Postgresql compare consecutive rows and insert identical row if there are no values

I have a table that gives me data every 15 minute and I need that range of time. I noticed that sometimes I don't have data for 3/4 hours but I need to duplicate the last row available with the missing timestamp.

Example:

product_id	total_revenue	timestamp
1	50	01-01-2021 00:00:00
2	17	01-01-2021 00:00:00
3	30	01-01-2021 00:00:00
1	67	01-01-2021 00:15:00
2	31	01-01-2021 00:15:00
1	67	01-01-2021 00:30:00
2	31	01-01-2021 00:30:00
3	33	01-01-2021 00:30:00

But I need an output like:

product_id	total_revenue	timestamp
1	50	01-01-2021 00:00:00
2	17	01-01-2021 00:00:00
3	30	01-01-2021 00:00:00
1	67	01-01-2021 00:15:00
2	31	01-01-2021 00:15:00
3	30	01-01-2021 00:15:00
1	67	01-01-2021 00:30:00
2	31	01-01-2021 00:30:00
3	33	01-01-2021 00:30:00

I have a select statement like:

select product_id,total_revenue,timestamp from revenue

(I calculate the difference between two consecutive rows too).

Does anybody know how to help me?

Solution

One method uses generate_series() and lead():

with tt as (
      select product_id, total_revenue, timestamp,
             lead(timestamp) over (partition by product_id order by timestamp) as next_timestamp
      from t
     )
select tt.product_id, coalesce(gs.ts, tt.timestamp),
       tt.total_revenue
from tt left join lateral
     generate_series(timestamp, next_timestamp - interval '15 minute', interval '15 minute') gs(ts);

Note: My guess is that you also want this extended to the most recent timestamp in the table:

with tt as (
      select product_id, total_revenue, timestamp,
             lead(timestamp) over (partition by product_id order by timestamp) as next_timestamp,
             max(timestamp) over () as max_timestamp
      from t
     )
select tt.product_id, coalesce(gs.ts, tt.timestamp),
       tt.total_revenue
from tt left join lateral
     generate_series(timestamp,
                     coalesce(next_timestamp - interval '15 minute', max_timestamp),
                     interval '15 minute'
                    ) gs(ts);

Also, if the timestamps are not exactly at 15 minute intervals, then I would suggest that you ask a new question with explanation and more realistic sample data.