Simple Join Indexes and Partial Query Covering
It is also possible for a join index to be used to partially cover a query to improve query performance. For example, if you wanted to count the number of orders made by customers in the European region during October, you might use the following query:
FROM cust, ord, orditm, location
WHERE ord.ordid = orditm.ordid
AND cust.customerid = ord.customerid
AND cust.loc = location.loc
AND location.region = 'EUROPE'
AND EXTRACT(MONTH, ord.orddate) = 10
GROUP BY cust.customerid;
In this example, the query includes the location table which is not included in the join index. Teradata Database can still use the join index to partially cover the query by joining the contents of the join index with the location table.
See “Partial Query Coverage” on page 505 for more information about partial query coverage.