Find difference between timestamps in amount of custom intervals in PostgreSQL - postgresql

Question

I would like to find difference between two timestamps (with timezone) in amount of custom intervals. So function should be like custom_diff(timestamptz from, timestamptz to, interval custom).
Keep in mind, that it is not equivalent to (to-from)/custom (custom_diff('2016-08-01 00:00:00','2016-09-01 00:00:00','1 day') is exactly 31, but ('2016-08-01 00:00:00','2016-09-01 00:00:00')/'1 day')='1 month'/'1 day' and is ambiguous).
Also I understand that in general there is no exact result of such operation (custom_diff('2016-08-01 00:00:00','2016-09-01 00:00:00','1 month 1 day') so it is possible to have group of function (round-to-nearest, round-to-lower, round-to-upper and truncating, all of them should return integer number).
Is there any standard/common way for such calculation in PostgreSQL (PL/pgSQL)? My main interesting is round-to-nearest function.
The best way I have invented is to iteratively add/substract interval custom to/from timestamptz from and compare with timestamptz to. Also it can be optimized by initially finding approximate result (for example divide [difference in seconds between timestamps] for [approximation of interval custom in seconds]) to reduce amount of iterations.
UPD 1:
Why
SELECT EXTRACT(EPOCH FROM (timestamp '2016-08-01 10:00'
- timestamp '2016-08-01 00:00'))
/ EXTRACT(EPOCH FROM interval '1 day');
is a wrong solution: lets try yourself:
SELECT EXTRACT(EPOCH FROM ( TIMESTAMPTZ '2016-01-01 utc' -
TIMESTAMPTZ '1986-01-01 utc' ))
/ EXTRACT(EPOCH FROM INTERVAL '1 month');
Result is 365.23.... Then check result:
SELECT ( TIMESTAMPTZ '1986-01-01 utc' + 365 * INTERVAL '1 month' )
AT TIME ZONE 'utc';
Result is 2016-06-01 00:00:00.000000. Of cause 365 is wrong result, because timestamps in this example describe exactly 30 years and in any year always exactly 12 months, so right answer is 12*30=360.
UPD 2:
My solution is
CREATE OR REPLACE FUNCTION custom_diff(
_from TIMESTAMPTZ, _to TIMESTAMPTZ, _custom INTERVAL, OUT amount INTEGER)
RETURNS INTEGER
LANGUAGE plpgsql
AS $function$
DECLARE
max_iterations INTEGER :=10;
t INTEGER;
BEGIN
amount:=0;
WHILE max_iterations > 0 AND NOT (
extract(EPOCH FROM _to) <= ( extract(EPOCH FROM _from) + extract(EPOCH FROM _from + _custom) ) / 2
AND
extract(EPOCH FROM _to) >= ( extract(EPOCH FROM _from) + extract(EPOCH FROM _from - _custom) ) / 2
) LOOP
-- RAISE NOTICE 'iter: %', max_iterations;
t:=EXTRACT(EPOCH FROM ( _to - _from )) / EXTRACT(EPOCH FROM _custom);
_from:=_from + t * _custom;
amount:=amount + t;
max_iterations:=max_iterations - 1;
END LOOP;
RETURN;
END;
$function$
but I does not sure that it is correct and still waiting for sugestion about existing/common solution.

Answer 1

You can get exact result after extracting the epoch from both intervals:
SELECT EXTRACT(EPOCH FROM (timestamp '2016-08-01 10:00'
- timestamp '2016-08-01 00:00'))
/ EXTRACT(EPOCH FROM interval '1 day'); -- any given interval
If you want rounded (truncated) result, a simple option is to cast both to integer. Integer division cuts off the remainder.
SELECT EXTRACT(EPOCH FROM (ts_to - ts_from))::int
/ EXTRACT(EPOCH FROM interval '1 day')::int; -- any given interval
You can easily wrap the logic into a IMMUTABLE SQL function.
You are drawing the wrong conclusions from what you read in the manual. The result of a timestamp subtraction is an exact interval, storing only days and seconds (not months). So the result is exact. Try my query, it isn't "ambiguous".
You can avoid involving the data type interval:
SELECT EXTRACT(EPOCH FROM ts_to) - EXTRACT(EPOCH FROM ts_from))
/ 86400 -- = 24*60*60 -- any given interval as number of seconds
But the result is the same.
Aside:
"Exact" is an elusive term when dealing with timestamps. You may have to take DST rules and other corner cases of your time zone into consideration. You might convert to UTC time or use timestamptz before doing the math.