Recursive queries with two aggregate columns - postgresql

Question

I want to compute certain statistics montly in a postgres database
WITH RECURSIVE totals(start, t_end, null_count, not_null_count) AS (
VALUES (date_trunc('month', current_date + interval '1 month'),
date_trunc('month', current_date + interval '2 months'),
0::bigint)
UNION
SELECT start - interval '1 month' as start, start as t_end,
(SELECT count(*) filter (WHERE flag IS NULL) FROM tbl
WHERE created_at >= start
and created_at < t_end
and deleted_at < current_timestamp
) as null_count,
(SELECT count(*) filter (WHERE flag IS NOT NULL) FROM tbl
WHERE created_at >= start
and created_at < t_end
and deleted_at < current_timestamp
) as not_null_count
FROM totals
WHERE start > current_date - interval '1 year'
)
select * from totals
This would give me what I want, but would scan tbl twice.
Is there a way to to this scanning only once, like one would do in a plain query
SELECT count(*) filter (WHERE flag IS NULL) null_count,
count(*) filter (WHERE flag IS NOT NULL) not_null_count, FROM tbl
WHERE created_at >= start
AND created_at < t_end
AND deleted_at < current_timestamp
I know I could group by date_trunk('month', created_at) but doing that causes the query to sort the rows, and this is very costly in this case.

Answer 1

Yes. You can update your query to -
WITH RECURSIVE totals(start, t_end, null_count, not_null_count) AS (
VALUES (date_trunc('month', current_date + interval '1 month'),
date_trunc('month', current_date + interval '2 months'),
0::bigint)
UNION
SELECT start - interval '1 month' as start, start as t_end,
count(*) filter (WHERE flag IS NULL) as null_count,
count(*) filter (WHERE flag IS NOT NULL) as not_null_count
FROM tbl
JOIN totals ON created_at >= start
and created_at < t_end
WHERE start > current_date - interval '1 year'
AND deleted_at < current_timestamp
)
select * from totals