In SICStus Prolog, predicates are indexed on their first
arguments. This means that when a predicate is called with
an instantiated first argument, a hash table is used to gain
fast access to only those clauses having a first argument
with the same primary functor as the one in the predicate
call. If the first argument is atomic, only clauses
with a matching first argument are accessed. Indexes are
maintained automatically by the built-in predicates manipulating
the Prolog database (for example, assert/1,
retract/1, and compile/1.
Keeping this feature in mind when writing programs can help speed their execution. Some hints for program structuring that will best use the indexing facility are given below. Note that dynamic predicates as well as static predicates are indexed. The programming hints given in this section apply equally to static and dynamic code.