FIX: Decrease in performance occurs with repeated execution of SELECT-SQL in Visual FoxPro for Windows 5.0 and in Visual FoxPro for Windows 6.0

There is a slow degradation of Visual FoxPro's performance with repeated execution of SELECT-SQL statements. The results returned by SYS(1016) and the time required to process SELECT-SQL statements gradually increase with repeated execution of SELECT-SQL statements.

This is caused by repeated execution of a SELECT-SQL statement containing WHERE and OR clauses, as in the following example:

Here are three ways to workaround this problem:

• Use the HAVING clause in the SELECT-SQL statement rather than the WHERE clause. For example:

  SELECT DISTINCT * ;
    FROM <table name> ;
    HAVING <condition 1> ;
    OR <condition 2> ;
    OR <condition 3> ;
    ORDER BY <order> ;
    INTO CURSOR <destination>
  					

• Use the INLIST() function in the SELECT-SQL statement, in place of the OR. For example:

  SELECT DISTINCT * ;
    FROM <table name> ;
    WHERE INLIST(eExpression,<condition 1>, ;
    <condition 2>, ;
    <condition 3>, ;
    ORDER BY <order> ;
    INTO CURSOR <destination>;
  					

• Use a UNION for each OR condition. For example:

  SELECT DISTINCT * ;
    FROM <table name> ;
    WHERE <condition 1> ;
    UNION SELECT DISTINCT * ;
    FROM <table name> ;
    WHERE <condition 2> ;
    UNION SELECT DISTINCT * ;
    FROM <table name> ;
    WHERE <condition 3> ;
    INTO CURSOR <destination>
  					

Microsoft has confirmed that this is a bug in the Microsoft products that are listed at the beginning of this article.

This bug was corrected in Visual Studio 6.0 Service Pack 3.

For more information about Visual Studio service packs, please see the following articles in the Microsoft Knowledge Base:

The amount by which the result returned by SYS(1016) increases is dependent on the version of Visual FoxPro and the number of ORs contained in the SELECT-SQL statement. With Visual FoxPro 5.0, the amount of increase in the result returned by SYS(1016) is 16 times the number of ORs contained in the SELECT-SQL statement. In Visual FoxPro 6.0, the amount of increase in the result returned by SYS(1016) is 24 times the number of ORs contained in the SELECT-SQL statement. This behavior does not occur with Visual FoxPro 3.0.

Steps to Reproduce Behavior

1. Create a program named Test.prg using the following code:

   CLOSE ALL
   CLEAR ALL
   OPEN DATABASE "c:\program files\devstudio\vfp\samples\data\testdata"
   startval=VAL(SYS(1016))
   FOR i=1 TO 10
     SELECT * FROM customer WHERE .T. OR .T. INTO CURSOR test
     endval=VAL(SYS(1016))
     ? ALLTRIM(STR(endval)) +  ;
       " - " + ALLTRIM(STR(startval)) + ;
       " = " + ALLTRIM(STR(endval-startval))
     startval=endval
   NEXT
   					

2. From the Command prompt, type the following:

   DO TEST
   					

3. Observe that the startval and endval increase and that the difference between endval and startval is 16 in Visual FoxPro 5.0 and 24 in Visual FoxPro 6.0.