A few days ago I posted a question on general concerning the new feature
SKIP LOCKED in PostgreSQL 9.5-dev.
For the orginal question can be found here:
http://www.postgresql.org/message-id/54F723C3.1020206@boerngen-schmidt.de
What I'm trying to archieve is to match a point from my data which meets
certain conditions to a given point. The given points are selected by
'Select id as "start" ....', then the left lateral join should go
through each given point and find a corresponding point which is not in
the same region and between 5 and 10 km away. If I did understand the
lateral join right, it will be executed like a loop for every previously
selected row in a non lateral statement.
The Lateral statement does return a result, which I do not expect. I
returns an end point multiple times for diverent start points. Why? I
thought, that the selected point by the lateral is locked by the FOR
UPDATE and if the lateral selects a point that is already locked it will
be skipped by the SKIP LOCKED.
SELECT start, destination, ST_Distance(start_geom, end_geom) AS distance_meter FROM ( SELECT id as "start",
geomas start_geom FROM de_sim_points_start WHERE NOT used AND rs = '057700032032' ORDER BY RANDOM()
LIMIT200 FOR UPDATE SKIP LOCKED ) AS s
LEFT JOIN LATERAL ( SELECT id as destination, geom as end_geom FROM de_sim_points_end WHERE NOT used AND rs
IN( SELECT sk.rs FROM de_commuter_kreise ck INNER JOIN de_shp_kreise sk ON sk.rs = ck.rs
ANDST_DWithin((SELECT ST_Union(geom) FROM
de_shp WHERE rs ='057700032032'), sk.geom, 5000) UNION SELECT cg.rs FROM de_commuter_gemeinden cg
INNER JOIN de_shp_gemeinden sg ON sg.rs = cg.rs AND ST_DWithin((SELECT ST_Union(geom) FROM
de_shp WHERE rs = '057700032032'), sg.geom, 5000) ) AND NOT ST_DWithin(geom, start_geom, 2000) AND
ST_DWithin(geom,
start_geom, 5000) FOR UPDATE SKIP LOCKED LIMIT 1
) AS e ON TRUE
What I think the query is doing:
1. It SELECTs the startpoints
2. Then for each row it selects in the lateral join a corresponding endpoint
2.1 skipping already locked endpoints and find the next not lock one
2.2 While it selects this endpoint it is lock due to the FOR UPDATE
3. Result is presented
BUT what happens is, that I get the same endpoint a couple of times. It
seems to me that the lateral join does not evaluate the SKIP LOCKED
right, since this endpoint occurs multiple times. Is this a bug or a
feature?
p.s.
I also would be glad if you guys could tell me a better way to match
points as I would really help me with my mastersthesis.
- Benjamin