PostgreSQL use value from previous row if missing

Question

I have a following query:

WITH t as (
  SELECT date_trunc(\'hour\', time_series) as trunc 
  FROM generate_series(\'2013-02-27 22:00\'::timestamp, \'2013-02-         


        

Answer 1

Try:

WITH t as (
  SELECT time_series as trunc 
    FROM generate_series('2013-02-27 22:00'::timestamp, '2013-02-28 2:00', 
                         '1 hour') as time_series
)
SELECT DISTINCT ON(t.trunc) t.trunc, e.id
  FROM t
  JOIN event e
    ON e.created < t.trunc 
 ORDER BY t.trunc, e.created DESC

If it is too slow - tell me. I will give you a faster query.

Answer 2

You ca mix a self join and windows functions

Simplifying I take this table with this sample values:

create table t ( a int, b int);    
insert into t values 
( 1, 1),
( 2, Null),
( 3, Null),
( 4, 2 ),
( 5, Null),
( 6, Null);

In your query a is trunc_u and b is your id. The query is:

with cte as (    
    select 
      t1.a, 
      coalesce( t1.b, t2.b, 0) as b,
      rank() OVER 
       (PARTITION BY t1.a ORDER BY t2.a DESC) as pos
    from t t1 
    left outer join t t2
      on t2.b is not null and
         t2.a < t1.a    
)
select a, b
from cte
where pos = 1;

And results:

| A | B |
---------
| 1 | 1 |
| 2 | 1 |
| 3 | 1 |
| 4 | 2 |
| 5 | 2 |
| 6 | 2 |