[postgis-users] GIST index speed
Steve Kondik
shade at chemlab.org
Fri Jun 6 09:37:16 PDT 2008
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Steve Kondik wrote:
> Mark Cave-Ayland wrote:
>> Steve Kondik wrote:
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>>>
>>> I'm seeing this same kind of slowness. I am in the process of migrating
>>> an application from Oracle to Postgres. One query that seems to take
>>> longer than it should is a very simple point-in-bbox query which takes 3
>>> seconds. There are 32000 rows in the table, and the query is as simple
>>> as it can be:
>>>
>>> select count(*) from geography where type='Z' and centroid && (select
>>> geometry from geography where id=69495);
>>>
>>> The second geography is the USA, so this query gets all zipcodes inside
>>> the bbox, using precalculated centroids. The query plan looks fine as
>>> well:
>>>
>>> Aggregate (cost=16.56..16.57 rows=1 width=0) (actual
>>> time=2407.123..2407.125 rows=1 loops=1)
>>> InitPlan
>>> -> Index Scan using geography_pkey on geography (cost=0.00..8.28
>>> rows=1 width=4528) (actual time=0.027..0.030 rows=1 loops=1)
>>> Index Cond: (id = 69495)
>>> -> Index Scan using idx_geography_centroid_z on geography
>>> (cost=0.00..8.28 rows=1 width=0) (actual time=1.417..2355.049 rows=29687
>>> loops=1)
>>> Index Cond: (centroid && $0)
>>> Filter: (centroid && $0)
>>> Total runtime: 2407.194 ms
>>> (8 rows)
>>>
>>>
>>> Any thoughts? This query takes 200ms on Oracle. It seems like
>>> something is wrong for it to take 3 seconds on PostGIS. It's faster for
>>> smaller geometries (like a state), but I don't understand why since its
>>> just point-inside-bbox.
>
>> Hi Steve,
>
>> Well from the above you can see fairly easily that the time is
>> disappearing into the second index scan. The above output doesn't look
>> quite right somehow (it's missing at least one node indicator) - can you
>> try cutting and pasting the exact output from EXPLAIN into an email
>> again? Also we'll need the output of "\d geography" from within psql.
>
>
>> ATB,
>
>> Mark.
>
>
>
> Sure thing. It's taking quite a bit longer now because we're doing some
> bulk loading. When nothing else is running, the query takes about 4
> seconds.
>
>
> health_central_22=> explain analyze select count(*) from geography where
> type='Z' and centroid && (select geometry from geography where id=69495);
>
> QUERY PLAN
>
> -
> ---------------------------------------------------------------------------------------------------------------------------------------------------
> Aggregate (cost=16.56..16.57 rows=1 width=0) (actual
> time=16534.739..16534.741 rows=1 loops=1)
> InitPlan
> -> Index Scan using geography_pkey on geography (cost=0.00..8.28
> rows=1 width=4528) (actual time=0.076..0.080 rows=1 loops=1)
> Index Cond: (id = 69495)
> -> Index Scan using idx_geography_centroid_z on geography
> (cost=0.00..8.28 rows=1 width=0) (actual time=25.453..16192.427
> rows=29687 loops=1)
> Index Cond: (centroid && $0)
> Filter: (centroid && $0)
> Total runtime: 16534.809 ms
> (8 rows)
>
> health_central_22=> \d geography
> Table "public.geography"
> Column | Type | Modifiers
> --------------+------------------------+-----------
> type | character(1) | not null
> id | bigint | not null
> centroid | geometry |
> geometry | geometry |
> name | character varying(32) | not null
> population | bigint |
> abbreviation | character varying(2) |
> po_name | character varying(100) |
> id_geo_state | bigint |
> Indexes:
> "geography_pkey" PRIMARY KEY, btree (id)
> "idx_geography_2" btree (name)
> "idx_geography_3" btree (abbreviation)
> "idx_geography_4" btree (id_geo_state)
> "idx_geography_centroid" gist (centroid)
> "idx_geography_centroid_c" gist (centroid) WHERE type = 'C'::bpchar
> "idx_geography_centroid_s" gist (centroid) WHERE type = 'S'::bpchar
> "idx_geography_centroid_z" gist (centroid) WHERE type = 'Z'::bpchar
> "idx_geography_geometry" gist (geometry) CLUSTER
> "idx_geography_geometry_c" gist (geometry) WHERE type = 'C'::bpchar
> "idx_geography_geometry_r" gist (geometry) WHERE type = 'R'::bpchar
> "idx_geography_geometry_s" gist (geometry) WHERE type = 'S'::bpchar
> "idx_geography_geometry_z" gist (geometry) WHERE type = 'Z'::bpchar
> Foreign-key constraints:
> "fk_geography_1" FOREIGN KEY (id_geo_state) REFERENCES geography(id)
>
Adding force_2d speeds this up immensely. Not sure I understand why
this is necessary, but I'm still getting used to PostGIS.
health_central_22=> explain analyze select id,name from geography where
type='Z' and centroid && (select force_2d(geometry) from geography where
id=69495);
QUERY
PLAN
-
------------------------------------------------------------------------------------------------------------------------------------------
Index Scan using idx_geography_centroid on geography (cost=8.29..16.57
rows=1 width=14) (actual time=0.263..125.338 rows=29687 loops=1)
Index Cond: (centroid && $0)
Filter: ((centroid && $0) AND (type = 'Z'::bpchar))
InitPlan
-> Index Scan using geography_pkey on geography (cost=0.00..8.29
rows=1 width=4441) (actual time=0.158..0.161 rows=1 loops=1)
Index Cond: (id = 69495)
Total runtime: 171.229 ms
(7 rows)
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