Comments on: Frequency Histogram 4

By: Randolf Geist

Randolf Geist — Fri, 10 Jun 2011 11:26:49 +0000

> I had also made a comment in the same line about Oracle extracting the first 20 bytes, and I can’t think where I got that from.

May be 0×20 = 32?

Randolf

By: Jonathan Lewis

Jonathan Lewis — Fri, 10 Jun 2011 11:11:45 +0000

Randolf,

Thanks for pointing out the error – now fixed. I had also made a comment in the same line about Oracle extracting the first 20 bytes, and I can’t think where I got that from.

You’re right about the “less than six characters” of course. I actually gave an example of this with the “NEW / NEVÿÿÿ” switch in the example, but I’ve added in a comment to that effect where I make the point about the 6th character possibly being wrong.

By: Randolf Geist

Randolf Geist — Thu, 09 Jun 2011 20:27:45 +0000

Jonathan,

I just came across this while I was working on a different issue – I think your description above how Oracle arrives at the numeric representation is not consistent with what you’ve outlined in “Cost Based Oracle – Fundamentals”.

The first step should read: Take the first 15 bytes, not six (Of course it’s obvious where the six comes from, but the result will not be consistent with Oracle’s result when using only the first six bytes).

By the way, for a original string less than six (single-byte) characters the last character can be incorrect due to the rounding applied, so you don’t end up necessarily with five correctly extracted characters sometimes.

Randolf

By: Eugen IACOB

Eugen IACOB — Sun, 30 Jan 2011 21:18:29 +0000

Page 117 more exactly. That helped me explain a client strange behaviour. Thanks for this Jonathan!

By: Jonathan Lewis

Jonathan Lewis — Sun, 23 Jan 2011 14:21:46 +0000

I created a series of test cases, and proposed the simplest explanation for the results. I published some details in "Cost Based Oracle - Fundamentals."

By: Rajeshwaran

Rajeshwaran — Sat, 22 Jan 2011 08:39:21 +0000

How did you arrived at the algorithm described above?

Is that available in Oracle product documentation? if yes, can you provide link to that?

By: Jonathan Lewis

Jonathan Lewis — Fri, 21 Jan 2011 17:19:30 +0000

Rajeshwaran, This seems to be covered by my comment:

With a nice friendly single-byte character code, the first 5 characters will be extracted correctly, and the sixth will be pretty close to the original.

I stopped at six characters because I knew that the 7th onwards would be garbage.

By: Rajeshwaran

Rajeshwaran — Fri, 21 Jan 2011 15:46:18 +0000


rajesh@10GR2> SELECT endpoint_number,
  2    endpoint_number - NVL(prev_endpoint,0) frequency,
  3    hex_values,
  4    chr(to_number(SUBSTR(hex_values, 2,2),'XX'))
  5    || chr(to_number(SUBSTR(hex_values, 4,2),'XX'))
  6    || chr(to_number(SUBSTR(hex_values, 6,2),'XX'))
  7    || chr(to_number(SUBSTR(hex_values, 8,2),'XX'))
  8    || chr(to_number(SUBSTR(hex_values,10,2),'XX'))
  9    || chr(to_number(SUBSTR(hex_values,12,2),'XX'))
 10    || chr(to_number(SUBSTR(hex_values,14,2),'XX'))
 11    || chr(to_number(SUBSTR(hex_values,16,2),'XX'))
 12    || chr(to_number(SUBSTR(hex_values,18,2),'XX'))
 13    || chr(to_number(SUBSTR(hex_values,20,2),'XX'))
 14    || chr(to_number(SUBSTR(hex_values,22,2),'XX'))
 15    || chr(to_number(SUBSTR(hex_values,24,2),'XX'))
 16    as column_values
 17  FROM
 18    (SELECT endpoint_number,
 19      lag(endpoint_number,1) over( order by endpoint_number ) prev_endpoint,
 20      TO_CHAR(endpoint_value,'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX')hex_values,
 21      endpoint_actual_value
 22    FROM user_tab_histograms
 23    WHERE table_name  = 'T'
 24    AND column_name = 'OBJECT_NAME'
 25    )
 26  ORDER BY endpoint_number ;

ENDPOINT_NUMBER  FREQUENCY HEX_VALUES                      COLUMN_VALUE
--------------- ---------- ------------------------------- ------------
              1          1  4144545F53544315EDB85726C00000 ADT_STC§f+W&
              2          1  41534F5F53452F1F9B9266BEA00000 ASO_SE/?++f+
              5          3  49525F5354474CEC7454244BE00000 IR_STGL8tT$K
              7          2  49585F53544751CD2477F0CDA00000 IX_STGQ-$w=-
              8          1  504B475F50523F5F66AC4457A00000 PKG_PR?_f¼DW
              9          1  52505449524141D86FB6896A200000 RPTIRAA+o¦+j
             10          1  52505449524141D86FB6896A200000 RPTIRAA+o¦+j

7 rows selected.

Elapsed: 00:00:00.90
rajesh@10GR2>

I am not getting the proper values in the column COLUMN_VALUE. Is this issue is due to Character set?

By: Frequency Histograms – 6 « Oracle Scratchpad

Frequency Histograms – 6 « Oracle Scratchpad — Wed, 20 Oct 2010 08:12:51 +0000

[...] under: Statistics,Troubleshooting,Tuning — Jonathan Lewis @ 6:01 pm UTC Oct 19,2010 In an earlier post on frequency histograms I described how Oracle creates an approximate histogram when dealing with character columns, and [...]

By: Frequency Histogram 5 « Oracle Scratchpad

Frequency Histogram 5 « Oracle Scratchpad — Wed, 13 Oct 2010 09:23:05 +0000

[...] an earlier post on frequency histograms I described how Oracle creates an approximate histogram when dealing with character columns, and [...]