CREATE TABLE users
(
userid bigserial NOT NULL,
location integer NOT NULL,
CONSTRAINT pk_users PRIMARY KEY (userid)
)
WITH (
OIDS=FALSE
);

INSERT INTO users (userid,location) SELECT -1*GENERATE_SERIES(1,1000000),-1;

CREATE INDEX idx_users_location
  ON users
  USING btree
  (location);

UPDATE users SET location=76543 WHERE userid=-100092;
UPDATE users SET location=76543 WHERE userid=-997000;