Tokutek provides an "engine" for MySQL. Each table in MySQL has associated with it a engine that does the storing and retreaval. Like all software it's optimized for certain use cases. Tokutek used a fractal tree index to allow faster lookups. I think they could reindex a table without locking .
They more recently did an engine for mongodb
A description from the Boston MySQL group a few years back (I was at the talk:)
They more recently did an engine for mongodb A description from the Boston MySQL group a few years back (I was at the talk:)
http://www.tokutek.com/2012/01/fractal-tree-indexes-and-mead...