Add section on testing index scan.

Author: bmomjian

doc/FAQ

@@ -1,7 +1,7 @@
 
                 Frequently Asked Questions (FAQ) for PostgreSQL
 
-   Last updated: Sun Jan 12 13:36:11 EST 2003
+   Last updated: Fri Jan 17 12:15:04 EST 2003
 
    Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
 
@@ -749,6 +749,10 @@
     ORDER BY col [ DESC ]
     LIMIT 1;
 
+   If you believe the optimizer is incorrect in choosing a sequential
+   scan, use SET enable_seqscan TO 'off' and run tests to see if an index
+   scan is indeed faster.
+   
    When using wild-card operators such as LIKE or ~, indexes can only be
    used in certain circumstances:
      * The beginning of the search string must be anchored to the start