WordPress DB Abstraction
Full Database Abstraction Layer
As a need to flatten database schema or completely replace meta table with NoSQL solution.
Next Level Cache
The only database replacement plugin that speeds up execution, and it is still maintained. Installed as a “drop-in” it overrides the class that controls access to the database. Next Level Cache intercepts queries and selectively caches them.
Where does Next Level Cache store it’s data? The cache is stored in the wp_options table and is loaded upfront with one large query instead of many small ones.
SQLite Integration is also PDO integration
ProjectNami is mostly WordPress powered by Microsoft SQL, built mostly for SQL Azure. Very is also interesting but it seems it is a complete rewrite of code.
WP DB Abstraction is PDO oriented, but unmaintained.
PDO (SQLite) For WordPress is also unmaintained and probably an ancestor of SQLite Integration plugin.
What about NoSQL, for example Mongo?
The only real effort was just mentioned here with references to this virtimus/mysql2mongo.
There once existed Mongopress but it was long abandoned.
Database class replacements of built-in database functions, for other performance reasons.
HyperDB by Automattic is only alive project, used