I have this trip GPS sampling table:
SELECT * FROM trajecttories_splitted;
user_id | session_id | timestamp | lat | lon | alt
---------+----------------+------------------------+-----------+------------+-----
1 | 20081023025304 | 2008-10-23 02:53:04+01 | 39.984702 | 116.318417 | 492
1 | 20081023025304 | 2008-10-23 02:53:10+01 | 39.984683 | 116.31845 | 492
1 | 20081023025304 | 2008-10-23 02:53:15+01 | 39.984686 | 116.318417 | 492
1 | 20081023025304 | 2008-10-23 02:53:20+01 | 39.984688 | 116.318385 | 492
1 | 20081023025304 | 2008-10-23 02:53:25+01 | 39.984655 | 116.318263 | 492
1 | 20081023025304 | 2008-10-23 02:53:30+01 | 39.984611 | 116.318026 | 493
1 | 20081023025304 | 2008-10-23 02:53:35+01 | 39.984608 | 116.317761 | 493
1 | 20081023025304 | 2008-10-23 02:53:40+01 | 39.984563 | 116.317517 | 496
1 | 20081023025304 | 2008-10-23 02:53:45+01 | 39.984539 | 116.317294 | 500
1 | 20081023025304 | 2008-10-23 02:53:50+01 | 39.984606 | 116.317065 | 505
Then for ease of analyses, I added a column sampling_rate to keep track of the GPS sampling intervals between consecutive rows.
Now I want to set the values of the new column to the difference: kind of row[sampling_rate]=timestamp -LAG(timestamp)
So I use:
UPDATE trajectories_splitted
SET sampling_rate=timestamp -LAG(timestamp) OVER (
PARTITION BY user_id
ORDER BY session_id
)
ERROR: window functions are not allowed in UPDATE
LINE 2: SET sampling_rate=timestamp -LAG(timestamp) OVER (
Expected results:
user_id | session_id | timestamp | lat | lon | alt | sampling_rate
---------+----------------+------------------------+-----------+------------+-----+---------------
1 | 20081023025304 | 2008-10-23 02:53:04+01 | 39.984702 | 116.318417 | 492 |
1 | 20081023025304 | 2008-10-23 02:53:10+01 | 39.984683 | 116.31845 | 492 | 6
1 | 20081023025304 | 2008-10-23 02:53:15+01 | 39.984686 | 116.318417 | 492 | 5
1 | 20081023025304 | 2008-10-23 02:53:20+01 | 39.984688 | 116.318385 | 492 | 10
1 | 20081023025304 | 2008-10-23 02:53:25+01 | 39.984655 | 116.318263 | 492 | 5
1 | 20081023025304 | 2008-10-23 02:53:30+01 | 39.984611 | 116.318026 | 493 | 5
1 | 20081023025304 | 2008-10-23 02:53:35+01 | 39.984608 | 116.317761 | 493 | 5
1 | 20081023025304 | 2008-10-23 02:53:40+01 | 39.984563 | 116.317517 | 496 | 5
1 | 20081023025304 | 2008-10-23 02:53:45+01 | 39.984539 | 116.317294 | 500 | 5
1 | 20081023025304 | 2008-10-23 02:53:50+01 | 39.984606 | 116.317065 | 505 | 5
EDIT
Based on the first answer below, postgres reports the error:
ERROR: column "sampling_rate" is of type timestamp with time zone but expression is of type interval
LINE 2: set sampling_rate = ts.timestamp - tts.prev_timestamp
If it might help, this is the table structure:
\d trajectories_splitted
Table "postgres.trajectories_splitted"
Column | Type | Collation | Nullable | Default
---------------+--------------------------+-----------+----------+---------
user_id | integer | | |
session_id | bigint | | not null |
timestamp | timestamp with time zone | | not null |
lat | double precision | | not null |
lon | double precision | | not null |
alt | double precision | | |
sampling_rate | timestamp with time zone | | |
Indexes:
"trajectories_splitted_pkey" PRIMARY KEY, btree (session_id, "timestamp")
"traj_splitted" btree (user_id, session_id, "timestamp")
You need to calculate the value in a subquery and join:
update trajectories_splitted ts
set sampling_rate = ts.timestamp - tts.prev_timestamp
from (select ts.*,
lag(timestamp) over (partition by user_id, session_id order by timestamp) as prev_timestamp
from trajectories_splitted ts
) tts
where tts.user_id = ts.user_id and tts.session_id = ts.session_id and
tts.timestamp = ts.timestamp