SERIAL order and INSERT order

Hi

I have a table defined as follows:

CREATE TABLE mytable (
seqnumber BIGSERIAL;
...
)

and I have multiple threads inserting into the table.

If multiple threads insert at the same time, will the sequence
numbers generated always appear in the table in order?

e.g.
thread 1 does INSERT getting serial number 1
thread 2 does INSERT getting serial number 2

and both complete successfully (no rollbacks).

Are there any cases where an observer might see a row with serial
number 2 but not yet see serial number 1, perhaps because thread 1
hasn't completed its insert yet? For example SELECT * WHERE
seqnumber < 3; would return only a row with seqnumber=2?

thanks