Postgres 14

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create table test(day integer primary key, growth_rate float, target float) ;

insert into test values
(1,	.2,	null),
(2,	.3,	null),
(3,	.4,	null),
(4,	.2,	null),
(5,	.1,	1000);
5 rows affected

CREATE OR REPLACE FUNCTION division (x anyelement, y anyelement, z anyelement)
RETURNS anyelement LANGUAGE sql AS $$
SELECT COALESCE(x,y)/z $$ ;

CREATE OR REPLACE AGGREGATE division_agg (x anyelement, z anyelement)
(sfunc = division, stype = anyelement) ;
SELECT day
     , growth_rate
     , COALESCE(division_agg(target, growth_rate) OVER (ORDER BY day DESC ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING), target)
  FROM test


day
growth_rate
coalesce


5
0.1
1000

4
0.2
10000

3
0.4
50000

2
0.3
125000

1
0.2
416666.6666666667