XnView uses an SQLite database for storing this info and accessing it; Windows uses a flat file for the image metadata and another for the actual image thumbnails.

A list of 5 best popular Open Source, free database software, Mysql, PostgreSQl, Firebird, Berkley DB and SQlite, along with comparison.

AFAIK, the contact database on iOS is an SQLite database file. I checked the backup for an iPhone running iOS 4 (the oldest backup I have), and the file is called 'AddressBook.sqlitedb', but ...