule-based and cost based. The cost
> based
> is more sophisticated. You first populate statistics on your tables. When
> creating an execution plan for your SQL the CBO will consider those
> statistics. Does that answer your questions?
>
> Dennis Williams
> DBA, 80%OCP, 100% DBA
> Lifetouch, Inc.
> dwilliams_at_lifetouch.com
>
>
> -----Original Message-----
> Sent: Friday, May 30, 2003 1:25 AM
> To: Multiple recipients of list ORACLE-L
>
>
>
>
>
>
> hai gurus
>
> please tell use of using reverse key index
> and what exactly cost based optimizer
> thanks in advance
> manjunath

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