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.