/usr/share/doc/postfix-mysql/README.Debian is in postfix-mysql 2.9.1-4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | The postfix-doc package contains documentation on how to configure this
map type. See /usr/share/doc/postfix/html/MYSQL_README.html
|