Does PostgreSQL guarantee unique timestamps?

8-byte integers are now the default for Postgres 8.4, so it allows microsecond values to be stored.

I don't care too much about real microsecond precision (probably depends on OS capabilities?!) - But does Postgres guarantee, that the timestamp values (inserted by current_timestamp) are always different between any two transaction开发者_高级运维s?

No.

I did a small test, inserting current_timestamp from 5 parallell clients in a table, 3 of 3463 records got the same timestamp.

Yes, precision depends on the OS. Within a single transaction the timestamps are all the same when using CURRENT_TIMESTAMP, when using CLOCK_TIMESTAMP they might be different. Different transactions can have different CURRENT_TIMESTAMPS, depends on how fast you can insert/update.

If you need uniqueness, add a unique constraint.

Even if it does, it seems dangerous to rely on the values being unique. If you need a unique key you are better off using an alternative such as a serial column.

Luckily no, since that would mean a "please make this slower than it has to be" option.

If you really need what is really a serial, but which looks like a timestamp, then add some microseconds on based on the result of a serial!

I agree with the recommendation of adding a serial. But if what you are trying to do doesn't merit a schema change, you could use the system column xmin to separate rows with the same timestamp but from different transactions.

  SELECT ts, xmin, count(1)
  FROM foo
  GROUP BY ts, xmin