query plans

Sort  (cost=28944864.69..28944865.19 ROWS=200 width=8)
  Sort KEY: x.t1
  ->  HashAggregate  (cost=28944855.04..28944857.04 ROWS=200 width=8)
        ->  Nested Loop LEFT JOIN  (cost=19163.59..28288859.49 ROWS=131199111 width=8)
              JOIN FILTER: ((t.fecha_hora >= x.t1) AND (t.fecha_hora < (x.t1 + '00:15:00'::INTERVAL)))
              ->  FUNCTION Scan ON generate_series x  (cost=0.00..10.00 ROWS=1000 width=8)
              ->  Materialize  (cost=19163.59..67574.47 ROWS=1180792 width=8)
                    ->  Bitmap Heap Scan ON tweets t  (cost=19163.59..57057.51 ROWS=1180792 width=8)
                          ->  Bitmap INDEX Scan ON tweets_date_idx  (cost=0.00..18868.39 ROWS=1180792 width=0)


Sort  (cost=897629.28..897629.78 ROWS=200 width=8)
  Sort KEY: x.t1
  ->  HashAggregate  (cost=897619.64..897621.64 ROWS=200 width=8)
        ->  Nested Loop LEFT JOIN  (cost=756.33..877896.30 ROWS=3944667 width=8)
              JOIN FILTER: ((t.fecha_hora >= x.t1) AND (t.fecha_hora < (x.t1 + '00:15:00'::INTERVAL)))
              ->  FUNCTION Scan ON generate_series x  (cost=0.00..10.00 ROWS=1000 width=8)
              ->  Materialize  (cost=756.32..29074.05 ROWS=35502 width=8)
                    ->  Bitmap Heap Scan ON tweets t  (cost=756.32..28757.54 ROWS=35502 width=8)
                          Recheck Cond: ((fecha_hora >= '2014-12-02 00:00:00'::TIMESTAMP WITHOUT TIME zone) AND (fecha_hora <= '2014-12-03 00:00:00'::TIMESTAMP WITHOUT TIME zone))
                          ->  Bitmap INDEX Scan ON tweets_date_idx  (cost=0.00..747.45 ROWS=35502 width=0)
                                INDEX Cond: ((fecha_hora >= '2014-12-02 00:00:00'::TIMESTAMP WITHOUT TIME zone) AND (fecha_hora <= '2014-12-03 00:00:00'::TIMESTAMP WITHOUT TIME zone))