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  Performance Tips:

  Mach SIx achieves performance over conventional query methods, by
  narrowing down the records that must be tested in the database to satisfy
  the query by searching the index files first.  If the indexed portion of
  the query has not effectively reduced the number of target records to a
  reasonable percentage of the source table, gains in query performance
  will be marginal at best.  Remember to target the smallest possible set
  of records with the indexed portion of your queries.  This way maximum
  efficiency will be obtained.

  Set the current index order to 0 with the SET ORDER TO command.

  If you do not need the result of your query in the current index order,
  set the index order to natural order.  This can be accomplished by
  issuing a SET TAG TO, SET TAGORDER TO, or SET ORDER TO.  By querying the
  database in natural record order you eliminate the need for Mach SIx to
  create any temporary indexes and as a result the increase the speed of
  the query.

  Enclose ranges in parenthesis.

  Any time you specify a range in a query condition, make sure you enclose
  it in parenthesis.  This way Mach SIx will be sure to evaluate the two
  expressions together, setting upper and lower bounds on its index scan.
  When a either bound is exceeded the scan of the index terminates.
  Otherwise, two separate scans to opposite ends of the index would be
  necessary to satisfy the range.  In the following example, the range
  of ages is bound in parenthesis.

      COUNT FOR  State = "CA" .AND. (Age >= 20 .AND. Age <= 30)

  Optimizing with DELETED records.

  When Mach SIx optimizes a fully indexed COUNT query, it has the potential
  to completely satisfy the query directly from the index files, with one
  exception.  Normally if SET DELETED is ON, Mach SIx is forced to visit
  all the records found in the indexes to test their deletion status.
  However, you can eliminate this requirement by simply maintaining an
  index or tag on DELETED() for the database files you will frequently
  query.  This way COUNTS will truly be 100% optimizable!  Not only will
  COUNTS execute faster, but other commands as well, especially for
  databases with a large number of deleted records.  The following code
  demonstrates creating an index on DELETED().


       // Open Account database
       USE Account VIA "SIXCDX"     // or VIA "DBFNSX"

       // Create deleted tag in structural CDX for faster queries
       INDEX ON Deleted() TAG Delete OF Account


  Direct temporary files to a local hard or ram drive.

  When optimizing the COPY, DISPLAY, LIST, and TOTAL commands, Mach SIx
  must create  a temporary index file to sort the resulting collection of
  data when the current index order is not set to 0.

  To speed creation of temporary indexes, set the Mach SIx temporary file
  path to a local hard or RAM drive.  You can accomplish this one of two
  ways.  First, you can set the path with the DOS environment variable
  M6TEMP or call the m6_SetTemp() function with the path as the
  parameter.  The path set by m6_SetTemp() will have precedence over the
  environment variable.

  The following example demonstrates setting the temporary file path
  to root directory of ram drive D:

  From DOS, type the following command:

      SET M6TEMP=D:\

  Or, in your program call:

      m6_SetTemp("D:\")

  Specify expressions in "most likely to fail first" order.

  Mach SIx was designed with the same short circuiting technology handling
  logical conditions found in the Clipper compiler.  If at any time an
  expression joined by .AND. returns 0 records, that portion of the query
  terminates immediately and resumes with the next .OR.'ed expression if
  one exists.

  List index keys to the left of the operator, and constants on the right.

  While Mach SIx will recognize an index key listed on either side of the
  relational operator ( =, #, >, <, etc..), it has been designed to
  indentify the key on the left first.  The last place it checks is on
  the right.  Unless it's absolutely necessary, always list index keys
  on the left and constants on the right.  See the following example.

     USE Customers

     SET INDEX TO State, Name, Age, Amount

     COUNT TO nCount FOR Amount = 50.00   // This one will be processed
                                          // in one active index key scan

     COUNT TO nCount FOR 50.00 = Amount   // This one will require two
                                          // scans to locate the Amount
                                          // index or tag