如何将时间戳从一个范围缩放到另一范围？

Question

I've got table with timestamps of item changes:

• ID: INT
• item: INT - foreign key
• created: TIMESTAMP

I need for certain item to squeeze created timestamps to some range. Ie.:

• src range: 1.10.2015 00:00 - 4.10.2015 23:59
• dst range: 1.10.2015 00:00 - 1.10.2015 23:59
• so, sample TIMESTAMP would look like:
  • 2.10.2015 00:00 -> 1.10.2015 6:00
  • 3.10.2015 00:00 -> 1.10.2015 12:00
  • 4.10.2015 00:00 -> 1.10.2015 18:00
  • 4.10.2015 12:00 -> 1.10.2015 21:00

I need to keep year, month, day, hour, minute, seconds part of resulting timestamp. Must not round seconds/minutes to zeroes.

Scale precision doesn't matter much, but order of changes by created timestamp must be kept same as it was before scaling . I don't care much about other scaling details: whether it's start/final time, inclusive/exclusive, or whether it scales to range of 6:00/5:59 hours.

I can do it with some external application that would transform it on its own, then updates timestamps. However, I now need to do it using SQL only. Is it possible? It may be postgres specific.

You may assume, there won't be collision after scaling applies.

Answer 1

something like this?

select id, item, (' ['||created||', '||created_l- '00:00.000001'::time||') ' )::tsrange as some_range from (
select id, item, created  , lead(created) over(order by created)  created_l from my_table  order by created) q1

Answer 2

with r (src, dst) as ( values
    (
        tsrange('2015-10-01', '2015-10-05', '[)'),
        tsrange('2015-10-01', '2015-10-02', '[)')
    ),
    (
        tsrange('2015-10-06', '2015-10-10', '[)'),
        tsrange('2015-10-02', '2015-10-03', '[)')
    )
), t (id, src) as ( values
    (1,'2015-10-02 00:00'::timestamp),
    (2,'2015-10-03 00:00'),
    (3,'2015-10-04 00:00'),
    (4,'2015-10-04 12:00'),
    (5,'2015-10-06 11:00'),
    (6,'2015-10-07 15:00')
)
select
    t.id, t.src,
    lower(r.dst) + rank() over(
            partition by r.dst order by t.src
    ) * interval '1 second' as squeezed
from
    t
    inner join
    r on t.src <@ r.src
;
 id |         src         |      squeezed       
----+---------------------+---------------------
  1 | 2015-10-02 00:00:00 | 2015-10-01 00:00:01
  2 | 2015-10-03 00:00:00 | 2015-10-01 00:00:02
  3 | 2015-10-04 00:00:00 | 2015-10-01 00:00:03
  4 | 2015-10-04 12:00:00 | 2015-10-01 00:00:04
  5 | 2015-10-06 11:00:00 | 2015-10-02 00:00:01
  6 | 2015-10-07 15:00:00 | 2015-10-02 00:00:02