If you want to enable cdb support for Postfix on Gentoo, "USE=cdb emerge -v postfix" will always pull in cdb, instead of tinycdb.

Another thing to note is that tinycdb conflicts with cdb.

In order to use tinycdb in place of cdb, you can first emerge tinycdb. This will unmerge cdb for you. So, if you had cdb installed, you must find out whether you have the green light for tinycdb. Run "equery d dev-db/cdb" to check for dependencies.

After you have installed tinycdb, you can then do "USE=cdb emerge -v postfix". To make this cdb flag permanent, add the following line into your /etc/portage/package.use:

mail-mta/postfix cdb