/etc/portage/modules
The /etc/portage/modules file can be used to override the metadata cache implementation. In practice, portdbapi.auxdbmodule is the only variable that the user will want to override.
/etc/portage/modulesModules exampleportdbapi.auxdbmodule = portage.cache.sqlite.database
After changing the portdbapi.auxdbmodule setting, it may be necessary to transfer or regenerate metadata cache. Users of the rsync tree need to run emerge --metadata if they have enabled FEATURES="metadata-transfer" in make.conf.
In order to regenerate metadata for repositories not distributing pre-generated metadata cache, run emerge --regen (see emerge).
When using something like the sqlite module and want to keep all metadata in that format alone (useful for querying), enable FEATURES="metadata-transfer" in make.conf.
Troubleshooting
equery
Problem: equery u portage stopped working (UnicodeDecodeError: 'ascii'...), but equery h qt5 still works.
Solution: cleanup Portage profile's deps:
root #rm -rf /var/db/repos/gentoo/profiles/desc
root #emerge --metadata