HOWTO: Use SYS(3054) to Optimize a Query

• Microsoft Visual FoxPro for Windows, versions 5.0, 6.0

SUMMARY

Visual FoxPro 5.0 added a new SYS() function that allows you to see the Rushmore optimization level of query. This information can help you optimize your queries in relation to their use of Rushmore.

SYS(3054) actually accepts the following three different parameters:

• 1 - to turn on reporting of filter optimization



• 11 - to turn on reporting of join optimization



• 0 - to turn off the reporting of optimization levels

MORE INFORMATION

To demonstrate how this function works, follow the steps below:

1. Copy the Customer.dbf and Orders.dbf files from the \Samples\Data folder to another folder where you can make changes to the tables. For the purpose of this example, copy only the .dbf files as they are the only files that you need. After you have copied the files, open them exclusively so you can remove the database container references and the Structural CDXs. When you open the tables, a dialog box opens stating that the Testdata.dbc could not be opened, click delete (you do not need the .dbc for this example). If you have SET SAFETY ON, you also get a dialog box stating that the Structural CDX file could not be found, click ignore.



2. Now you can type the following in the Command window:

   
         ? SYS(3054,1)
         SELECT * FROM customer WHERE customer.country = 'USA' 

   The following is returned:

   
         Rushmore optimization level for table customer: none 




3. Next, add an index tag on country field and repeat the query above:

   
         INDEX on country TAG country 

   NOTE: Be sure to re-select the customer table first (SELECT customer) so you are operating on the original data. Now, you should see the following:

   
         Using index tag Country to rushmore optimize table customer
         Rushmore optimization level for table customer: full 




4. Now, if you try the following query:

   
         SELECT * FROM customer WHERE customer.country = 'USA' and ;
            customer.maxordamt>2000 

   You see the following:

   
         Using index tag Country to rushmore optimize table customer
         Rushmore optimization level for table customer: partial 

   Adding a tag on maxordamt makes the second query fully optimizable.



5. Next, if you join the two tables:

   
         SELECT * FROM customer, orders WHERE ;
            customer.cust_id = orders.cust_id 

   You see the following:

   
         Rushmore optimization level for table customer: none
         Rushmore optimization level for table orders: none 




6. Now, add tags to both tables for the cust_id field:

   
         SELECT customer
         INDEX ON cust_id TAG cust_id
         SELECT orders
         INDEX ON cust_id TAG cust_id 

   Try the query again:

   
         SELECT * FROM customer, orders WHERE ;
            customer.cust_id = orders.cust_id 

   You see the following:

   
         Rushmore optimization level for table customer: none
         Rushmore optimization level for table orders: none 

   Since the query is joining the tables and not doing any filtering, this is correct. Since there is no filter, there is no filter optimization.



7. Next, issue the following command:

   
         ? SYS(3054,11) 

   Try the query again and you see the following:

   
         Rushmore optimization level for table customer: none
         Rushmore optimization level for table orders: none
         Joining table customer and table orders using index tag Cust_id 




8. Now, try this query:

   
      SELECT * FROM customer, orders WHERE customer.cust_id=orders.cust_id ;
         AND customer.country = 'USA' AND orders.order_amt>500 

   You see the following:

   
         Rushmore optimization level for table customer: full
         Rushmore optimization level for table orders: none
         Joining table customer and table orders using index tag Cust_id 

   Since there is no tag for orders.order_amt, that filter expression cannot be optimized. Add a tag to orders for the order_amt field and the optimization for orders is full.

Additional query words: query performance

Keywords : kbFont kbPrinting kbVFp500 kbVFp600
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Issue type : kbhowto