To manage indexes effectively,
it is important to know how SAS decides whether to use an index and
which index to use.
To find out whether
an index is being used, specify the SAS option MSGLEVEL=I in an OPTIONS
statement before the PROC SQL statement. This option enables SAS to
write informational messages about index usage (and other additional
information) to the SAS log. The default setting MSGLEVEL=N displays
notes, warnings, and error messages only.
To force SAS to use
the best available index, to use a specific index, or not to use an
index at all, include either the SAS data set option IDXWHERE= or
IDXNAME= in your PROC SQL query.