accuracy of the histogram

Hi, I have a question about the accuracy of the endpoint of histogram values.

Endpoint histogram values are normalized to the default double precision that corresponds to 15 decimal digits. Is it possible to increase this value?
I ask this question because we struggle to collect histograms for a column that contains 18 numbers. After collecting the histograms, the endpoint values seen in dba_histograms are truncated after 15 digits resulting from bad estimates CBO and poor execution plans.

Andreas

user6778570 wrote:
I was also thinking to transform the number of varchar2 columns (18) and then let the application do a conversion in number (since Oracle properly treats the 32 when calculating histograms byte strings, as you mentioned).

Andreas,

using my test case, I can confirm that in 10.2.0.1/3 the histogram works when you store the number as a character, even if implicit conversion of TO_NUMBER is applied. By "works", I mean that the optimizer can use the histogram information to arrive at a correct cardinality estimate, which is not the case when I use the original number data type for the same data (precision > 15). I used a character set of database-byte, multibyte character sets could lead to different results, due to the limitation of 32 bytes (it is 32 bytes, not characters, by the way, I was not sure of in my previous post).

So it could be useful to evaluate, but you must also be aware of the implications of the use of a data type "daft". One of the most important threats is obviously that the implicit conversion is based on the NLS client settings, so you might find yourself with clients where statements fail due to errors of conversion if they have a different NLS parameter of one who has stored the data in the database.

Kind regards
Randolf

Oracle related blog stuff:
http://Oracle-Randolf.blogspot.com/

SQLTools ++ for Oracle (Open source Oracle GUI for Windows):
http://www.sqltools-plusplus.org:7676 /.
http://sourceforge.NET/projects/SQLT-pp/

Tags: Database

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