Amavis spamassassin per-user configuration without using a database

In the main.cf, set

smtpd_data_restrictions = reject_unauth_pipelining,permit_sasl_authenticated,check_sender_access pcre:/etc/postfix/content_filter.pcre

Do not set content_filter= in main.cf!

/etc/postfix/content_filter.pcre should be something like:

/^/ FILTER smtp-amavis:[127.0.0.1]:10024

Do not forget to run

postmap /etc/postfix/content_filter.pcre

master.cf should be something like:

127.0.0.1:10025 inet n  -   n -   -  smtpd
    -o content_filter=
    -o local_recipient_maps=
    -o relay_recipient_maps=
    -o smtpd_data_restrictions=
    -o smtpd_restriction_classes=
    -o smtpd_client_restrictions=
    -o smtpd_helo_restrictions=
    -o smtpd_sender_restrictions=
    -o smtpd_recipient_restrictions=permit_mynetworks,reject
    -o mynetworks=127.0.0.0/8
    -o smtpd_error_sleep_time=0
    -o smtpd_soft_error_limit=1001
    -o smtpd_hard_error_limit=1000
smtp-amavis unix -      -       n       -       16       lmtp
    -o lmtp_data_done_timeout=1200
    -o lmtp_send_vforward_command=yes
    -o disable_dns_lookups=yes

Hope this helps. See http://www.postfix.org/postconf.5.html for further info.

is it ever possible to force SA and amavis to use database?

It should be since the 2.7.0 version of Amavisd-new which introduces the @sa_userconf_maps configuration parameter. From the Amavisd-new release notes:

SpamAssassin will be requested to load a user preferences configuration through its read_scoreonly_config() or load_scoreonly_sql() method, which otherwise (in spamd) serves to load user's .spamassassin/user_prefs file or SQL preferences when switching users. See SpamAssassin documentation file sql/README for SQL details.

[...]

Example:

@sa_userconf_maps = (
  { 'user1@example.com' =>
      '/etc/mail/spamassassin/special_user_config',
    '.example.org' => 'sql:',
  }
);

If not, what will be performace influence when i drop amavis and use SA directly with postfix?

Spamassassin/spamd would be able to filter mail, but Amavisd-new does a lot more than that. Your impact would not be not so much performance-wise (since Amavisd-new does just invoke the appropriate SpamAssassin functions, similar to what spamd is doing too) but you would lose Amavisd-new specific functionality like the lovers maps, the destiny configurations, the antivirus scanner integration, the attachment file type recognition, ...