I need to calculate value of some column X based on some other columns of the current record and the value of X for the previous record (using some partition and order). Basically I need to implement query in the form
SELECT <some fields>,
<some expression using LAG(X) OVER(PARTITION BY ... ORDER BY ...) AS X
FROM <table>
This is not possible because only existing columns can be used in window function so I'm looking way how to overcome this.
Here is an example. I have a table with events. Each event has type and time_stamp.
create table event (id serial, type integer, time_stamp integer);
I wan't to find "duplicate" events (to skip them). By duplicate I mean the following. Let's order all events for given type by time_stamp ascending. Then
- the first event is not a duplicate
- all events that follow non duplicate and are within some time frame after it (that is their
time_stamp is not greater then time_stamp of the previous non duplicate plus some constant TIMEFRAME) are duplicates
- the next event which
time_stamp is greater than previous non duplicate by more than TIMEFRAME is not duplicate
- and so on
For this data
insert into event (type, time_stamp)
values
(1, 1), (1, 2), (2, 2), (1,3), (1, 10), (2,10),
(1,15), (1, 21), (2,13),
(1, 40);
and TIMEFRAME=10 result should be
time_stamp | type | duplicate
-----------------------------
1 | 1 | false
2 | 1 | true
3 | 1 | true
10 | 1 | true
15 | 1 | false
21 | 1 | true
40 | 1 | false
2 | 2 | false
10 | 2 | true
13 | 2 | false
I could calculate the value of duplicate field based on current time_stamp and time_stamp of the previous non-duplicate event like this:
WITH evt AS (
SELECT
time_stamp,
CASE WHEN
time_stamp - LAG(current_non_dupl_time_stamp) OVER w >= TIMEFRAME
THEN
time_stamp
ELSE
LAG(current_non_dupl_time_stamp) OVER w
END AS current_non_dupl_time_stamp
FROM event
WINDOW w AS (PARTITION BY type ORDER BY time_stamp ASC)
)
SELECT time_stamp, time_stamp != current_non_dupl_time_stamp AS duplicate
But this does not work because the field which is calculated cannot be referenced in LAG:
ERROR: column "current_non_dupl_time_stamp" does not exist.
So the question: can I rewrite this query to achieve the effect I need?
An alternative to a recursive approach is a custom aggregate. Once you master the technique of writing your own aggregates, creating transition and final functions is easy and logical.
State transition function:
create or replace function is_duplicate(st int[], time_stamp int, timeframe int)
returns int[] language plpgsql as $$
begin
if st is null or st[1] + timeframe <= time_stamp
then
st[1] := time_stamp;
end if;
st[2] := time_stamp;
return st;
end $$;
Final function:
create or replace function is_duplicate_final(st int[])
returns boolean language sql as $$
select st[1] <> st[2];
$$;
Aggregate:
create aggregate is_duplicate_agg(time_stamp int, timeframe int)
(
sfunc = is_duplicate,
stype = int[],
finalfunc = is_duplicate_final
);
Query:
select *, is_duplicate_agg(time_stamp, 10) over w
from event
window w as (partition by type order by time_stamp asc)
order by type, time_stamp;
id | type | time_stamp | is_duplicate_agg
----+------+------------+------------------
1 | 1 | 1 | f
2 | 1 | 2 | t
4 | 1 | 3 | t
5 | 1 | 10 | t
7 | 1 | 15 | f
8 | 1 | 21 | t
10 | 1 | 40 | f
3 | 2 | 2 | f
6 | 2 | 10 | t
9 | 2 | 13 | f
(10 rows)
Read in the documentation: 37.10. User-defined Aggregates and CREATE AGGREGATE.
Naive recursive chain knitter:
-- temp view to avoid nested CTE
CREATE TEMP VIEW drag AS
SELECT e.type,e.time_stamp
, ROW_NUMBER() OVER www as rn -- number the records
, FIRST_VALUE(e.time_stamp) OVER www as fst -- the "group leader"
, EXISTS (SELECT * FROM event x
WHERE x.type = e.type
AND x.time_stamp < e.time_stamp) AS is_dup
FROM event e
WINDOW www AS (PARTITION BY type ORDER BY time_stamp)
;
WITH RECURSIVE ttt AS (
SELECT d0.*
FROM drag d0 WHERE d0.is_dup = False -- only the "group leaders"
UNION ALL
SELECT d1.type, d1.time_stamp, d1.rn
, CASE WHEN d1.time_stamp - ttt.fst > 20 THEN d1.time_stamp
ELSE ttt.fst END AS fst -- new "group leader"
, CASE WHEN d1.time_stamp - ttt.fst > 20 THEN False
ELSE True END AS is_dup
FROM drag d1
JOIN ttt ON d1.type = ttt.type AND d1.rn = ttt.rn+1
)
SELECT * FROM ttt
ORDER BY type, time_stamp
;
Results:
CREATE TABLE
INSERT 0 10
CREATE VIEW
type | time_stamp | rn | fst | is_dup
------+------------+----+-----+--------
1 | 1 | 1 | 1 | f
1 | 2 | 2 | 1 | t
1 | 3 | 3 | 1 | t
1 | 10 | 4 | 1 | t
1 | 15 | 5 | 1 | t
1 | 21 | 6 | 1 | t
1 | 40 | 7 | 40 | f
2 | 2 | 1 | 2 | f
2 | 10 | 2 | 2 | t
2 | 13 | 3 | 2 | t
(10 rows)
This feels more like a recursive problem than windowing function. The following query obtained the desired results:
WITH RECURSIVE base(type, time_stamp) AS (
-- 3. base of recursive query
SELECT x.type, x.time_stamp, y.next_time_stamp
FROM
-- 1. start with the initial records of each type
( SELECT type, min(time_stamp) AS time_stamp
FROM event
GROUP BY type
) x
LEFT JOIN LATERAL
-- 2. for each of the initial records, find the next TIMEFRAME (10) in the future
( SELECT MIN(time_stamp) next_time_stamp
FROM event
WHERE type = x.type
AND time_stamp > (x.time_stamp + 10)
) y ON true
UNION ALL
-- 4. recursive join, same logic as base
SELECT e.type, e.time_stamp, z.next_time_stamp
FROM event e
JOIN base b ON (e.type = b.type AND e.time_stamp = b.next_time_stamp)
LEFT JOIN LATERAL
( SELECT MIN(time_stamp) next_time_stamp
FROM event
WHERE type = e.type
AND time_stamp > (e.time_stamp + 10)
) z ON true
)
-- The actual query:
-- 5a. All records from base are not duplicates
SELECT time_stamp, type, false
FROM base
UNION
-- 5b. All records from event that are not in base are duplicates
SELECT time_stamp, type, true
FROM event
WHERE (type, time_stamp) NOT IN (SELECT type, time_stamp FROM base)
ORDER BY type, time_stamp
There are a lot of caveats with this. It assumes no duplicate time_stamp for a given type. Really the joins should be based on a unique id rather than type and time_stamp. I didn't test this much, but it may at least suggest an approach.
This is my first time to try a LATERAL join. So there may be a way to simplify that moe. Really what I wanted to do was a recursive CTE with the recursive part using MIN(time_stamp) based on time_stamp > (x.time_stamp + 10), but aggregate functions are not allowed in CTEs in that manner. But it seems the lateral join can be used in the CTE.
