my_base.h:

  Add comment about what kind of key value prefixes HA_READ_KEY_PREFIX_LAST etc. allow as a search key

include/my_base.h

@@ -49,6 +49,21 @@
 
 	/* The following is parameter to ha_rkey() how to use key */
 
+/* We define a complete-field prefix of a key value as a prefix where the
+last included field in the prefix contains the full field, not just some bytes
+from the start of the field. A partial-field prefix is allowed to
+contain only a few first bytes from the last included field.
+
+Below HA_READ_KEY_EXACT, ..., HA_READ_BEFORE_KEY can take a
+complete-field prefix of a key value as the search key. HA_READ_PREFIX
+and HA_READ_PREFIX_LAST could also take a partial-field prefix, but
+currently (4.0.10) they are only used with complete-field prefixes. MySQL uses
+a padding trick to implement LIKE 'abc%' queries.
+
+NOTE that in InnoDB HA_READ_PREFIX_LAST will NOT work with a partial-field
+prefix because InnoDB currently strips spaces from the end of varchar
+fields! */
+
 enum ha_rkey_function {
   HA_READ_KEY_EXACT,			/* Find first record else error */
   HA_READ_KEY_OR_NEXT,			/* Record or next record */