select * from depesz;

nixternal wrote about boost library for c++.

with it he was able to find the answer to title question in miliseconds (he didn’t specify how many, but let’s assume that is was less than 10 ms).

so i decided to check how fast can i do it in postgresql …

first, i will need list of all days from given date range (1901-01-01 to 2000-12-31):

select
'1901-01-01'::date + i as date
FROM
generate_series(0, '2000-12-31'::date - '1901-01-01'::date) as i
;

now i need to check which of these days are 1st days of month:

select
'1901-01-01'::date + i as date
FROM
generate_series(0, '2000-12-31'::date - '1901-01-01'::date) as i
WHERE
extract(day FROM '1901-01-01'::date + i) = 1
;

and the “sunday” question:

select
'1901-01-01'::date + i as date
FROM
generate_series(0, '2000-12-31'::date - '1901-01-01'::date) as i
WHERE
extract(day FROM '1901-01-01'::date + i) = 1 and extract(dow FROM '1901-01-01'::date + i) = 0
;

great, the code works ok, so now, let’s get count, and time the query:

# SELECT
=# count(*)
=# FROM
=# generate_series(0, '2000-12-31'::date - '1901-01-01'::date) as i
=# WHERE
=# extract(day FROM '1901-01-01'::date + i) = 1 AND extract(dow FROM '1901-01-01'::date + i) = 0
=# ;
count
-------
171
(1 row)
Time: 59.698 ms

not bad, but i’d like to get the time a bit lower.

perhaps i should go with single comparison with “to_char” ?

# SELECT
=# count(*)
=# FROM
=# generate_series(0, '2000-12-31'::date - '1901-01-01'::date) as i
=# WHERE
=# to_char('1901-01-01'::date + i, 'D DD') = '1 01'
=# ;
count
-------
171
(1 row)
Time: 113.314 ms

oops. single to_char() is slower than 2 separate extract()s.

what else can i do?

i know. let’s use the fact that i can add intervals, and not only days. theoretically if i’ll add ‘1 month’ i should stay on 1st day of the month. quick test:

# select '1901-01-01'::date + '1 month'::interval * i from generate_series(1,10) i;
?column?
---------------------
1901-02-01 00:00:00
1901-03-01 00:00:00
1901-04-01 00:00:00
1901-05-01 00:00:00
1901-06-01 00:00:00
1901-07-01 00:00:00
1901-08-01 00:00:00
1901-09-01 00:00:00
1901-10-01 00:00:00
1901-11-01 00:00:00
(10 rows)

looks cool.

so, how to use it?

basic approach:

# SELECT
count(*)
FROM
generate_series(0, ('2000-12-31'::date - '1901-01-01'::date)/28) as i
WHERE
extract(day FROM '1901-01-01'::date + i * '1 month'::INTERVAL) = 1 AND extract(dow FROM '1901-01-01'::date + i * '1 month'::INTERVAL) = 0
AND '1901-01-01'::date + i * '1 month'::INTERVAL < '2000-12-31'::date
;
count
-------
171
(1 row)
Time: 6.083 ms

shows success

now, it would be cool to de-uglify the query, and perhaps make it so it will need to have dates specified only once:

# select count(*)
from (
select
cast(start.date + '1 month'::INTERVAL * generate_series(0, (finish.date - start.date)/28) as date) as date,
finish.date as "finish"
from
(select '2000-12-31'::date as date) as finish, (select '1901-01-01'::date as date) as start
) x
where 1 = extract(day from x.date) and 0 = extract(dow from x.date) AND x.date < x.finish
;
count
-------
171
(1 row)
Time: 4.905 ms

it’s a bit faster this some as date + interval additions are done only once for each month.

in general - i was able to write a 5ms query to calculate “number of sundays since 1901″, in interpreted language. and the language/engine is not even calendar centric (designed to work mostly with dates).

it’s just one of the things why i love postgresql.