Because the MR model is so simple, MR frameworks do not provide built-in indexes. The programmer must implement any indexes
that they may desire to speed up access to the data inside of their
application. This is not easily accomplished, as the framework’s
data fetching mechanisms must also be instrumented to use these
indexes when pushing data to running Map instances. Once more,
this is an acceptable strategy if the indexes do not need to be shared
between multiple programmers, despite requiring every MR programmer re-implement the same basic functionality.