请教一个逆地理编码的 PostgresSQL 语句优化问题

一个内网项目，需要获取到距离 GPS 坐标最近的两条路，道路 polyline 保存在 PostgresSQL+PostGIS 数据库里。现在我的查询语句是这样的形式：

 SELECT name, st_distance ( STGeomFromText ('POINT(108.862531 34.288909)') :: geography, geom :: geography ) as distance, id, st_asgeojson(geom) FROM road1_polyline WHERE name is not null UNION SELECT name, st_distance ( ST_GeomFromText ('POINT(108.862531 34.288909)') :: geography, geom :: geography ) as distance , id, st_asgeojson(geom) FROM road2_polyline WHERE name is not null ORDER BY distance ASC LIMIT 100 

总的数据记录数大概在 6w，执行时间 4-8s，explain 的输出如下：

Limit (cost=155093.26..155093.51 rows=100 width=358) -> Sort (cost=155093.26..155226.28 rows=53207 width=358) Sort Key: (_st_distance('0101000020E6100000513239B533375B40914259F8FA244140'::geography, (national_highway_polyline_clip.geom)::geography, '0'::double precision, true)) -> HashAggregate (cost=152527.66..153059.73 rows=53207 width=358) Group Key: national_highway_polyline_clip.name, (_st_distance('0101000020E6100000513239B533375B40914259F8FA244140'::geography, (national_highway_polyline_clip.geom)::geography, '0'::double precision, true)), national_highway_polyline_clip.id, (st_asgeojson(national_highway_polyline_clip.geom, 15, 0)) -> Append (cost=0.00..151995.59 rows=53207 width=358) -> Seq Scan on national_highway_polyline_clip (cost=0.00..6681.75 rows=2381 width=54) Filter: (name IS NOT NULL) -> Seq Scan on country_road_polyline_clip (cost=0.00..18560.35 rows=6620 width=58) Filter: (name IS NOT NULL) -> Seq Scan on city_fast_road_polyline_clip (cost=0.00..508.69 rows=180 width=68) Filter: (name IS NOT NULL) -> Seq Scan on highway_polyline_clip (cost=0.00..7240.14 rows=2566 width=70) Filter: (name IS NOT NULL) -> Seq Scan on level_nine_road_polyline_clip (cost=0.00..6616.24 rows=2362 width=59) Filter: (name IS NOT NULL) -> Seq Scan on other_road2_polyline_clip (cost=0.00..16308.71 rows=5172 width=59) Filter: (name IS NOT NULL) -> Seq Scan on provincial_highway_polyline_clip (cost=0.00..13497.61 rows=4811 width=59) Filter: (name IS NOT NULL) -> Seq Scan on sub_country_road2_polyline_clip (cost=0.00..82050.03 rows=29115 width=59) Filter: (name IS NOT NULL) 

求教如何优化？