[postgis-users] Odd explain analyze estimates?

[Ron Mayer]

I have a query that runs quite quickly using a hash join when run
standalone.  When I use this query as a subquery the planner always 
seems to pick a differnt plan with an order of magnitude worse 
performance.

This bad plan is chosen even when the outer sql statement is
a trivial expression like this:
   select * from (query) as a;
which I believe should be a no-op.



If I'm reading the "explain analyze" output correctly; it seems that 
depending on the context; in one case the optimizer thinks the 
comparison on GEOMETRIES will be extremely selective (returning 1 row)
while in the other case it think it will be quite unselective (returning 
15000+ rows).   I'm pretty sure that the two expressions are identical,
though, and can't guess why the estimates vary so drastically.  Am I
reading that explain analyze output correctly?  Or am I doing something
else wrong?   Thanks.


  Explain analyze output follows.
  Thanks,
  Ron



============================================================================

fli=# explain analyze SELECT * from (select * from userfeatures.points join icons using (iconid) where the_geom && setSRID('BOX3D(-123.40 25.66,-97.87 43.17)'::BOX3D, -1 )) as upf ;
                                                                                                              QUERY PLAN                                                                                                              
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=0.00..446.42 rows=1 width=120) (actual time=-0.096..7928.546 rows=15743 loops=1)
   Join Filter: ("outer".iconid = "inner".iconid)
   ->  Seq Scan on points  (cost=0.00..444.43 rows=1 width=82) (actual time=0.096..132.255 rows=15743 loops=1)
         Filter: (the_geom && '010300000001000000050000009A99999999D95EC0295C8FC2F5A839409A99999999D95EC0F6285C8FC295454048E17A14AE7758C0F6285C8FC295454048E17A14AE7758C0295C8FC2F5A839409A99999999D95EC0295C8FC2F5A83940'::geometry)
   ->  Seq Scan on icons  (cost=0.00..1.44 rows=44 width=42) (actual time=0.006..0.242 rows=44 loops=15743)
 Total runtime: 8005.766 ms
(6 rows)

fli=# explain analyze               select * from userfeatures.points join icons using (iconid) where the_geom && setSRID('BOX3D(-123.40 25.66,-97.87 43.17)'::BOX3D, -1 );
                                                                                                              QUERY PLAN                                                                                                              
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Hash Join  (cost=1.55..682.84 rows=15789 width=120) (actual time=0.641..320.002 rows=15743 loops=1)
   Hash Cond: ("outer".iconid = "inner".iconid)
   ->  Seq Scan on points  (cost=0.00..444.43 rows=15794 width=82) (actual time=0.067..94.307 rows=15743 loops=1)
         Filter: (the_geom && '010300000001000000050000009A99999999D95EC0295C8FC2F5A839409A99999999D95EC0F6285C8FC295454048E17A14AE7758C0F6285C8FC295454048E17A14AE7758C0295C8FC2F5A839409A99999999D95EC0295C8FC2F5A83940'::geometry)
   ->  Hash  (cost=1.44..1.44 rows=44 width=42) (actual time=0.530..0.530 rows=0 loops=1)
         ->  Seq Scan on icons  (cost=0.00..1.44 rows=44 width=42) (actual time=0.026..0.287 rows=44 loops=1)
 Total runtime: 397.003 ms
(7 rows)