cancancan with union

with_union

-- execution time: 74ms

SELECT DISTINCT proposals.*
FROM (SELECT "proposals".*
      FROM "proposals"
      WHERE "proposals"."private" = 'f'
      UNION SELECT "proposals".*
            FROM "proposals"
            WHERE "proposals"."visible_outside" = 't'
      UNION SELECT "proposals".*
            FROM "proposals"
              INNER JOIN "group_proposals" ON "group_proposals"."proposal_id" = "proposals"."id"
              INNER JOIN "groups" ON "groups"."id" = "group_proposals"."group_id"
              INNER JOIN "group_participations" ON "group_participations"."group_id" = "groups"."id"
              INNER JOIN "participation_roles"
                ON "participation_roles"."id" = "group_participations"."participation_role_id"
            WHERE "group_participations"."user_id" = 55 AND "participation_roles"."view_proposals" = 't'
      UNION SELECT "proposals".*
            FROM "proposals"
              INNER JOIN "area_proposals" ON "area_proposals"."proposal_id" = "proposals"."id"
              INNER JOIN "group_areas" ON "group_areas"."id" = "area_proposals"."group_area_id"
              INNER JOIN "area_participations" ON "area_participations"."group_area_id" = "group_areas"."id"
              INNER JOIN "area_roles" ON "area_roles"."id" = "area_participations"."area_role_id"
            WHERE "area_participations"."user_id" = 55 AND "area_roles"."view_proposals" = 't'
      UNION SELECT "proposals".*
            FROM "proposals"
              INNER JOIN "group_proposals" ON "group_proposals"."proposal_id" = "proposals"."id"
              INNER JOIN "groups" ON "groups"."id" = "group_proposals"."group_id"
              INNER JOIN "group_participations" ON "group_participations"."group_id" = "groups"."id"
            WHERE "group_participations"."user_id" = 55 AND "group_participations"."participation_role_id" = 2
      UNION SELECT "proposals".*
            FROM "proposals" EXCEPT SELECT "proposals".*
                                    FROM "proposals"
                                    WHERE "proposals"."private" = 't' AND "proposals"."visible_outside" = 'f' AND
                                          "proposals"."area_private" = 't') AS proposals;

without_union

-- execution time: ~1 minute
SELECT DISTINCT proposals.*
FROM "proposals"
  LEFT OUTER JOIN "group_proposals" ON "group_proposals"."proposal_id" = "proposals"."id"
  LEFT OUTER JOIN "groups" ON "group_proposals"."group_id" = "groups"."id"
  LEFT OUTER JOIN "group_participations" ON "group_participations"."group_id" = "groups"."id"
  LEFT OUTER JOIN "area_proposals" ON "area_proposals"."proposal_id" = "proposals"."id"
  LEFT OUTER JOIN "group_areas" ON "area_proposals"."group_area_id" = "group_areas"."id"
  LEFT OUTER JOIN "area_participations" ON "area_participations"."group_area_id" = "group_areas"."id"
  LEFT OUTER JOIN "area_roles" ON "area_participations"."area_role_id" = "area_roles"."id"
  LEFT OUTER JOIN "participation_roles" ON "group_participations"."participation_role_id" = "participation_roles"."id"

WHERE
  (("group_participations"."user_id" = 676 AND "group_participations"."participation_role_id" = 2) OR
   (("area_participations"."user_id" = 676 AND "area_roles"."view_proposals" = 't') OR
    (NOT ("proposals"."private" = 't' AND "proposals"."visible_outside" = 'f' AND "proposals"."area_private" = 't')
     AND (("group_participations"."user_id" = 676 AND "participation_roles"."view_proposals" = 't') OR
          (("proposals"."visible_outside" = 't') OR ("proposals"."private" = 'f'))))));