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21.3.2 Writing a Procedure »
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21.3.1. PROCEDURE ANALYSE

ANALYSE([max_elements[,max_memory]])

ANALYSE() is defined in the sql/sql_analyse.cc source file, which serves as an example of how to create a procedure for use with the PROCEDURE clause of SELECT statements. ANALYSE() is built in and is available by default; other procedures can be created using the format demonstrated in the source file.

ANALYSE() examines the result from a query and returns an analysis of the results that suggests optimal data types for each column that may help reduce table sizes. To obtain this analysis, append PROCEDURE ANALYSE to the end of a SELECT statement: 

SELECT ... FROM ... WHERE ... PROCEDURE ANALYSE([max_elements,[max_memory]])

For example: 

SELECT col1, col2 FROM table1 PROCEDURE ANALYSE(10, 2000);

The results show some statistics for the values returned by the query, and propose an optimal data type for the columns. This can be helpful for checking your existing tables, or after importing new data. You may need to try different settings for the arguments so that PROCEDURE ANALYSE() does not suggest the ENUM data type when it is not appropriate.

The arguments are optional and are used as follows:

  • max_elements (default 256) is the maximum number of distinct values that ANALYSE() notices per column. This is used by ANALYSE() to check whether the optimal data type should be of type ENUM; if there are more than max_elements distinct values, then ENUM is not a suggested type.

  • max_memory (default 8192) is the maximum amount of memory that ANALYSE() should allocate per column while trying to find all distinct values.


User Comments

Posted by Wagner Bianchi on November 11 2009 11:20am[Delete] [Edit]

I did some tests using a table with 1000000 rows and this function PROCEDURE ANALYSE() returned all values in ENUM data type.

mysql> SELECT id, ativada, cumprida FROM t1 PROCEDURE ANALYSE(1000000,256)\G

Posted by Gary Smith on November 13 2009 11:35am[Delete] [Edit]

I think you may be misunderstanding the syntax here. Let's say we've got a table called charac which has five characters in it:

mysql> select * from charac;

+--------+
| charac |
+--------+
| A      |
| B      |
| C      |
| D      |
| E      |
+--------+
5 rows in set (0.00 sec)

If we select * from charac show procedure(), we're passing the default values, so we'll get everything back as enum:

mysql> select * from charac procedure analyse()\G
*************************** 1. row ***************************
Field_name: world.charac.charac
Min_value: A
Max_value: E
Min_length: 1
Max_length: 1
Empties_or_zeros: 0
Nulls: 0
Avg_value_or_avg_length: 1.0000
Std: NULL
Optimal_fieldtype: ENUM('A','B','C','D','E') NOT NULL
1 row in set (0.00 sec)

The first argument refers to the number of elements, and the next argument refers to the total memory assigned. So, if we do this:

mysql> select * from charac procedure analyse(5,24)\G
*************************** 1. row ***************************
Field_name: world.charac.charac
Min_value: A
Max_value: E
Min_length: 1
Max_length: 1
Empties_or_zeros: 0
Nulls: 0
Avg_value_or_avg_length: 1.0000
Std: NULL
Optimal_fieldtype: CHAR(1) NOT NULL

Then it's suggested CHAR(1) for the field which is perhaps more applicable. Hope this helps.

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