Declaration
function QueryTest( sExpression: String ): Integer;
Description
Tests if a given xBase expression will result in an optimized query set or not. Some queries may result in partial optimization.
The function is useful in determining whether a permanent index should be built using the tested expression if the query syntax is used often enough to warrant the addition of an index.
Parameters
sExpression: A standard xBase expression that defines the query filter conditions and evaluates as logical True or False.
An index used in query optimization does not have to be the controlling index and the query conditional expression may use fields that reference more than one index.
Conditional expressions that contain references to aliased fields in tables linked to the current work area via SetRelation are not optimized.
To clear a query, pass the query expression as a NULL string ('' or #0).
Clearing a query also clears filters set with SetFilter.
Return Value
QueryTest returns one of the following defined constants:
OPTIMIZE_NONE 0
OPTIMIZE_PART 1
OPTIMIZE_FULL 2
If NONE, no indexes are available that match the query condition. If PART is returned, part of the query expression may be used in constructing a bitmap. When records are read, the other part (not optimized) is evaluated as a standard filter within the optimized set. If FULL optimization is possible, all parts of the expression will utilize open index file(s) to construct the query set.
Delphi Example
// Test if query can be optimized
procedure Form1.Button1Click(Sender: TObject);
var
qResult: Integer;
begin
if QueryBox.Text = '' then
begin
ShowMessage( 'No query to test' );
Exit;
end;
qResult := ApTbl.QueryTest( QueryBox.Text );
case qResult of
OPTIMIZE_NONE: TestBox.Text := 'NONE';
OPTIMIZE_PART: TestBox.Text := 'PART';
OPTIMIZE_FULL: TestBox.Text := 'FULL';
end;
end;
C++Builder Example
// Test if query can be optimized
void __fastcall TForm1::Button1Click(TObject *Sender)
{
int qResult;
if (QueryBox->Text == "")
{
ShowMessage( "No query to test" );
return;
}
qResult = ApTbl->QueryTest( QueryBox->Text );
switch( qResult )
{
case OPTIMIZE_NONE:
TestBox->Text = "NONE";
break;
case OPTIMIZE_PART:
TestBox->Text = "PART";
break;
case OPTIMIZE_FULL:
TestBox->Text = "FULL";
break;
}
}
See Also