SELECT
  t1.id,
  t1.tags,
  t1.linestring
FROM
    (SELECT
      relations.id,
      relations.tags,
      ways.linestring
    FROM
      public.relations,
      public.relation_members,
      public.ways
    WHERE
      relation_members.relation_id = relations.id AND
      ways.id = relation_members.member_id AND
      relation_members.member_type = 'W' AND
      relations.tags->'boundary' = 'administrative' AND
      relations.tags->'admin_level' = '6') as t1
WHERE
  (t1.linestring && (SELECT geom FROM search ) AND ST_Intersects(t1.linestring, (SELECT geom FROM search )))
;

-----------------------------------------------------------------

"QUERY PLAN"
"Nested Loop  (cost=2.48..19.61 rows=1 width=947)"
"  InitPlan 1 (returns $0)"
"    ->  Seq Scan on search  (cost=0.00..1.24 rows=24 width=32)"
"  InitPlan 2 (returns $1)"
"    ->  Seq Scan on search  (cost=0.00..1.24 rows=24 width=32)"
"  ->  Nested Loop  (cost=0.00..16.81 rows=1 width=915)"
"        ->  Index Scan using idx_ways_linestring on ways  (cost=0.00..8.53 rows=1 width=915)"
"              Index Cond: (linestring && $0)"
"              Filter: st_intersects(linestring, $1)"
"        ->  Index Scan using idx_relation_members_member_id_and_type on relation_members  (cost=0.00..8.28 rows=1 width=16)"
"              Index Cond: ((relation_members.member_id = ways.id) AND (relation_members.member_type = 'W'::bpchar))"
"  ->  Index Scan using pk_relations on relations  (cost=0.00..0.30 rows=1 width=40)"
"        Index Cond: (relations.id = relation_members.relation_id)"
"        Filter: (((relations.tags -> 'boundary'::text) = 'administrative'::text) AND ((relations.tags -> 'admin_level'::text) = '6'::text))"