A multitable join index stores and maintains the joined rows of two or more tables and, optionally, aggregates selected columns.
Multitable join indexes are for join queries that are performed frequently enough to justify defining a prejoin on the joined columns.
A multitable join index is useful for queries where the index structure contains all the columns referenced by one or more joins, thereby allowing the index to cover that part of the query, allowing retrieval of the requested data from the index rather than accessing its underlying base tables. For obvious reasons, an index with this property is often called a covering index.