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  The Mach SIx Query Optimizer

  The Mach SIx Query Optimizer is an integrated system that provides
  high-speed query optimization for SET FILTER and other selected Clipper
  commands when used with a FOR condition.  Mach SIx optimizes queries by
  following a procedural set of steps that determine the best access plan
  to retrieve the requested set of data.  Mach SIx's primary focus is on
  the existence of index key expressions in the FOR criteria of its
  supported commands and functions.  Using this information, Mach SIx is
  able to directly search the index files to satisfy all or part of the
  query without ever having to access the database.

  Once it becomes necessary to query the physical database, Mach SIx only
  queries the set of records found to meet the condition based on the data
  in the index files.  This means that including just one indexed
  expression, in the FOR condition, has the potential to narrow the scope of
  the query to a very small set of the original file.  This translates to
  incredible speed gains for any optimized command that contains one or more
  index key expressions.  Queries that used to take minutes, now return in
  seconds.  In some cases, you will experience performance increases  that
  are magnitudes above standard Clipper.  It's completely done for you in
  the background.

  If Mach SIx is unable to optimize the query condition, it simply
  turns control over to standard Clipper, but when it can, you'll see
  Clipper sail, and sail like it never has before.