[...]

> I could have sworn that PostgreSQL had this (by timestamp)
> but I can't find any documentation to support this "memory".
> At the time I was thinking "hey, that's not right!" ...
> (violation of the "no hidden information" rule in relational
> theory.) My apparently fake memory involves a WHERE
> clause with a timestamp not normally accessible causing rows
> to appear out of nowhere. If anyone else remembers this too,
> and can find said documentation, you'd make my day.

[...]
Looks like the no-longer-supported "time travel" feature in
PostgreSQL, does
it not ?

SELECT name, population
FROM cities['epoch', 'now']
WHERE name = 'Mariposa';

http://www.postgresql.org/docs/6.3/interactive/c0503.htm