BEGIN;
CREATE TABLE dist ( x INTEGER NOT NULL );
INSERT INTO dist SELECT random()*50 FROM generate_series( 1, 5000000 );
COMMIT;
CREATE INDEX dist_x ON dist(x);
VACUUM ANALYZE dist;
EXPLAIN ANALYZE SELECT DISTINCT x FROM dist;
HashAggregate (cost=84624.00..84624.51 rows=51 width=4) (actual time=1840.141..1840.153 rows=51 loops=1)
-> Seq Scan on dist (cost=0.00..72124.00 rows=5000000 width=4) (actual time=0.003..573.819 rows=5000000 loops=1)
Total runtime: 1848.060 ms
O PG não pode (ainda) usar um índice para distinto (ignorando os valores idênticos), mas você pode fazer isso:
CREATE OR REPLACE FUNCTION distinct_skip_foo()
RETURNS SETOF INTEGER
LANGUAGE plpgsql STABLE
AS $$
DECLARE
_x INTEGER;
BEGIN
_x := min(x) FROM dist;
WHILE _x IS NOT NULL LOOP
RETURN NEXT _x;
_x := min(x) FROM dist WHERE x > _x;
END LOOP;
END;
$$ ;
EXPLAIN ANALYZE SELECT * FROM distinct_skip_foo();
Function Scan on distinct_skip_foo (cost=0.00..260.00 rows=1000 width=4) (actual time=1.629..1.635 rows=51 loops=1)
Total runtime: 1.652 ms
