Dovecot auth error

[Mathieu R.]

I just tried to configure a new dovecot/postfix server, and i end up with a
dovecot auth error at startup.
I can't find a solution by myself.
Below are details, thanks in advance for your precious help, and excuse my
poor english :

dovecot --version
2.2.27 (c0f36b0) (Debian)

Dovecot -n :
https://400iso.net/public/dov.txt


grep -v '^ *\(#.*\)\?$' /etc/dovecot/dovecot-sql.conf
driver = mysql
connect = host=127.0.0.1 dbname=postfix user=postfix password=password
default_pass_scheme = MD5-CRYPT
user_query = SELECT '/srv/vmail/%d/%n' AS home, 3000 AS uid, 3000 AS gid,
CONCAT('*:bytes=', CAST(quota AS CHAR)) AS quota_rule FROM mailbox WHERE
username = '%u' AND active='1'
password_query = SELECT password FROM mailbox WHERE username = '%u'


Here is part of the server's log :

Nov  4 20:57:49 vps81550 postfix/postscreen[21578]: CONNECT from
[209.85.215.51]:47485 to [149.56.x.x]:25
Nov  4 20:57:49 vps81550 postfix/dnsblog[21583]: addr 209.85.215.51 listed
by domain dnsbl.sorbs.net as 127.0.0.6
Nov  4 20:57:55 vps81550 postfix/postscreen[21578]: PASS OLD
[209.85.215.51]:47485
Nov  4 20:57:55 vps81550 postfix/smtpd[21585]: connect from
mail-lf0-f51.google.com[209.85.215.51]
Nov  4 20:57:55 vps81550 postfix/smtpd[21585]: Untrusted TLS connection
established from mail-lf0-f51.google.com[209.85.215.51] TLSv1.2 with cipher
ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)
Nov  4 20:57:55 vps81550 dovecot: auth: Fatal: sql: driver not set in
configuration file /etc/dovecot/dovecot-sql.conf.ext
Nov  4 20:57:55 vps81550 dovecot: master: Error: service(auth): command
startup failed, throttling for 2 secs
Nov  4 20:57:55 vps81550 postfix/smtpd[21585]: fatal: no SASL
authentication mechanisms
Nov  4 20:57:56 vps81550 postfix/master[21528]: warning: process
/usr/lib/postfix/sbin/smtpd pid 21585 exit status 1
Nov  4 20:57:56 vps81550 postfix/master[21528]: warning:
/usr/lib/postfix/sbin/smtpd: bad command startup -- throttling
-- 

Mathieu R.