Oracle Scratchpad

July 14, 2013

12c histograms

Filed under: 12c,Histograms,Oracle,Statistics — Jonathan Lewis @ 7:11 pm BST Jul 14,2013

There are a few enhancements in 12c that might make a big difference to performance for a small investment in effort. One of the important enhancements comes from changes in histograms – which improve speed of collection with accuracy of results. The changes are so significant that I chose the topic as my presentation at OpenWorld last year.


September 13, 2012

Histogram Generation

Filed under: Histograms,Oracle,Statistics — Jonathan Lewis @ 5:21 pm BST Sep 13,2012

This really could be published in the Errata and Addenda of “Cost Based Oracle – Fundamentals”, but it’s more convenient to publish the whole thing here and just add a link to the Errata pages.

In chapter 7, on page 156, I gave an example of the type of SQL that Oracle runs (in the dbms_stats package) to generate a histogram. A sample of the code, and the plan from the tkprof output, is listed below:

January 3, 2012


Filed under: CBO,Histograms,Oracle,Statistics — Jonathan Lewis @ 5:56 pm BST Jan 3,2012

A recent comment on a note I wrote some time ago about faking histograms asked about the calculations of selectivity in the latest versions of Oracle. As I read the question, I realised that I had originally supplied a formula for calculating cardinality, rather than selectivity, so I thought I’d supply a proper example.

We’ll start with a script to create some data and stats – and I’m going to start with a script I wrote in Jan 2001 (which is why it happens to use the analyze command rather than dbms_stats.gather_table_stats, even though this example comes from an instance of

October 19, 2010

Frequency Histograms – 6

Filed under: Histograms,Statistics,Troubleshooting,Tuning — Jonathan Lewis @ 6:01 pm BST Oct 19,2010

In an earlier post on frequency histograms I described how Oracle creates an approximate histogram when dealing with character columns, and warned you that the strategy could lead to a couple of anomalies if you were unlucky. I’ve already published a note about one such anomaly that can occur with fairly long character strings, this note describes another anomaly that could appear in less extreme cases. Again, we start by constructing a data set.


October 13, 2010

Frequency Histogram 5

Filed under: Histograms,Oracle,Statistics,Troubleshooting — Jonathan Lewis @ 9:21 am BST Oct 13,2010

In an earlier post on frequency histograms I described how Oracle creates an approximate histogram when dealing with character columns, and warned you  that the strategy could lead to a couple of anomalies if you were unlucky. This note describes one such anomaly. We start with a slightly curious data set:

October 5, 2010

Frequency Histogram 4

Filed under: Histograms,Oracle,Statistics,Troubleshooting — Jonathan Lewis @ 6:25 pm BST Oct 5,2010

In an earlier note on interpreting the content of frequency histograms I made a throwaway comment about the extra complexity of interpreting frequency histograms on character-based columns. This note starts to examine some of the complications.

The driving problem behind character columns is that they can get quite large – up to 4,000 bytes – so the content of an “accurate histogram” could become quite large, and Oracle seems to have taken a strategic decision (at some point in history) to minimise this storage. As a result we can see an algorithm that works roughly as follows:

September 24, 2010

Frequency Histogram 3

Filed under: Histograms,Oracle,Statistics — Jonathan Lewis @ 6:45 pm BST Sep 24,2010

Here’s the output I get from querying dba_tab_histograms for a column that has been given a frequency histogram by a call to dbms_stats.gather_table_stats().

September 20, 2010

Frequency Histograms 2

Filed under: Histograms,Statistics — Jonathan Lewis @ 6:18 pm BST Sep 20,2010

I find it convenient occasionally to “translate” a frequency histogram into a report of the underlying data (sometimes to see how closely the histogram matches the real data). To demonstrate the type of query I use I’ve created a data set with a small number of distinct values and generated a frequency histogram on the data set. This is what the data and histogram look like:

March 23, 2010

Fake Histograms

Filed under: CBO,Histograms,Oracle,Statistics — Jonathan Lewis @ 8:49 pm BST Mar 23,2010

I got an email a few days ago asking me how to create a “height-balanced” histogram using the dbms_stats.set_column_stats() procedure.

I posted an example of creating a frequency histogram a few months ago – and it doesn’t take much to turn this into an example for the height-balanced type; the key change apppears in three critical lines.


May 28, 2009

Frequency Histograms

Filed under: CBO,Histograms,Statistics,Tuning — Jonathan Lewis @ 7:34 pm BST May 28,2009

In a recent article on changes to the calculation of cardinality with frequency histograms I made a comment about writing programs to create fake but realistic and complete frequency histograms to work around the problems of dramatic changes in execution plan that can occur if you let Oracle create the histograms by gathering stats.

As a follow-up to that comment, here’s a little script to demonstrate the method (for a numeric column), and a few points to remember when using this method.


April 23, 2009

Histogram change

Filed under: CBO,Histograms,Statistics,Troubleshooting — Jonathan Lewis @ 7:17 pm BST Apr 23,2009

When you upgrade from to or 11g, watch out for SQL that depends on the existence of frequency histograms.

In (and earlier versions of Oracle) if you have a frequency histogram on a column, and then use a predicate of the form:

where colX = {value not found in histogram}

then the optimizer would calculate a cardinality of one for this predicate (having recorded a density of 1/(2 * num_rows) in the data dictionary (see comments 21 and 22 of this note on dbms_xplan).


April 25, 2008

Cardinality Change

Filed under: CBO,Execution plans,Histograms,Statistics,Troubleshooting — Jonathan Lewis @ 8:13 pm BST Apr 25,2008

Here’s an interesting point that’s just been raised on the Oracle-L list-server:

Metalink bug: 5483301.8

If a column has a frequency histogram and a query uses an equality predicate over the column looking for a value that does not exist in the histogram then the CBO was estimating a cardinality of 1. This could favor Nested Loops too much.

The fix introduces a notable change in behavior: instead of estimating a cardinality of 1 then with this fix CBO estimates a cardinality of (0.5 * the lowest cardinality found in the histogram). This can result in plan changes.


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