Postgres 9.3

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create table demo ( id bigint, value int)
;

CREATE TABLE

insert into demo(value)
  select generate_series(1,10000)
;

INSERT 0 10000

create index demo_idx
  on demo(value, id)
;

CREATE INDEX

vacuum analyze demo;

VACUUM

explain (verbose)
select id from demo
  where value = 42
;

QUERY PLAN

Index Only Scan using demo_idx on public.demo  (cost=0.29..4.30 rows=1 width=8)

  Output: id

  Index Cond: (demo.value = 42)

EXPLAIN