osm_mapnik=> explain analyze select way from planet_polygon limit 100000;
                                                             QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..6051.34 rows=100000 width=1692) (actual time=0.015..628.778 rows=100000 loops=1)
   ->  Seq Scan on planet_polygon  (cost=0.00..829926.48 rows=13714748 width=1692) (actual time=0.013..572.862 rows=100000 loops=1)
 Total runtime: 657.149 ms
(3 rows)



osm_mapnik=> explain analyze select way from planet_polygon where ST_IsValid(way) limit 100000;
                                                             QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..93154.04 rows=100000 width=1692) (actual time=0.110..8719.331 rows=100000 loops=1)
   ->  Seq Scan on planet_polygon  (cost=0.00..4258613.48 rows=4571582 width=1692) (actual time=0.109..8650.261 rows=100000 loops=1)
         Filter: st_isvalid(way)
 Total runtime: 8754.785 ms
(4 rows)



osm_mapnik=> explain analyze select ST_PointOnSurface(way) from planet_polygon where ST_IsValid(way) limit 100000;
                                                             QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..118154.04 rows=100000 width=1692) (actual time=0.516..30578.418 rows=100000 loops=1)
   ->  Seq Scan on planet_polygon  (cost=0.00..5401508.98 rows=4571582 width=1692) (actual time=0.513..30505.566 rows=100000 loops=1)
         Filter: st_isvalid(way)
 Total runtime: 30616.056 ms
(4 rows)