Filling gaps in postgresql - postgresql

Question

I have Actions table, which has rows ordered by time
| time | session |
|----------|-----------|
| 16:10:10 | session_1 |
| 16:13:05 | null |
| 16:16:43 | null |
| 16:23:12 | null |
| 16:24:01 | session_2 |
| 16:41:32 | null |
| 16:43:56 | session_3 |
| 16:51:22 | session_4 |
I want to write a select which will put previous meaningful value instead of nulls
How to get this result with postgresql?
| time | session |
|----------|-----------|
| 16:10:10 | session_1 |
| 16:13:05 | session_1 |
| 16:16:43 | session_1 |
| 16:23:12 | session_1 |
| 16:24:01 | session_2 |
| 16:41:32 | session_2 |
| 16:43:56 | session_3 |
| 16:51:22 | session_4 |

Answer 1

update Actions a
set session = (
select session
from Actions
where time = (
select max(time) from Actions b
where b.time < a.time and session is not null
)
) where session is null;
I tried this with 'time' as int and 'session' as int [easier to add data].
drop table Actions;
create table Actions (time int, session int);
insert into Actions values (1,10),(2,null),(3,null),(4,2),(5,null),(6,3),(7,4);
select * from Actions order by time;
update Actions a ...;
select * from Actions order by time;
EDIT
Response to your modified question.
select a1.time, a2.session
from Actions a1
inner join
Actions a2
on a2.time = (
select max(time) from Actions b
where b.time <= a1.time and session is not null
)