Hello,
I have a rather big query which should match id's from various tables
together. To be able to use multiprocessing I'm currently using
postgresql 9.5-dev, because of the SKIP LOCKED feature.
SELECT
start,
destination,
ST_Distance(start_geom, end_geom) AS distance_meter
FROM (
SELECT id as "start", geom as start_geom
FROM de_sim_points_start
WHERE NOT used AND rs = '057700032032'
ORDER BY RANDOM()
LIMIT 200
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 AND ST_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?
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