4.14.6 Indexing databases

The indexing capabilities of SWI-Prolog are described in section 2.17. Summarizing, SWI-Prolog creates indexes for any applicable argument, pairs of arguments and indexes on the arguments of compound terms when applicable. Extended JIT indexing is not widely supported among Prolog implementations. Programs that aim at portability should consider using term_hash/2 and term_hash/4 to design their database such that indexing on constant or functor (name/arity reference) on the first argument is sufficient. In some cases, using the predicates below to add one or more additional columns (arguments) to a database predicate may improve performance. The overall design of code using these predicates is given below. Note that as term_hash/2 leaves the hash unbound if Term is not ground. This causes the lookup to be fast if Term is ground and correct (but slow) otherwise.

:- dynamic
    x/2.

assert_x(Term) :-
    term_hash(Term, Hash),
    assertz(x(Hash, Term)).

x(Term) :-
    term_hash(Term, Hash),
    x(Hash, Term).

[det]term_hash(+Term, -HashKey)

If Term is a ground term (see ground/1), HashKey is unified with a positive integer value that may be used as a hash key to the value. If Term is not ground, the predicate leaves HashKey an unbound variable. Hash keys are in the range 0 ... 16,777,215, the maximal integer that can be stored efficiently on both 32 and 64 bit platforms.

This predicate may be used to build hash tables as well as to exploit argument indexing to find complex terms more quickly.

The hash key does not rely on temporary information like addresses of atoms and may be assumed constant over different invocations and versions of SWI-Prolog.^{92Last change: version 5.10.4} Hashes differ between big and little endian machines. The term_hash/2 predicate is cycle-safe.^{bugAll arguments that (indirectly) lead to a cycle have the same hash key.}

[det]term_hash(+Term, +Depth, +Range, -HashKey)

As term_hash/2, but only considers Term to the specified Depth. The top-level term has depth 1, its arguments have depth 2, etc. That is, Depth = 0 hashes nothing; Depth = 1 hashes atomic values or the functor and arity of a compound term, not its arguments; Depth = 2 also indexes the immediate arguments, etc.

HashKey is in the range [0 ...Range-1]. Range must be in the range [1 ... 2147483647].

[det]variant_sha1(+Term, -SHA1)

Compute a SHA1-hash from Term. The hash is represented as a 40-byte hexadecimal atom. Unlike term_hash/2 and friends, this predicate produces a hash key for non-ground terms. The hash is invariant over variable-renaming (see =@=/2) and constants over different invocations of Prolog.^{bugThe hash depends on word order (big/little-endian) and the wordsize (32/64 bits).}

This predicate raises an exception when trying to compute the hash on a cyclic term or attributed term. Attributed terms are not handled because subsumes_chk/2 is not considered well defined for attributed terms. Cyclic terms are not supported because this would require establishing a canonical cycle. That is, given A=[a|A] and B=[a,a|B], A and B should produce the same hash. This is not (yet) implemented.

This hash was developed for lookup of solutions to a goal stored in a table. By using a cryptographic hash, heuristic algorithms can often ignore the possibility of hash collisions and thus avoid storing the goal term itself as well as testing using =@=/2.

[det]variant_hash(+Term, -HashKey)

Similar to variant_sha1/2, but using a non-cryptographic hash and produces an integer result like term_hash/2. This version does deal with attributed variables, processing them as normal variables. This hash is primarily intended to speedup finding variant terms in a set of terms. ^{bugAs variant_sha1/2, cyclic terms result in an exception.}