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+Prefetch User Database
+======================
+
+Prefetch userdb can be used to combine passdb and userdb lookups into a single
+lookup. It's usually used with <SQL> [AuthDatabase.SQL.txt], <LDAP>
+[AuthDatabase.LDAP.txt] and <checkpassword>
+[PasswordDatabase.CheckPassword.txt] passdbs.
+
+Prefetch basically works by requiring that the passdb returns the userdb
+information in <extra fields> [PasswordDatabase.ExtraFields.txt] with 'userdb_'
+prefixes. For example if a userdb typically returns 'uid', 'gid' and 'home'
+fields, the passdb would have to return 'userdb_uid', 'userdb_gid' and
+'userdb_home' fields.
+
+If you're using <LDA.txt> or <LMTP.txt>, you still need a valid userdb which
+can be used to locate the users. You can do this by adding a normal SQL/LDAP
+userdb *after the userdb prefetch*. The order of definitions is significant.
+See below for examples.
+
+LDAP: 'auth_bind=yes' with 'auth_bind_userdn'-template is incompatible with
+prefetch, because no passdb lookup is done then. If you want zero LDAP lookups,
+you might want to use <static userdb> [UserDatabase.Static.txt] instead of
+prefetch.
+
+SQL example
+-----------
+
+'dovecot.conf':
+
+---%<-------------------------------------------------------------------------
+passdb {
+  driver = sql
+  args = /etc/dovecot/dovecot-sql.conf.ext
+}
+userdb {
+  driver = prefetch
+}
+# The userdb below is used only by lda.
+userdb {
+  driver = sql
+  args = /etc/dovecot/dovecot-sql.conf.ext
+}
+---%<-------------------------------------------------------------------------
+
+'dovecot-sql.conf.ext':
+
+---%<-------------------------------------------------------------------------
+password_query = SELECT userid AS user, password, \
+  home AS userdb_home, uid AS userdb_uid, gid AS userdb_gid \
+  FROM users \
+  WHERE userid = '%u'
+
+# For LDA:
+user_query = SELECT home, uid, gid FROM users WHERE userid = '%u'
+---%<-------------------------------------------------------------------------
+
+LDAP example
+------------
+
+'dovecot.conf':
+
+---%<-------------------------------------------------------------------------
+passdb {
+  driver = ldap
+  args = /etc/dovecot/dovecot-ldap.conf.ext
+}
+userdb {
+  driver = prefetch
+}
+# The userdb below is used only by LDA.
+userdb {
+  driver = ldap
+  args = /etc/dovecot/dovecot-ldap.conf.ext
+}
+---%<-------------------------------------------------------------------------
+
+'dovecot-ldap.conf.ext':
+
+---%<-------------------------------------------------------------------------
+pass_attrs = uid=user, userPassword=password, \
+  homeDirectory=userdb_home, uidNumber=userdb_uid, gidNumber=userdb_gid
+
+# For LDA:
+user_attrs = homeDirectory=home, uidNumber=uid, gidNumber=gid
+---%<-------------------------------------------------------------------------
+
+(This file was created from the wiki on 2019-06-19 12:42)