Speed up query where results with count(*) = 0 are included - sql

Question

I have a table squitters with, amongst others, a column parsed_time. I want to know the number of records per hour for the last two days and used this query:
SELECT date_trunc('hour', parsed_time) AS hour , count(*)
FROM squitters
WHERE parsed_time > date_trunc('hour', now()) - interval '2 day'
GROUP BY hour
ORDER BY hour DESC;
This works, but hours with zero records do not appear in the result. I want to have hours
with zero records also in the result with a count equal to zero, so I wrote this query using the generate_series function:
SELECT bins.hour, count(squitters.parsed_time)
FROM generate_series(date_trunc('hour', now() - interval '2 day'), now(), '1 hour') bins(hour)
LEFT OUTER JOIN squitters ON bins.hour = date_trunc('hours', squitters.parsed_time)
GROUP BY bins.hour
ORDER BY bins.hour DESC;
This works, in the results are hour-bins with counts equal to zero, but is considerably slower.
How can I have the speed of the first query with the count=zero results of the second query?
(btw. there is an index on parsed_time)

Answer 1

You could try and change the join condition so no date function is applied on column parsed_time:
SELECT b.hour, COUNT(s.parsed_time) cnt
FROM generate_series(date_trunc('hour', now() - interval '2 day'), now(), '1 hour') b(hour)
LEFT OUTER JOIN squitters s
ON s.parsed_time >= b.hour
AND s.parsed_time < b.hours + interval '1 hour'
GROUP BY b.hour
ORDER BY b.hour DESC;
Alternatively, you could also try using a correlated subquery (or a lateral join) instead of a left join - this avoids the need for outer aggregation:
SELECT
b.hour,
(
SELECT COUNT(*)
FROM squitters s
WHERE s.parsed_time >= b.hour AND s.parsed_time < b.hours + interval '1 hour'
) cnt
FROM generate_series(date_trunc('hour', now() - interval '2 day'), now(), '1 hour') b(hour)
ORDER BY b.hour desc

Answer 2

You could take advantage of Common Table Expressions to divide your problem into small chunks:
WITH cte AS (
--First query your table
SELECT date_trunc('hour', parsed_time) AS sq_hour , count(*)
FROM squitters
WHERE parsed_time > date_trunc('hour', now()) - interval '2 day'
GROUP BY hour
ORDER BY hour DESC
), series AS (
--Create the series without the data returned from 1st query
SELECT
bins.series_hour,
0
FROM
generate_series(date_trunc('hour', now() - interval '2 day'), now(), '1 hour') bins(series_hour)
WHERE
series_hour not in (SELECT sq_hour FROM cte)
)
--Union the result
SELECT * FROM cte
UNION
SELECT * FROM series
ORDER BY 1