Teradata - Statistics

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Teradata optimizer comes up with an execution strategy for every SQL query. This execution strategy is based on the statistics collected on the tables used within the SQL query. Statistics on the table is collected using COLLECT STATISTICS command. Optimizer requires environment information and data demographics to come up with optimal execution strategy.

Environment Information

• Number of Nodes, AMPs and CPUs
• Amount of memory

Data Demographics

• Number of rows
• Row size
• Range of values in the table
• Number of rows per value
• Number of Nulls

There are three approaches to collect statistics on the table.

• Random AMP Sampling
• Full statistics collection
• Using SAMPLE option

Collecting Statistics

COLLECT STATISTICS command is used to collect statistics on a table.

Syntax

Following is the basic syntax to collect statistics on a table.

COLLECT [SUMMARY] STATISTICS   
INDEX (indexname) COLUMN (columnname) 
ON <tablename>;

Example

The following example collects statistics on EmployeeNo column of Employee table.

COLLECT STATISTICS COLUMN(EmployeeNo) ON Employee;

When the above query is executed, it produces the following output.

*** Update completed. 2 rows changed. 
*** Total elapsed time was 1 second.

Viewing Statistics

You can view the collected statistics using HELP STATISTICS command.

Syntax

Following is the syntax to view the statistics collected.

HELP STATISTICS <tablename>; 

Example

Following is an example to view the statistics collected on Employee table.

HELP STATISTICS employee;

When the above query is executed, it produces the following result.

  Date       Time      Unique Values           Column Names 
--------   -------- -------------------- ----------------------- 
16/01/01   08:07:04         5                       * 
16/01/01   07:24:16         3                   DepartmentNo 
16/01/01   08:07:04         5                   EmployeeNo

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