JOIN INDEX:
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Join Index is nothing but pre-joining 2 or more tables or
views which are commonly joined in order to reduce the
joining overhead.
So teradata uses the join index instead of resolving the
joins in the participating base tables.
They increase the efficiency and performance of join queries.
They can have different primary indexes than the base tables
and also are automatically updated as and when the base rows
are updated. they can have repeating values.

There are 3 types of join indexes:
1)Single table join index - here the rows are distributed  
based on the foreign key hash value of the base table.
2) Multi table join index - joining two tables.
3) Aggregate join index - performing the aggregates but only
sum and count.

Join Index is nothing but pre-joning 2 or more tables or 
views which are commonly joined in order to reduce the 
joining overhead.
So teradata usdes the join index instead of resolving the 
joins in the participating base tables.
They increase the efficiency and performance of join 
queries.
They can have different primary indexes than the base tables
and also are automatically updated as and when the base rows
are updated. they can have repeating values.

There are 3 types of join indexes:
1) Single table join index - here the rows are distributed 
based on the foreign key hash value of the base table.
2) Multi table join index - joining two tables.
3) Aggregate join index - performing the aggregates but 
only sum and count.

Join index stores the joined rows to two or more tables in
the permanent space. Not beneficiary always. Space
consumptions may be a problem. Can be used whenever large
tables are consistently and frequently joined and space may
not be a problem.