SWI, like most prologs provides fast first-argument indexing. Accessing a large database of facts via other arguments can be very slow, as a sequential scan is used. SWI provides index/1, but it doesn’t appear to be very effective.
The index_util module provides faster indexing by rewriting fact clauses to provide multiple entry points.
From the documentation:
This is designed to be a swap-in replacement for index/1. Indexing a
fact with M arguments on N of those arguments will generate N sets of
facts with arguments reordered to take advantage of first-argument
indexing. The original fact will be rewritten.
For example, calling:
materialize_index(my_fact(1,0,1)).
will retract all my_fact/3 facts and generate the following clauses in its place:
my_fact(A,B,C) :-
nonvar(A),
!,
my_fact__ix_1(A,B,C).
my_fact(A,B,C) :-
nonvar(C),
!,
my_fact__ix_3(C,A,B).
my_fact(A,B,C) :-
my_fact__ix_1(A,N,C).
here my_fact__ix_1 and my_fact__ix_3 contain the same data as the original my_fact/3 clause. In the second case, the arguments have been reordered
Speed Improvements
Some users have reported perfomance gains of 1000x. For example, this post
Limitations
- Single key indexing only. Could be extended for multikeys.
- Reindexing is not a good idea. It could be smarter about this.
- Should not be used on dynamic databases.
Does not have to be used with fact (unit clauses) – but the clauses should enumerable
