Program:  Superbase 4
     Version:  1.3x
     Topic:    Optimizing filters in queries
     Date:     October 13, 1992

     Summary : Filters in queries ( and updates ) can be considerably
     optimized. There are two levels of optimization: basic
     optimization ( always present ) and the use of SET QUERY ON to
     switch on full/advanced optimization. Basic Optimization accepts
     th e filter as is, and it analyses the filter from left to right,
     looking for any point in the filter where optimization will be
     BROKEN. If such a break occurs, further optimization attempts
     cease. In Advanced Optimization, Superbase does not necessarily
     reject a filter instruction line at the first point optimization
     is broken.  Advanced Query  Optimization attempts to resolve
     variables into literal values, convert functions and calculations
     to their result, handle multiple and outer join expressi ons and
     respect the OR operator. Further, Superbase may choose not to use
     a specific ORDER designation in a  filter to access records, in
     preference to another indexed field in the filter that Superbase
     may decide will produce a quicker response.

     Rules for Basic Optimization

     1.  The index to be used for optimization must be the index that
     is being used by the selection commands.

     2.  Superbase scans from left to right looking for optimizable
     expressions.

     3. Certain syntax elements such as OR, ( ) 's and FUNCTIONS stop
     this scanning at the place where they occur, and so should be put
     to the right of any optimizable expressions.

     4. A maximum of two expressions can be optimized (but these must
     refer to the current index field and be linked with AND).

     5. Superbase optimizes >, <, <=, >= and =  (but apparently not >
     or < for text fields) and optimizes LIKE with simple wildcards
     (as long as it is of the form  LIKE "text*", rather than LIKE
     "*text").