What kind of database do `updatedb` and `locate` use?

The locate program of findutils scans one or more databases of filenames and displays any matches. This can be used as a very fast find command if the file was present during the last file name database update. There are many kinds of databases nowadays, - relational databases (with query language e.g. SQL), - NoSQL databases - document-oriented databases (e.g. MongoDB) - Key-value database (e.g. Redis) - Column-oriented databases (e.g. Cassandra) - Graph database So what kind of database does updatedb update and locate use?