RE: Histograms - SIZE clause & num

From: Charudatta Joshi <joshic_at_mahindrabt.com>
Date: Mon, 2 Aug 2004 19:11:12 +0530
Message-ID: <MHEAIPLKCACENJKNJIALCEOLCDAA.joshic@mahindrabt.com>

Thanks Wolfgang,

Yes, doubling the size parameter created frequency based histograms. Seems very weird though. BTW, I am seeing exactly the same behaviour in 8.1.7.4 environment.

And unless one is aware of this quirk, this creation of height-based histograms instead of f.b. histograms will affect the query execution plans adversely, isn't it? Hasn't any bug been raised on this? Or is some kind of 'feature'?

Good point about using analyze to delete related index statistics.

Thanks & Regards,
Charu.

-----Original Message-----
From: oracle-l-bounce_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org]On Behalf Of Wolfgang Breitling Sent: Monday, August 02, 2004 6:35 PM
To: oracle-l_at_freelists.org
Subject: Re: Histograms - SIZE clause & num_buckets anomaly

I have noticed that too that Oracle 9 collects a height balanced histogram when you gather statistics with size=<num_distinct> instead of a frequency histogram as you'd expect (and as Oracle 8 did). At least sometimes. I haven't done enough testing to have recognized a pattern or rule. In my case, for example, I needed to crank up size to 38 or 39 (forgot exactly which one) (~ 50% more) in order to get the frequency histogram. Btw, if you use "analyze ... for columns owner size 11" you'll get your frequency histogram. Analyze hasn't changed from 8 to 9, but gather_table_stats has.
Also, in order to delete statistics I still use analyze because delete_table_stats doesn't delete index stats. It doesn't matter in your case, I just thought I mention it.

At 05:32 AM 8/2/2004, you wrote:
>Hi all,
>
>Version:- 9.2.0.1 Enterprise Edition
>OS: Win2K
>
>
>** Check the number of buckets.
>SQL> select num_distinct, num_buckets
> 2 from user_tab_col_statistics
> 3 where table_name = 'T4' and column_name = 'OWNER';
>
>NUM_DISTINCT NUM_BUCKETS
>------------ -----------
> 11 4
>
>How come there are only four buckets created even though I specified size
as
>11? Could it be histogram compression?

Regards

Wolfgang Breitling
Centrex Consulting Corporation
www.centrexcc.com

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Received on Mon Aug 02 2004 - 08:36:47 CDT

RE: Histograms - SIZE clause & num_buckets anomaly