install fails

When enabling the module, I get

* warning: pg_query() [function.pg-query]: Query failed: ERROR: function ifnull(character varying, character varying) does not exist LINE 1: ... notify_hash) SELECT c.cid, 0, md5(concat(c.mail, ifnull(u.m... ^ HINT: No function matches the given name and argument types. You might need to add explicit type casts. in /var/www/drupal/includes/database.pgsql.inc on line 139.
    * user warning: query: INSERT INTO comment_notify (cid, notify, notify_hash) SELECT c.cid, 0, md5(concat(c.mail, ifnull(u.mail, u.init), c.uid, c.name, c.nid)) FROM comments c LEFT OUTER JOIN users u on c.uid = u.uid in /var/www/drupal/sites/all/modules/comment_notify/comment_notify.install on line 12.

The module appears in Site Configuration though. The error message is
warning: Invalid argument supplied for foreach() in /var/www/drupal/includes/form.inc on line 1192.