Who knows this SELECT in PostgreSQL

Dear gurus, I'm sure someone knows immediately how to translate this
mysql to postgresql:

SELECT GROUP_CONCAT(dbmail_messageblks.messageblk
ORDER BY dbmail_messageblks.is_header DESC SEPARATOR '')
FROM dbmail_messages
JOIN dbmail_messageblks ON
(dbmail_messages.physmessage_id=dbmail_messageblks.physmessage_id)
WHERE dbmail_messages.message_idnr='${MSG_UID}'
GROUP BY dbmail_messages.message_idnr"

It should simply return the whole message which can be truncated to
pieces into dbmail_messageblks and return it as a single entity. I found
this for mysql and would like to rewrite the shell script to postgresql,
but am stuck at this select.

mfg zmi
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