FIX: Performance Degradation with Repeated SQL SELECTs

ID: Q191172

The information in this article applies to:

Microsoft Visual FoxPro for Windows, versions 5.0, 5.0a, 6.0

SYMPTOMS

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

CAUSE

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


SELECT * ;
  FROM <TABLE NAME> ;
  WHERE .T. ;
  OR .T. ;
  INTO CURSOR TEST

RESOLUTION

Here are three ways to workaround this problem:

Use the HAVING clause in the SQL Select 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 SQL Select 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>

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products 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:

Q194022 INFO: Visual Studio 6.0 Service Packs, What, Where, Why

Q194295 HOWTO: Tell That Visual Studio 6.0 Service Packs Are Installed

MORE INFORMATION

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 SQL Select 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 SQL Select 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 SQL Select statement. This behavior does not occur with Visual FoxPro 3.0.

Steps to Reproduce Behavior

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

From the Command prompt, type the following:


DO TEST

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.

Additional query words:

Keywords : kbDatabase kbSQL kbVFp500 kbVFp500abug kbVFp500bug kbVFp600 kbVFp600bug kbVS600sp3fix kbGrpFox
Version : WINDOWS:5.0,5.0a,6.0
Platform : WINDOWS
Issue type : kbbug