Can window function LAG reference the column which value is being calculated?

Question

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). Bas

Answer 1

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.