Grouping by date, with 0 when count() yields no lines - postgresql

Question

I'm using Postgresql 9 and I'm fighting with counting and grouping when no lines are counted.
Let's assume the following schema :
create table views {
date_event timestamp with time zone ;
event_id integer;
}
Let's imagine the following content :
2012-01-01 00:00:05 2
2012-01-01 01:00:05 5
2012-01-01 03:00:05 8
2012-01-01 03:00:15 20
I want to group by hour, and count the number of lines. I wish I could retrieve the following :
2012-01-01 00:00:00 1
2012-01-01 01:00:00 1
2012-01-01 02:00:00 0
2012-01-01 03:00:00 2
2012-01-01 04:00:00 0
2012-01-01 05:00:00 0
.
.
2012-01-07 23:00:00 0
I mean that for each time range slot, I count the number of lines in my table whose date correspond, otherwise, I return a line with a count at zero.
The following will definitely not work (will yeld only lines with counted lines > 0).
SELECT extract ( hour from date_event ),count(*)
FROM views
where date_event > '2012-01-01' and date_event <'2012-01-07'
GROUP BY extract ( hour from date_event );
Please note I might also need to group by minute, or by hour, or by day, or by month, or by year (multiple queries is possible of course).
I can only use plain old sql, and since my views table can be very big (>100M records), I try to keep performance in mind.
How can this be achieved ?
Thank you !

Answer 1

Given that you don't have the dates in the table, you need a way to generate them. You can use the generate_series function:
SELECT * FROM generate_series('2012-01-01'::timestamp, '2012-01-07 23:00', '1 hour') AS ts;
This will produce results like this:
ts
---------------------
2012-01-01 00:00:00
2012-01-01 01:00:00
2012-01-01 02:00:00
2012-01-01 03:00:00
...
2012-01-07 21:00:00
2012-01-07 22:00:00
2012-01-07 23:00:00
(168 rows)
The remaining task is to join the two selects using an outer join like this :
select extract ( day from ts ) as day, extract ( hour from ts ) as hour,coalesce(count,0) as count from
(
SELECT extract ( day from date ) as day , extract ( hour from date ) as hr ,count(*)
FROM sr
where date>'2012-01-01' and date <'2012-01-07'
GROUP BY extract ( day from date ) , extract ( hour from date )
) AS cnt
right outer join ( SELECT * FROM generate_series ( '2012-01-01'::timestamp, '2012-01-07 23:00', '1 hour') AS ts ) as dtetable on extract ( hour from ts ) = cnt.hr and extract ( day from ts ) = cnt.day
order by day,hour asc;

Answer 2

This query will give you the output what your are looking for,
select to_char(date_event, 'YYYY-MM-DD HH24:00') as time, count (to_char(date_event, 'HH24:00')) as count from views where date(date_event) > '2012-01-01' and date(date_event) > '2012-01-07' group by time order by time;