A Join Index (JI) is an indexing structure containing columns from one or more base tables.
Some queries can be satisfied by examining only the JI when all referenced columns are stored in the index. Such queries are said to be covered by the JI. Other queries may use the JI to qualify a few rows, then refer to the base tables to obtain requested columns that are not stored in the JI. Such queries are said to be partially-covered by the index.
Because Teradata Database supports multitable, partially-covering JIs, all types of JIs, except the aggregate JI, can be joined to their base tables to retrieve columns that are referenced by a query but are not stored in the JI. Aggregate JIs can be defined for commonly-used aggregation queries.
Much like SIs, JIs impose additional processing on insert and delete operations and update operations which change the value of columns stored in the JI. The performance trade-off considerations are similar to those for SIs.