[postgis-users] Query PLanning for spatial containment

[Burgholzer, Robert (DEQ)]

I just noticed something interesting, perhaps its trivial and well understood, but this is the first time I figured it out (older postgis and postgresql to boot).  I am doing a spatial containment query on two tables, table "a" being a point table in SRID 4326, with GIST index, and table "b" being a polygon with GIST index in SRID 26918, using the "transform" function to bring them into a common projection.  If I apply the transform to the polygon layer, the query planner (and indeed the query) is verrrry slow, whereas, if I apply the transform to the point layer, things go along much more swiftly.  Now I know, so I just thought I'd share, if anyone has any suggestions of course, I would be delighted to hear them.

/r/b

For example:
  postgis_version
---------------------------------------
 1.5 USE_GEOS=1 USE_PROJ=1 USE_STATS=1 / PostgreSQL 8.3


                                          QUERY PLAN
-----------------------------------------------------------------------------------------------
 Nested Loop  (cost=0.00..16874.46 rows=75352 width=82234)
   Join Filter: contains(b.the_geom, transform(a.the_geom, 26918))
   ->  Seq Scan on sc_cbp53 b  (cost=0.00..65.09 rows=1 width=81986)
         Filter: ((riverseg)::text = 'PS3_5100_5080'::text)
   ->  Seq Scan on vpdes_discharge_no_ms4_cache a  (cost=0.00..13418.55 rows=226055 width=248)
(5 rows)

vpdes=# explain select * from vpdes_discharge_no_ms4_cache as a, sc_cbp53 as b where contains(transform(b.the_geom,4326),a.the_geom);                                               QUERY PLAN
-----------------------------------------------------------------------------------------------
 Nested Loop  (cost=0.00..18815543.18 rows=109033851 width=82234)
   Join Filter: contains(transform(b.the_geom, 4326), a.the_geom)
   ->  Seq Scan on vpdes_discharge_no_ms4_cache a  (cost=0.00..13418.55 rows=226055 width=248)
   ->  Seq Scan on sc_cbp53 b  (cost=0.00..61.47 rows=1447 width=81986)
(4 rows)

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