HOWTO: Optimize SQL Using the FORCE Clause and SYS(3054)

ID: Q155788

The information in this article applies to:

Microsoft Visual FoxPro for Windows, versions 5.0, 6.0

SUMMARY

Visual FoxPro 5.0 and later have a new clause, the FORCE clause, in the SELECT - SQL command. You can use the FORCE clause to override the default query optimization that Visual FoxPro performs.

The FORCE clause specifies that tables are joined in the order in which they appear in the FROM clause. If the FORCE clause is omitted, Visual FoxPro attempts to optimize the query. However, Visual FoxPro might execute the query faster by including the FORCE clause to disable the Visual FoxPro query optimization.

MORE INFORMATION

When a SQL statement is processed, the SQL optimizer determines the most efficient way to join the tables. There are times when you have a better understanding of the data structures than FoxPro. The FORCE clause, tells Visual FoxPro that the SELECT statement is optimized and that it does not need to be further changed.

In order to use FORCE clause, you must use the ANSI join syntax within the FROM clause. Any conditions within the WHERE clause are not subject to the FORCE clause. When the FORCE clause is positioned, it is global to all listed joins. The order in which the Join tables appear in the statement is the order in which they are joined. The following statements, therefore, may produce different performance results:


   SELECT * ;
   FROM FORCE customer JOIN(orders JOIN orditems ;
   ON orditems.order_id=orders.order_id) ;
   ON orders.customer_id=customer.customer_id

   SELECT * ;
   FROM FORCE customer JOIN orders ;
   ON orders.customer_id=customer.customer_id ;
   JOIN orditems ;
   ON orditems.order_id=orders.order_id

In the first SELECT statement, the orders and orditems tables are joined and the result is joined to the customer table. In the second statement, the customer table is joined with the orders table first. The result is then joined with the orditems table.

In order to make the decision to Force a join condition, you can use the SYS(3054) function to determine the extent to which the query is optimized by Rushmore technology. For example, you can issue the following command before the SELECT statement to determine the internal optimization plan for the query:


   ? SYS(3054,1)
   SELECT * FROM customer JOIN orders;
   ON customer.customer_id=orders.customer_id ;
   WHERE UPPER(country)="USA"

SYS(3054) returns information, such as the index tag used to optimize the join as well as the optimization level for each table, whether it be 'full,' 'partial,' or 'none.'

Sometimes, you can determine the best optimization level by trial and error. Try running the SQL statement with and without the FORCE clause to see if there is a significant change in performance. Make sure to clear the environment between SELECT statements so that caching does not effect the outcome of successive queries.

Additional query words:

Keywords : kbDatabase kbSQL kbVFp500 kbVFp600
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Issue type : kbhowto