Re: dbms_stats and number of buckets

From: Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
Date: Thu, 19 Oct 2017 08:31:31 +0000
Message-ID: <LOXP123MB0086C9B215C1C80A5743F0C9A5420_at_LOXP123MB0086.GBRP123.PROD.OUTLOOK.COM>

I'd guess the "bad cardinality" is about 20,000 for PENDIENTE and PENDIENTE2.

You either have to gather stats on just that column at estimate_percent => 100 at the right time of day, or you can fake a histogram ( sample here for a numeric column - just use the chararray for your requirment: https://jonathanlewis.wordpress.com/2009/05/28/frequency-histograms/ ).

12c could handle this properly with its auto_sample_size.

Regards
Jonathan Lewis

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From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> on behalf of Ls Cheng <exriscer_at_gmail.com> Sent: 19 October 2017 09:02:49
To: Oracle Mailinglist
Subject: dbms_stats and number of buckets

Hi

I am trying to generate histogram in one of columns in a message table, there are 5 differente values in the column as following

select estado, count(*) from hl7.hl7_pending group by estado;

ESTADO                             COUNT(*)
-------------------------------- ----------
OMITIDO                               40155
PENDIENTE                               145
ENVIADO                             8801353
PROCESADO                            363466
PENDIENTE2                              227

When I run dbms_stats as follows ( I tried different combination for mthod_opt)

exec dbms_stats.gather_table_stats('HL7', 'HL7_PENDING', method_opt => 'FOR ALL COLUMNS SIZE 1 FOR COLUMNS SIZE 10 ESTADO')

exec dbms_stats.gather_table_stats('HL7', 'HL7_PENDING', method_opt => 'FOR ALL COLUMNS SIZE skewonly')

The number of buckets shows 3 only instead of 5

select table_name, column_name, histogram, num_buckets   from dba_tab_columns
 where table_name = 'HL7_PENDING'
   and column_name = 'ESTADO';

TABLE_NAME                     COLUMN_NAME                    HISTOGRAM       NUM_BUCKETS
------------------------------ ------------------------------ --------------- -----------
HL7_PENDING                    ESTADO                         FREQUENCY                 3

the column is varchar32 and NOT NULL

And the query shows bad cardinality of course. This is RAC 11.2.0.4 with July 217 PSU. Any idea?

Thanks

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Received on Thu Oct 19 2017 - 10:31:31 CEST