Индекс

CREATE INDEX idx_top_views
  ON album_photos
  USING btree
  (views, sex) WHERE participate_top;


Запрос

select photo_id from album_photos
where 
	participate_top and sex=1
order by views DESC
LIMIT 1


Explain запроса

Limit  (cost=1.47..1.48 rows=1 width=8)
  ->  Sort  (cost=1.47..1.48 rows=1 width=8)
        Sort Key: views
        ->  Seq Scan on album_photos  (cost=0.00..1.46 rows=1 width=8)
              Filter: (participate_top AND (sex = 1))

CREATE INDEX idx_top_sex_views
  ON album_photos
  USING btree
  ( sex, (-views)) WHERE participate_top;

select photo_id from album_photos
where 
	participate_top and sex= 1 
order by (-views) --DESC
LIMIT  1