Обсуждение: BUG #3607: timestamp is behaving strangely

The following bug has been logged online:

Bug reference:      3607
Logged by:          Cade Cairns
Email address:      cadec@otii.com
PostgreSQL version: 8.1.9
Operating system:   Mac OS X 10.4.10
Description:        timestamp is behaving strangely
Details:

test=> select '2007-05-11 09:33:01.167885'::timestamp without time zone,
('epoch'::timestamp without time zone + '1178875981.167885
seconds'::interval)::timestamp without time zone;
         timestamp          |         timestamp
----------------------------+----------------------------
 2007-05-11 09:33:01.167885 | 2007-05-11 09:33:01.167885
(1 row)

test=> select '2007-05-11 09:33:01.167885'::timestamp without time zone =
('epoch'::timestamp without time zone + '1178875981.167885
seconds'::interval)::timestamp without time zone;
 ?column?
----------
 f
(1 row)

Why?

"Cade Cairns" <cadec@otii.com> writes:
> test=> select '2007-05-11 09:33:01.167885'::timestamp without time zone,
> ('epoch'::timestamp without time zone + '1178875981.167885
> seconds'::interval)::timestamp without time zone;
>          timestamp          |         timestamp
> ----------------------------+----------------------------
>  2007-05-11 09:33:01.167885 | 2007-05-11 09:33:01.167885
> (1 row)

> test=> select '2007-05-11 09:33:01.167885'::timestamp without time zone =
> ('epoch'::timestamp without time zone + '1178875981.167885
> seconds'::interval)::timestamp without time zone;
>  ?column?
> ----------
>  f
> (1 row)

If you're using floating-point timestamps (which is the default build)
you are up against the limit of accuracy of a standard float8 value.
IOW that's not really exactly 09:33:01.167885, but close to it.
Checking this example shows that the two float timestamps internally
are 232191181.16788501 and 232191181.16788507.

            regards, tom lane