[ Thanks to Dmitri
Popov for this link. ]
“By default, digiKam uses two SQLite databases for storing
essential data: digikam4.db and thumbnails-digikam.db. And to make
the application run fast and smoothly, it’s a good idea to check
and optimize the databases every now and then. To do this on Ubuntu
or its derivatives, you need to install the sqlite3 package using
the sudo apt-get install sqlite3 command. Once you’ve done that,
back up the digikam4.db and thumbnails-digikam.db databases. Open
then the terminal and switch to the directory where the databases
are stored.”