[postgis-users] ST_Intersection very slow.

[Rémi Cura]

Hey,
you could try to not use CASE (so separate the spatial join from the
processing, which is easy to parallelize (assuming you have more than one
core )).

First generate the table with
----
CREATE TABLE psatial_mapping_between_lancover_and_taz AS
SELECT row_number() over() as row_id, p.your_primary_key, n.your_primary_key
 FROM lancover_polygons_snap AS p
   INNER JOIN tazjan2 AS n
    ON (ST_Intersects(p.geom, n.the_geom)
      AND NOT ST_Touches(p.geom, n.the_geom) );
----

Then when you have your table, you can do your processing (put into a SQL
function , it is cleaner),
and you parallelize on the "row_id".

Basically you get min(row_id), ax(row_id), you spearate it into N parts,
then execute the parts with your K processes.
(both bash and python have utilities for this).

Of course if it is actually the spatial join that is slow, you can also
parallelise this.
(by cutting lancoverinto N pieces for instance )
Cheers,
Rémi-C

2015-02-19 19:45 GMT+01:00 John Abraham <jea at hbaspecto.com>:

> So I've was running this query for 866000 s (10 days) before I decided to
> kill it:
>
> create table taz_and_lancover_10_fast_2 as
> SELECT p.lc_class, n.taz
>  , CASE
>    WHEN ST_CoveredBy(p.geom, n.the_geom)
>    THEN p.geom
>    ELSE
>     ST_Multi(
>       ST_Intersection(p.geom,n.geom)
>       ) END AS geom
>  FROM lancover_polygons_snap AS p
>    INNER JOIN tazjan2 AS n
>     ON (ST_Intersects(p.geom, n.the_geom)
>       AND NOT ST_Touches(p.geom, n.the_geom) );
>
> Explain shows this, it's using the spatial index:
>
> "Nested Loop  (cost=0.00..310492.35 rows=483105 width=9973)"
> "  Join Filter: (_st_intersects(p.geom, n.the_geom) AND ((NOT (p.geom &&
> n.the_geom)) OR (NOT _st_touches(p.geom, n.the_geom))))"
> "  ->  Seq Scan on tazjan2 n  (cost=0.00..843.09 rows=2709 width=9493)"
> "  ->  Index Scan using lancover_polygons_snap_geom_idx on
> lancover_polygons_snap p  (cost=0.00..21.67 rows=5 width=480)"
> "        Index Cond: (geom && n.the_geom)"
>
> There are 2709 rows in tazjan2 and 998031 rows in lancover_polygons_snap,
> so I appreciate that it's a bit of a large problem.  But MapInfo was able
> to do it interactively in a few days and Geomedia was also able to do it in
> about a day.
>
> Both MapInfo and Geomedia ran out of memory (8GB machines) until the
> problem was broken into two regions (North and South), but postgresql seems
> to be chewing on the problem using only 400MB.  The interactive approach in
> MapInfo was to divide lancover_polygons_snap by lc_class, to further divide
> each region into about 10 subproblems.  Perhaps subprobleming this is the
> way to go?  Can't the query subproblem it based in the indices or would I
> have to do that manually?
>
> One potential thing I've realized is that a few of the geometries in
> tazjan2 are multipolygons, not single polygons.  But it's only a few.
> There are a few very large and complex polygons in lancover_polygons_snap,
> but again, most of the 998031 are simple small polygons, about half would
> be ST_CoveredBy the polygons in tazjan2 and most of the rest would only
> overlap two or three of the polygons in tazjan2.
>
> I must be doing something wrong.  Any hints?
>
> I have
>  max_connections set to 100 (currently only 11 connections active)
>  work_mem was defaulting to 1MB, I just bumped it to 256MB (machine has
> 32GB but it's Postgresql9.1 (x86), i.e. 32bit version
>  shared_buffers was 32MB, I am trying 512MB, could go higher
>  effective_cache_size was defaulting to 128MB, I am trying 20GB.
>  random_page_cost was 4.0, this is VMWare virtual server.  I am trying 2.0
>
> I'm trying an "explain analyze" with just 5 rows of tazjan2, a 5 x 998031
> intersection problem with indices shouldn't take that long, should it?
> It's been running for a little over an hour.  I'm wondering if the few
> complex polygons in lancover_polygons_snap are causing the slowness?  Is
> there some fast way to divide complex polygons, e.g. apply a 1km grid over
> them?
>
> PostGISfullversion(): "POSTGIS="2.0.3 r11128" GEOS="3.3.8-CAPI-1.7.8"
> PROJ="Rel. 4.8.0, 6 March 2012" GDAL="GDAL 1.9.2, released 2012/10/08"
> LIBXML="2.7.8" LIBJSON="UNKNOWN" (core procs from "2.0.3 r11132" need
> upgrade) TOPOLOGY (topology procs from "2.0.3 r11132" need upgrade) RASTER
> (raster procs from "2.0.3 r11132" need upgrade)"
>
> --
> John Abraham
>
>
>
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