Atomar SQL Statement

Guys,

I want to assure that a SQL statement is atomar. It trys to check if an
email address is already found in a table, if yes, it returns the id
field of the entry. If not found, it inserts the entry with the
previously generated increment id and returns - again - the id.

My concern: in a multi threaded environment, can a second thread
interrupt this statement and eventually insert the same email address in
the table with a different id? Or is this statement atomar?

Any help would be highely appreciated.

CODE:

_id := nextval('email_id_increment');

INSERT INTO email_adr (email_id, email, lastupdate)

SELECT _id, 'sender(at)example(dot)com', now()::timestamp

WHERE NOT EXISTS (SELECT * FROM email_adr WHERE
email='sender(at)example(dot)com');

- Johann