The first step for SAS is to determine
whether there are any existing indexes that might be used to process
the WHERE expression. Specifically, SAS checks the variable in each
condition in the WHERE expression to determine whether the variable
is a key variable in an index.
SAS can use either a
simple index or a composite index to optimize a WHERE expression.
To be considered for use in optimizing a single WHERE condition, one
of the following requirements must be met:
the variable in the WHERE condition
is the key variable in a simple index
the variable in the WHERE condition is the first key ...