It is a index that is maintained in a system .It maintains rows joined on two or more tables. 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 all or part of the query
Benefits if using join index is
- to eliminate base table access
- Aggregate processing is eliminated by creating aggregate join index
- It reduces redistribution of data since data is materialized by JI.
- Reduces complex join conditions by using covering queries