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Oracle Histograms Simplified: Part 2

In my previous post Oracle Histograms Simplified: Part 1, I focused on what is histogram & why it is used. In this post I will explain different ways to create & drop histograms.

HISTOGRAM CREATION:

Creation of histogram is always depends on parameter method_opt that you specify during gathering statstics.

Typical syntax for method_opt is as follows:

First column part of clause has different options as given below:

1. ALL COULMNS :

As name suggests oracle considers all columns on table for histogram creation.

EXEC DBMS_STATS.GATHER_TABLE_STATS (OWNNAME=>'<OWNER>’,TABNAME=>'<TABLE_NAME>’,METHOD_OPT => ‘FOR ALL COLUMNS SIZE AUTO’,CASCADE = TRUE);

2. ALL INDEXED COLUMNS :

Oracle will consider only columns with indexes for histogram creation.

EXEC DBMS_STATS.GATHER_TABLE_STATS (OWNNAME=>'<OWNER>',TABNAME=>'<TABLE_NAME>',METHOD_OPT => 'FOR ALL INDEXED COLUMNS SIZE AUTO',CASCADE = TRUE);

3. ALL HIDDEN COLUMNS :

Oracle will cosider only hidden columns for histogram creation.

EXEC DBMS_STATS.GATHER_TABLE_STATS (OWNNAME=>'<OWNER>',TABNAME=>'<TABLE_NAME>',METHOD_OPT => 'FOR ALL HIDDEN COLUMNS SIZE AUTO',CASCADE = TRUE);

4. COLUMNS <COLUMN NAMES SEPERATED BY COMMA> :

Oracle will consider only given columns for histogram creation.

EXEC DBMS_STATS.GATHER_TABLE_STATS (OWNNAME=>'<OWNER>',TABNAME=>'<TABLE_NAME>',METHOD_OPT => 'FOR COLUMNS COLUMN1,COLUMN2 SIZE AUTO',CASCADE = TRUE);

Moving to next portion of syntax, which lets you controls histogram creation and its type.

Size clause has different options as given below:

1. INTEGER:

You can specify number of buckets. It must be in range of 1 to 254. 1 denotes no histogram with base column statistics (i.e. minimal information required for the Optimizer) while 254 is highest allowable value.

EXEC DBMS_STATS.GATHER_TABLE_STATS (OWNNAME=>'<OWNER>',TABNAME=>'<TABLE_NAME>',METHOD_OPT => 'FOR ALL COLUMNS SIZE 10',CASCADE = TRUE);

2. AUTO:

This most commonly used size in real time scenarios. The basic process is as follows:

  • You run DBMS_STATS for a table with the METHOD_OPT parameter set to the default SIZE AUTO.
  • You query the table.
  • The database keeps track of the predicates in the preceding query and updates the data dictionary table SYS.COL_USAGE$.
  • Then you run DBMS_STATS again, DBMS_STATS takes into consideration data in SYS.COL_USAGE$ to determine which columns require histograms based on queries ran on table.

This method has following drawbacks:

  • If you gather stats with auto option & do not query table at all, then Oracle will not create any histogram on column.
  • If queries are constantly being changed in particular environment, so even data in table almost the same, can cause changes in execution plans for the queries.
EXEC DBMS_STATS.GATHER_TABLE_STATS (OWNNAME=>'<OWNER>',TABNAME=>'<TABLE_NAME>',METHOD_OPT => 'FOR ALL COLUMNS SIZE AUTO',CASCADE = TRUE);

3. REPEAT :

It will just collect histograms on columns which already have histograms.

EXEC DBMS_STATS.GATHER_TABLE_STATS (OWNNAME=>'<OWNER>',TABNAME=>'<TABLE_NAME>',METHOD_OPT => 'FOR ALL COLUMNS SIZE REPEAT',CASCADE = TRUE);

4. SKEWONLY :

Oracle collects histograms on the columns with only skewed data.

EXEC DBMS_STATS.GATHER_TABLE_STATS (OWNNAME=>'<OWNER>',TABNAME=>'<TABLE_NAME>',METHOD_OPT => 'FOR ALL COLUMNS SIZE SKEWONLY',CASCADE = TRUE);

HISTOGRAM DELETION:

There are different methods for dropping histograms on table columns:

To completely delete all types of column statistics plus histograms for a column including minimal statistics:

EXEC DBMS_STATS.DELETE_COLUMN_STATS(OWNNAME=>'<OWNER>',TABNAME=>'<TABLE_NAME>','<COLUMN_NAME>');

This is generally not recommended as the optimizer needs at least the minimal (size 1) information for creating optimal execution plans.

To delete histograms from a column without touching base column statistics i.e. minimal information required for the Optimizer (This is recommended method for pre 11g databases).

EXEC DBMS_STATS.GATHER_TABLE_STATS(OWNNAME=>'<OWNER>',TABNAME=>'<TABLE_NAME>',METHOD_OPT=>'FOR COLUMNS <COLUMN_NAME> SIZE 1');

From Oracle 11g onwards, there is new syntax to delete the histogram for a column while still leaving behind the base column statistics:

EXEC DBMS_STATS.DELETE_COLUMN_STATS(OWNNAME=>'<OWNER>',TABNAME=>'<TABLE_NAME>',COLNAME=>'<COLUMN_NAME>',COL_STAT_TYPE=>'HISTOGRAM');

Additionally we can delete histograms for a partition:

EXEC DBMS_STATS.DELETE_COLUMN_STATS('<OWNER>',TABNAME=>'<TABLE_NAME>',COLNAME=>'<COLUMN_NAME>',PARTNAME=>'<PARTITION_NAME>', COL_STAT_TYPE=>'HISTOGRAM');

and to delete column histograms for the table and all its partitions:

 EXEC DBMS_STATS.DELETE_COLUMN_STATS(OWNNAME=>'<OWNER>',TABNAME=>'<TABLE_NAME>',COLNAME=>'<COLUMN_NAME>',CASCADE_PARTS=>TRUE,COL_STAT_TYPE=>'HISTOGRAM');

Hope so u will find this post very useful:-)

Visit Oracle Histograms Simplified: Part 1, in which I focussed on what is histogram & why it is used.

Cheers

Regards,
Adityanath

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