PostgreSQL works similarly with varchar and text columns. Rather than
limiting the amount of characters a column can hold simply use text
columns.

Because this sql is now PostgreSQL specific, there's no need to maintain
what was set in the mysql settings.

Remove all configuration for MySQL and configure PostgreSQL as the main
database.

All *_mysql_* options have been changed to *_db_* options.

Postgres requires the database user to have a password in order to
connect via localhost. The db_admin_password option is used to set the
password of the admin user (usually postgres).

When you're hosting a couple of hundred aliases, you will benefit.
It seems safer to create the index and deal with the slight overhead
for just a couple of records, as opposed to not having these
indexes and deal with serious performance issues if you have
lots of aliases on the machine.

Use the changed `mail_virtual_users` structure to ensure directories and
sieve rules are created for each defined account.

Thanks @pjan