Kak to u Vas nemnogo putano vse napisano.

No delo v tom, chto current_timestamp vsegda ukazyvaet na nachalo transactsii i ne meniatsia vnutri nee. Tak chto esli u vas transactsia dlitsia nedeliu, to current_timestamp v kontse transactsii budet imet' to je znachenie chto v ee nachale -- tak trebuet SQL standart.

JelisА если вы хотите время именно текущее("моментальное") время, а не время начала транзакции, используйте now()
Vy nepravy http://www.postgresql.org/docs/8.2/static/functions-datetime.html#FUNCTIONS-DATETIME-CURRENT
Iz doki:

now() is a traditional PostgreSQL equivalent to CURRENT_TIMESTAMP. transaction_timestamp() is likewise equivalent to CURRENT_TIMESTAMP, but is named to clearly reflect what it returns. statement_timestamp() returns the start time of the current statement (more specifically, the time of receipt of the latest command message from the client). statement_timestamp() and transaction_timestamp() return the same value during the first command of a transaction, but may differ during subsequent commands. clock_timestamp() returns the actual current time, and therefore its value changes even within a single SQL command. timeofday() is a historical PostgreSQL function. Like clock_timestamp(), it returns the actual current time, but as a formatted text string rather than a timestamp with time zone value.