PL/pgSQL: Comparing Successive Rows - postgresql

Question

I have the function, get_untracked_moves, below. My goal is to, for all data between two date ranges, find successive events which are farther than p_separation_distance apart.
E.g.:
If event 1 and event 2 are 40 m apart when p_separation_distance is 100m, a record would be returned with event 1's associated cont_name as the source_name and event 2's cont_name as the target_name.
CREATE FUNCTION get_untracked_moves(IN p_since_date TIMESTAMP WITHOUT TIME ZONE, IN p_before_date TIMESTAMP WITHOUT TIME ZONE, IN p_separation_distance INTEGER)
RETURNS TABLE ( id INTEGER,
asset_name CHARACTER VARYING,
source_name CHARACTER VARYING,
target_name CHARACTER VARYING,
source_time TIMESTAMP WITHOUT TIME ZONE,
target_time TIMESTAMP WITHOUT TIME ZONE,
source_lat DOUBLE PRECISION,
source_lon DOUBLE PRECISION,
target_lat DOUBLE PRECISION,
target_lon DOUBLE PRECISION ) AS $$
DECLARE
d_previous_location GEOMETRY;
d_previous_name CHARACTER VARYING;
d_previous_time TIMESTAMP WITHOUT TIME ZONE;
d_cur record;
BEGIN
-- Begin # 0,0
d_previous_location := st_setsrid(st_makepoint(0,0), 4326);
d_previous_name := '';
d_previous_time := NULL;
FOR d_cur
IN
SELECT
rank() OVER (PARTITION BY events.asset_id ORDER BY events.event_time) AS idx,
tags.id asset_id,
tags.name asset_name,
d_previous_name,
conts.name cont_name,
events.position,
events.event_time evt_time
FROM
events
JOIN
assets tags ON tags.id = events.asset_id
JOIN
assets conts ON conts.id = events.container_asset_id
WHERE
events.event_time >= p_since_date
AND
events.event_time <= p_before_date
LOOP
IF (d_previous_time = NULL) THEN
d_previous_time := events.event_time;
END IF;
IF (st_distancesphere(events.position, d_previous_location)>=p_separation_distance) THEN
RETURN NEXT;
END IF;
d_previous_location := events.position;
d_previous_name := conts.name;
d_previous_time := events.event_time;
END LOOP;
END;
$$
LANGUAGE plpgsql VOLATILE;
The function creates fine, but when I go to run it with:
select * from get_untracked_moves('2015-11-1', '2015-12-1', 10000);
I get:
ERROR: missing FROM-clause entry for table "events"
LINE 1: SELECT (st_distancesphere(events.position, d_previous_locati...
^
QUERY: SELECT (st_distancesphere(events.position, d_previous_location)>=p_separation_distance)
CONTEXT: PL/pgSQL function "get_untracked_moves" line 41 at IF
********** Error **********
ERROR: missing FROM-clause entry for table "events"
SQL state: 42P01
Context: PL/pgSQL function "get_untracked_moves" line 41 at IF
What am I missing here? I thought the inclusion of FROM events in my SELECT statement was enough.

Answer 1

Each pass of the loop is given the value of the record containing the corresponding row of the select result set. So events is not visible inside the loop. In instead use d_cur.position to refer to that column.
BTW, as commented to your question, you should really use the lag window function and get rid of the messy loop.
As a suggestion check this query:
select idx, asset_id, asset_name, previous_name, cont_name, position, evt_time
from (
select
rank() over (partition by e.asset_id order by e.event_time) as idx,
st_distancesphere(
e.position,
lag(e.position, 1, e.position) over (order by e.event_time)
) >= p_separation_distance as b,
t.id as asset_id,
t.name as asset_name,
lag(c.name, 1) as previous_name,
c.name as cont_name,
e.position,
e.event_time as evt_time
from
events e
inner join
assets tags on t.id = e.asset_id
inner join
assets c on c.id = e.container_asset_id
where
e.event_time >= p_since_date
and
e.event_time <= p_before_date
) s
where b