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PostgreSQL Timestamps - Indexing

I run a query where I am looking for a record, and another record after a while.

Table definition:

(
  id integer primary key,
  gpsstatus character(2),
  datetime timestamp without time zone,
  lat numeric(9,6),
  lon numeric(9,6),
  alt numeric(9,4),
  time integer,
  datafileid integer,
  shape geometry,
  speed double precision,
  dist double precision,
  shape_utm geometry,
  lokalitet character(128),
  cowid integer
)

There are indices in datetime, lokalitet, cowid, gpsstatus, gist-index in shape and shape_utm.

Points should be sampled every 5 seconds, so I tried to do

select <something more>,p1.timestamp 
from table p1, table p2 
where p1.timestamp + interval '5 secound' = p2.timestamp

This went fast enough, but then I found out that I had lost quite a lot of points due to jitter in the sample, so the points could be 4 to 6 seconds apart.

Then I tried:

where    (p2.timestamp, interval'0 second')
overlaps (p1.timestamp + interval '4 second', interval '2 second')

and it took forever. I also tried a simpler solution:

WHERE p1.timestamp + interval '4 second' <= p2.timestamp
AND   p1.timestamp + interval '6 second' >= p2.timestamp

which also turned out to be unusually slow.

The timestamp field has a normal index. Is there a special type of index - is it something else that will make this query usable?

Request at the moment:

SELECT
    p1.cowid,
    p1.datetime,
    st_distance(p1.shape_utm, lead(p1.shape_utm)
      OVER (ORDER BY p1.datetime)) AS meters_obs,
    st_distance(p1.shape_utm, lead(p1.shape_utm, 720)
      OVER (ORDER BY p1.datetime)) AS meters_hour,
    observation.observation
  FROM (gpspoint p1 LEFT JOIN observation
                           ON (observation.gpspointid = p1.id)),
       status
  WHERE p1.gpsstatus = status.id
    AND status.use = true;

, .

+3
1

, :

SELECT  p, LAG(p) OVER (ORDER BY timestamp) AS pp
FROM    table p
ORDER BY
        timestamp

4 6 , :

SELECT  p1.*, p2.*
FROM    table p1
LEFT JOIN
        table p2
ON      p2.timestamp BETWEEN p1.timestamp - '4 seconds'::INTERVAL
                         AND p1.timestamp - '6 seconds'::INTERVAL
ORDER BY
        p1.timestamp

, .

+6

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