To add auto increment to on column using the following code in the
migration class:

$systemSettingsTable->changeColumn('setting_id',
array('autoincrement' => true));
or
$settingIdColumn = $systemSettingsTable->getColumn('setting_id');
$settingIdColumn->setAutoincrement(true);

This works fine when MySQL is used. But when PostGreSQL is used, this
statement doesn't create any SQL statement.

The patch in attachment could perhaps resolve this problem some way.