Hi. Thanks for the comments. My data is right, and the result is exactly what i want, but as you say i think what causes the query to be slow is the ST_Intersection which creates the intersection between the vector grid (fishnet) and the country polygons. I will check with the postgis user list if they have any idea on how to speed up this query.
Best, Andreas 2011/3/8 Tom Lane <t...@sss.pgh.pa.us> > =?ISO-8859-1?Q?Andreas_For=F8_Tollefsen?= <andrea...@gmail.com> writes: > > This is a query i am working on now. It creates an intersection of two > > geometries. One is a grid of 0.5 x 0.5 decimal degree sized cells, while > the > > other is the country geometries of all countries in the world for a > certain > > year. > > Hm, are you sure your data is right? Because the actual rowcounts imply > that each country intersects about half of the grid cells, which doesn't > seem right. > > > priogrid=# EXPLAIN ANALYZE SELECT priogrid_land.gid, gwcode, > > ST_Intersection(pri > > ogrid_land.cell, cshapeswdate.geom) FROM priogrid_land, cshapeswdate > WHERE > > ST_Intersects(priogrid_land.cell, cshapeswdate.geom); > > QUERY > > PLAN > > > > -------------------------------------------------------------------------------- > > ------------------------------------------------------------------ > > Nested Loop (cost=0.00..12644.85 rows=43351 width=87704) (actual > > time=1.815..7 > > 074973.711 rows=130331 loops=1) > > Join Filter: _st_intersects(priogrid_land.cell, cshapeswdate.geom) > > -> Seq Scan on cshapeswdate (cost=0.00..14.42 rows=242 width=87248) > > (actual > > time=0.007..0.570 rows=242 loops=1) > > -> Index Scan using idx_priogrid_land_cell on priogrid_land > > (cost=0.00..7.1 > > 5 rows=1 width=456) (actual time=0.069..5.604 rows=978 loops=242) > > Index Cond: (priogrid_land.cell && cshapeswdate.geom) > > Total runtime: 7075188.549 ms > > (6 rows) > > AFAICT, all of the runtime is going into calculating the ST_Intersects > and/or ST_Intersection functions. The two scans are only accounting for > perhaps 5.5 seconds, and the join infrastructure isn't going to be > terribly expensive, so it's got to be those functions. Not knowing much > about PostGIS, I don't know if the functions themselves can be expected > to be really slow. If it's not them, it could be the cost of fetching > their arguments --- in particular, I bet the country outlines are very > large objects and are toasted out-of-line. There's been some past > discussion of automatically avoiding repeated detoastings in scenarios > like the above, but nothing's gotten to the point of acceptance yet. > Possibly you could do something to force detoasting in a subquery. > > regards, tom lane >
