Administración de PostgreSQL
Índices Parciales

Porción pequeña de tabla es interesante
alvh=# CREATE INDEX prefs ON refs(refsid) WHERE valor < 10000;
CREATE
alvh=#  EXPLAIN SELECT refsid FROM refs WHERE valor BETWEEN 8000 AND 9000;
NOTICE:  QUERY PLAN:
Index Scan using parcial_refs on refs  (cost=0.00..11.27 rows=41 width=4)
EXPLAIN