You're ending up with something that's basically a carthesian product of
closebuildings and closebuildingdescriptions.
Your query looks like a simple join would serve just fine, something like:
prepare getmydata(real,real,real) AS (
select image, data
from info
inner join buildings on (buildings.id = info.building_id)
inner join buildingdescriptions on (buildingdescriptions.id =
buildings.description_id)
where ST_DWithin(position, 'POINT($1 $2)', $3)
)
On 3 January 2013 12:43, Opel Fahrer <opelfahre...@yahoo.de> wrote:
> I'm a noob in writing efficient Postgres queries, so I wrote a first
> function to query multiple linked tables using the PostGIS extension. The
> query should fetch data from multiple tables and finally give me a table
> with two columns. Here's the code:
>
> [code]
> prepare getmydata(real,real,real) AS (
> with
> closeby(id) AS (
> select buildingid from info where ST_DWithin(position, 'POINT($1
> $2)', $3)
> ),
> closebuildings(descriptionid,image) AS (
> select descriptionid,image from buildings where id IN (select * from
> closeby)
> ),
> closebuildingdescriptions(data) AS (
> select data from buildingdescriptions where id IN (select
> descriptionid from closebuildings)
> )
> select image,data from closebuildings,closebuildingdescriptions;
> );
> execute getmydata(0.0,0.0,10.0);
> [/code]
>
> Actually the problem is that this query is VERY slow, even if the database
> content is small (taking around 15 minutes or so). The problem seems to be
> that postgres has to make sure that for the select statement both columns
> have equal length. If I only do "select image from closebuildings", the
> results are delivered in 0.1 secs, a "select data from
> closebuildingdescriptions" is delivered in 7.8 secs.
>
> I ran an "explain analyze" call, but I can't make any sense from the
> output:
>
> [code]
> "Nested Loop (cost=7816.51..2636821.06 rows=131352326 width=36) (actual
> time=117.125..6723.014 rows=12845056 loops=1)"
> " CTE closeby"
> " -> Seq Scan on info (cost=0.00..1753.11 rows=186 width=4) (actual
> time=0.022..5.821 rows=1579 loops=1)"
> " Filter: (("position" &&
> '0103000020797F000001000000050000007D3F35DEAC512041E3A59BBC9BE153417D3F35DEAC512041E3A59BBCA0E153417D3F35DED4512041E3A59BBCA0E153417D3F35DED4512041E3A59BBC9BE153417D3F35DEAC512041E3A59BBC9BE15341'::geometry)
> AND ('0101000020797F00007D3F35DEC0512041E3A59B3C9EE15341'::geometry &&
> st_expand("position", 10::double precision)) AND _st_dwithin("position",
> '0101000020797F00007D3F35DEC0512041E3A59B3C9EE15341'::geometry, 10::double
> precision))"
> " CTE closebuildings"
> " -> Hash Semi Join (cost=6.04..4890.03 rows=1351 width=8) (actual
> time=54.743..61.025 rows=3584 loops=1)"
> " Hash Cond: (closebuildings.id = closeby.buildingid)"
> " -> Seq Scan on closebuildings (cost=0.00..4358.52 rows=194452
> width=12) (actual time=0.042..31.646 rows=194452 loops=1)"
> " -> Hash (cost=3.72..3.72 rows=186 width=4) (actual
> time=7.073..7.073 rows=1579 loops=1)"
> " Buckets: 1024 Batches: 1 Memory Usage: 56kB"
> " -> CTE Scan on closeby (cost=0.00..3.72 rows=186
> width=4) (actual time=0.023..6.591 rows=1579 loops=1)"
> " CTE closebuildingdescriptions"
> " -> Nested Loop (cost=30.40..1173.37 rows=97226 width=516) (actual
> time=117.103..1890.902 rows=3584 loops=1)"
> " -> HashAggregate (cost=30.40..32.40 rows=200 width=4) (actual
> time=63.529..66.176 rows=3584 loops=1)"
> " -> CTE Scan on closebuildings (cost=0.00..27.02
> rows=1351 width=4) (actual time=54.746..62.316 rows=3584 loops=1)"
> " -> Index Scan using buildingdescriptions_pkey on
> buildingdescriptions (cost=0.00..5.69 rows=1 width=520) (actual
> time=0.506..0.507 rows=1 loops=3584)"
> " Index Cond: (id = closebuildings.descriptionid)"
> " -> CTE Scan on closebuildingdescriptions (cost=0.00..1944.52
> rows=97226 width=32) (actual time=117.115..1901.993 rows=3584 loops=1)"
> " -> CTE Scan on closebuildings (cost=0.00..27.02 rows=1351 width=4)
> (actual time=0.000..0.536 rows=3584 loops=3584)"
> "Total runtime: 7870.567 ms"
> [/code]
>
>
> If anyone can come up with a solution or a suggestion how to solve this, I
> would highly appreciate it.
>
> Cheers
>
>
>
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--
If you can't see the forest for the trees,
Cut the trees and you'll see there is no forest.
