For WHERE queries on
indexed variables, the SPD Engine determines the number of index segments
that contain one or more variable values that match one or more of
the conditions in the WHERE expression. Often, a substantial performance
gain can be realized if the WHERE expression is applied only to the
segments that contain observations satisfying the WHERE expression.
The SPD Engine uses
the value of MAXSEGRATIO= to determine at what point the cost of applying
the WHERE expression to every segment would be less than the cost
of continuing to identify candidate segments. When the calculated
ratio exceeds the ratio specified in MAXSEGRATIO=, the SPD Engine
stops identifying candidate segments and applies the WHERE expression
to all segments.
Note: For a few tables, 75% might
not be the optimal setting. To determine a better setting, run a performance
benchmark, adjust the percentage, and rerun the performance benchmark.
Comparing results shows you how the specific data population you
are querying responds to shifting the index-segment ratio.