osm_mapnik=> explain analyze select ST_Centroid(way) from planet_polygon where ST_IsValid(way) limit 100000;
                                                               QUERY PLAN
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 Limit  (cost=0.00..93404.04 rows=100000 width=1692) (actual time=117.619..13097.848 rows=100000 loops=1)
   ->  Seq Scan on planet_polygon  (cost=0.00..4270042.44 rows=4571582 width=1692) (actual time=117.617..13027.815 rows=100000 loops=1)
         Filter: st_isvalid(way)
 Total runtime: 13134.124 ms
(4 rows)