[postgis-users] Odd explain analyze estimates?

[strk at refractions.net]

I think this is still due to JOIN selectivity.
In the first query it cannot attempt a real estimate so a default
is returned.
In the second query it tries, and if I get it right it fails ;).

#define REALLY_DO_JOINSEL 0
in lwgeom_estimate.c around line 100, re-install and see how things
change.

--strk;

On Fri, Mar 04, 2005 at 03:46:49AM -0800, Ron Mayer wrote:
> 
> 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)
> 
> 
> 
> 
> 
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