Query Plans - Teradata Vantage

Query Plan: No Join Index Defined

Without a defined join index, the execution plan for this query typically redistributes the orders table into a spool, sort the spool on o_custkey, and then perform a merge join between the spool and the customer table.

Query Plan: Join Index Defined

Now consider the execution plan for this same query when the following join index has been defined:

CREATE JOIN INDEX OrdCustIdx AS
SELECT (o_custkey, c_name), (o_status, o_date, o_comment)
FROM orders, customer
WHERE o_custkey=c_custkey;

With this join index defined, the run plan for the query specifies a simple scan of the join index without accessing any of the underlying base tables and without having to join the tables on the predicate WHERE o_custkey = c_custkey.

In the join index defined for this example, (o_custkey, c_name) is the specified fixed part of the index and (o_status, o_date, o_comment) is the repeated portion. Therefore, assume the following specimen base table entries (where the ? character indicates a null).

You cannot collect statistics on a complex expression from a base table. If your applications frequently run queries that specify complex expressions in their predicates, consider creating a single-table join index that specifies a matching complex expression in its select list or column list, respectively. When creating an index, Vantage transforms the complex expression into a simple index column on which you can collect statistics.

If the complex expression specified by the index is a term that matches a predicate condition for a query made against the base table the on which the index is defined, statistics collected on the index expression can be mapped to the base table so the Optimizer can use the statistics to make more accurate single-table cardinality estimates.