As of Postgres v6.2, time travel is no longer supported. There are several reasons for this: performance impact, storage size, and a pg_time file which grows toward infinite size in a short period of time.
New features such as triggers allow one to mimic the behavior of time travel when desired, without incurring the overhead when it is not needed (for most users, this is most of the time). See examples in the contrib directory for more information.
Time travel is deprecated: The remaining text in this section is retained only until it can be rewritten in the context of new techniques to accomplish the same purpose. Volunteers? - thomas 1998-01-12
Postgres supports the notion of time travel. This feature allows a user to run historical queries. For example, to find the current population of Mariposa city, one would query:
SELECT * FROM cities WHERE name = 'Mariposa'; +---------+------------+----------+ |name | population | altitude | +---------+------------+----------+ |Mariposa | 1320 | 1953 | +---------+------------+----------+Postgres will automatically find the version of Mariposa's record valid at the current time. One can also give a time range. For example to see the past and present populations of Mariposa, one would query:
SELECT name, population
FROM cities['epoch', 'now']
WHERE name = 'Mariposa';
where "epoch" indicates the beginning of the system clock.
Note: On UNIX systems, this is always midnight, January 1, 1970 GMT.
If you have executed all of the examples so far, then the above query returns:
+---------+------------+ |name | population | +---------+------------+ |Mariposa | 1200 | +---------+------------+ |Mariposa | 1320 | +---------+------------+
The default beginning of a time range is the earliest time representable by the system and the default end is the current time; thus, the above time range can be abbreviated as ``[,].''