[postgis-users] ST_DWithin is not using SpatialIndex with Subquery

[Thomas Klemmer]

Hi hugues,

net_geom has a spatial index, the postam is just using the primary key on
this tabel since I am not useing any spatial filter to get the LINESTING
out of the table (just "where ogs_fid = 2" which is the primary key).

The cruzial part is the seq scen on the large point table which should not
be used since the ST_DWithin is a spatial filter thus the spatial index
should be used.

TK

2012/9/11 Francois Hugues <hugues.francois at irstea.fr>

> **
> I think your query does not use the index on net_geom because it is not a
> spatial index (not a gist one) on the geometry, but a btree index on the id.
>
> You should make the index (create index net_geom_gist on net_geom using
> gist(geom) ) and try again your query
>
> Hugues.
>
>
>
>  ------------------------------
> *De :* postgis-users-bounces at postgis.refractions.net [mailto:
> postgis-users-bounces at postgis.refractions.net] *De la part de* Thomas
> Klemmer
> *Envoyé :* mardi 11 septembre 2012 11:09
> *À :* postgis-users at postgis.refractions.net
> *Objet :* [postgis-users] ST_DWithin is not using SpatialIndex with
> Subquery
>
> Hi folks
>
> first of here some system informations:
>
> Server: Ubuntu 12.04 LTS, 16GB RAM 1TB 4x SSD HW Raid
>
> "PostgreSQL 9.1.5 on x86_64-unknown-linux-gnu, compiled by gcc
> (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3, 64-bit"
> "POSTGIS="2.0.0 r9605" GEOS="3.3.3-CAPI-1.7.4" PROJ="Rel. 4.7.1, 23
> September 2009" GDAL="GDAL 1.9.1, released 2012/05/15" LIBXML="2.7.8"
> LIBJSON="UNKNOWN" TOPOLOGY RASTER"
>
> Database:
>
> TableName:   PointTbl_v1
> Index:           idx_pdb_v1
> NumRows:    4.09806e+08
> TableSize:     280 GB
> IndexSize:     21 GB    [Gist on(point_pos)]
>
> GeometryCo:    point_pos
> Type:                POINT
> Dimensions:     2
> SRID:              4326
>
> /-----------------------------------------------
>
> TableName:    PointTbl_v5
> Index:             idx_pdb_v5
> NumRows:     4.16218e+07
> TableSize:     19 GB
> IndexSize:     2344 MB [Gist on(point_pos)]
> Primarykey:    false
>
> GeometryCo:   point_pos
> Type:              POINT
> Dimensions:    2
> SRID:             4326
>
> /-----------------------------------------------
>
> TableName:    NetTbl
> Index:            idx_net
> NumRows:     270615
> TableSize:     195 MB
> IndexSize:     17 MB
> Primarykey:   NetTbl_pk
>
>
> GeometryCo:    net_geom
> Type:                LINESTRING
> Dimensions:     2
> SRID:              4326
>
> Basically I'm trying to gather all points from PointTbl_v5 / PointTbl_v1
> that are close or on a LineString in NetTbl;
> Here ist the Query I'm trying to run:
>
> SELECT  ST_AsBinary(point_pos) AS point_pos, oid, ..., type
> FROM PointTbl_v5
> WHERE ST_DWithin(point_pos,(SELECT net_geom from NetTbl where ogc_fid =
> 500) ,5e-05);
>
> This query takes 319005 msec to return which is very slow due to the a seq
> scan done on PoinDB_v5
>
> EXPLAIN ANALYSE
>
> "Seq Scan on PointTbl_v5  (cost=10000000008.36..10013364820.01
> rows=13873927 width=202) (actual time=199926.978..318895.494 rows=5
> loops=1)"
> "  Filter: st_dwithin(point_pos, $0, 5e-05::double precision)"
> "  InitPlan 1 (returns $0)"
> "    ->  Index Scan using NetTbl_pk on NetTbl  (cost=0.00..8.36 rows=1
> width=847) (actual time=2.069..2.075 rows=1 loops=1)"
> "          Index Cond: (ogc_fid = 2)"
> "Total runtime: 318895.583 ms"
>
> The query:
>
> SELECT ST_AsText(net_geom) from NetTbl where ogc_fid = 2
>
> returns in 16 ms;
>
> If I instert the Geometry by Hand into the first query like this:
>
> SELECT  ST_AsBinary(point_pos) AS point_pos, oid, country, federalstate,
> district, town
> FROM PointTbl_v5
> WHERE ST_DWithin(point_pos, ST_GeomFromText(
> 'LINESTRING(....)',4326),5e-05);
>
> This query return in 63ms on the small table and 766ms on the bigger table.
>
> EXPLAIN ANALYSE
>
> "Index Scan using idx_pdb_v5 on PointTbl_v5  (cost=0.00..147.61 rows=1
> width=202) (actual time=0.047..1.050 rows=23 loops=1)"
> "  Index Cond: (point_pos && '0103.....A40'::geometry)"
> "  Filter: (('0102.....4A40'::geometry && st_expand(point_pos,
> 5e-05::double precision)) AND _st_dwithin(point_pos,
> '01020.....A40'::geometry, 5e-05::double precision))"
> "Total runtime: 1.080 ms"
>
> Does anybody have a clue why the first query with the subquery for the
> LineString Geometry withing ST_DWithin not using the Spatial index?
>
> seqscan_enabled is turned off...
>
> cheers Thomas
>
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