DIF is a collection of tools aimed at implementing a powerful indexing system for astronomical catalogues and other data with spherical coordinates, stored into MySQL / MariaDB databases.
DIF is able to use both HTM and HEALPix pixelization schemas simultaneously and it allows very fast query execution even on billion-row tables.
See also the documentation or the web page and the reference paper.
If you want to use almost all of the DIF capabilities but avoid to use MySQL / MariaDB source code, please use SID instead.