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 Updating the Statistics Information 

Various statistics are compiled for each database instance, such as the number of table entries, the size of the tables and indexes, and the value distribution (various values) of indexes and columns. This information is required by the SQL Optimizer to define the best strategy for executing complex SQL statements. The statistics information is stored in the database catalog.

If the database size or the values it contains have changed considerably, you have to update the statistics. Do this about once a week.

The statistics values can be updated for certain tables, columns, or for all base tables.

Prerequisites

The database instance is in the ONLINE operational state.

Procedure

Select the required database instance in the list of database instances. Choose Instance ® Tuning ® Optimizer Statistics.

The database system contains an internal list of the tables whose statistics need to be updated.

·        Updating statistics entries for all tables
You can update the statistics entries for all tables in this internal list. To do this, do not enter a search argument under
Search; instead, just enter *.
Updating statistics entries for selected tables
You can update the statistics entries for selected tables in this internal list. To do this, enter appropriate search arguments for the owner, table name, and column name under
Search.

·        You can determine the size of a sample. To do this, enter appropriate information under Estimate.

Use estimate value

Select this option.

Rows

Number of rows to be checked

Percent

Percentage of rows to be checked

The update is only performed across the number of rows or the specified percentage, based on the total number of database tables. This option, therefore, is faster.

The significance of the random sample depends on the size of the tables and the physical position of the data. Measurements have shown that for large tables (more than 1 million entries), samples of more than 20000 rows produce good results.

·        Independently of the internal list, you can display all tables for which statistics can be updated. To do this, select Select from tables under Advanced.

Updating Table Statistics

       1.      Choose Actions ® Search.

       2.      A list of the tables is displayed that matches your criteria under Search or Advanced. From the Update Statistics Dateand Update Statistics Time, you can tell when the statistics for this table were last updated.
If you want to update the statistics for one of the tables in the list, simply select the relevant entry. If you want to update the statistics for all of the tables, choose
Actions ® Mark All.
Select
Include update columns for marked tables (under Advanced) if you want to update column statistics as well as the table statistics. You can display the columns of a table by double clicking. Select the required columns.

       3.      Choose Actions  ® Execute to update the statistics for the selected tables.
If you have not selected any tables, the statistics of those tables are updated that are in the internal list of the database system.

 

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