[postgis-users] Index Use Craziness
Mose
mose.andre at gmail.com
Wed Mar 18 12:31:32 PDT 2009
Robert,
I have been haunted by similar issues but I haven't had time to figure out
some explanation I really believe in.
Does this post with a reproducible case seem similar? It seems like it to
me.
http://postgis.refractions.net/pipermail/postgis-devel/2008-September/003547.html
For now when these situations aren't working (the planner favors the spatial
index due to a weird estimate when it is really going to be lots) I am using
a constant geometry [e.g. geometryfromwkt(wkt geometry string)] for the join
condition. I realize the feasibility of this depends on your application,
or your desire to write a user-defined function.
Why does the planner get different estimates for the same geometry being
used in a constraint based on it coming from a table, or from a string in
the query? I think the reason this is happening might be apparent in
lwgeom_estimate.c, but I'm open to being totally wrong :).
-Mose
On Wed, Mar 18, 2009 at 11:32 AM, Paragon Corporation <lr at pcorp.us> wrote:
> Robert,
>
> Which version of PostgreSQL are you running? I wander if you are being
> bitten by one of these bugs
>
> --the description doesn't quite match any of these 2 - but it could be
> another side effect of these
> http://www.postgresql.org/docs/8.3/static/release-8-3-7.html
>
> Fix planner problem with sub-SELECT in the output list of a larger subquery
> (Tom)
>
> Or
> http://www.postgresql.org/docs/8.3/static/release-8-3-6.html
> Fix planner misestimation of selectivity when transitive equality is
> applied
> to an outer-join clause (Tom)
>
> This could result in bad plans for queries like ... from a left join b on
> a.a1 = b.b1 where a.a1 = 42 ...
>
>
>
> Hope that helps,
> Regina
>
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
> Burgholzer,Robert
> Sent: Wednesday, March 18, 2009 9:05 AM
> To: PostGIS Users Discussion
> Subject: [postgis-users] Index Use Craziness
>
> I am having much trouble getting postgres to use the proper index.
>
> I have a table with about 20 million daily rainfall observation points,
> with
> between 0 and 19,000 points on any given day. In order to speed up
> querying, I have created a GIST index on the geometry column, and an index
> on the date column.
>
> What I want to do is to summarize the observed rainfall in another shape
> table on any given day. If I just want to query points by date, this uses
> the date index, and performs very quickly (see "CASE 1" below).
>
> When I try to introduce a geometry operator, the GIST index takes over, and
> the index on the "thisdate" column is ignored. The weird thing is, I can
> hear it churning the heck out of my hard drive checking on all the points.
> (See "CASE 2" below).
>
> I even tried to sub-query the points for the given date, but the index did
> not seem to be fooled ("CASE 3"). It gave me the exact same query plan!!
> I
> am unsure if I really understand the way a sub-query functions given this
> result.
>
> Any help would be appreciated!
>
> Robert
>
> CASE 1:
> wsp=# explain select count(*) from precip_gridded where thisdate =
> '2009-01-12';
>
> QUERY PLAN
> ------------------------------------------------------------------------
> ----
> Aggregate (cost=479.43..479.44 rows=1 width=0)
> -> Index Scan using pg_tdix on precip_gridded (cost=0.00..451.92
> rows=11004
> width=0)
> Index Cond: (thisdate = '2009-01-12 00:00:00'::timestamp without
> time zone)
> (3 rows)
>
>
> CASE 2:
> explain select count(*) from precip_gridded where thisdate = '2009-01-12'
> and the_geom && setsrid((select extent(the_geom) from proj_seggroups where
> gid = 448),4326);
>
> QUERY PLAN
> ------------------------------------------------------------------------
> ----
> Aggregate (cost=100.23..100.24 rows=1 width=0)
> InitPlan
> -> Aggregate (cost=5.87..5.88 rows=1 width=2807)
> -> Index Scan using psg_giix on proj_seggroups
> (cost=0.00..5.87 rows=1 width=2807)
> Index Cond: (gid = 448)
> -> Index Scan using pg_gix on precip_gridded (cost=0.00..94.35
> rows=1 width=0)
> Index Cond: (the_geom && setsrid(($0)::geometry, 4326))
> Filter: ((thisdate = '2009-01-12 00:00:00'::timestamp without time
> zone) AND (the_geom && setsrid(($0)::geometry, 4326)))
> (8 rows)
>
>
> CASE 3: Sub-query by date to try and force the use of date index wsp=#
> explain select count(a.*) from (select * from precip_gridded where thisdate
> = '2009-01-12') as a where a.the_geom && setsrid((select
> extent(the_geom) from proj_seggroups where gid = 448),4326);
>
> QUERY PLAN
>
> ------------------------------------------------------------------------
> ----
> Aggregate (cost=100.23..100.24 rows=1 width=81)
> InitPlan
> -> Aggregate (cost=5.87..5.88 rows=1 width=2807)
> -> Index Scan using psg_giix on proj_seggroups
> (cost=0.00..5.87 rows=1 width=2807)
> Index Cond: (gid = 448)
> -> Index Scan using pg_gix on precip_gridded (cost=0.00..94.35
> rows=1 width=81)
> Index Cond: (the_geom && setsrid(($0)::geometry, 4326))
> Filter: ((thisdate = '2009-01-12 00:00:00'::timestamp without time
> zone) AND (the_geom && setsrid(($0)::geometry, 4326)))
> (8 rows)
>
>
> Robert W. Burgholzer
> Surface Water Modeler
> Office of Water Supply and Planning
> Virginia Department of Environmental Quality rwburgholzer at deq.virginia.gov
> 804-698-4405
> Open Source Modeling Tools:
> http://sourceforge.net/projects/npsource/
>
>
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