This example creates a secondary index on a join index named order_join_line:
CREATE INDEX shipidx (l_shipdate) ON order_join_line;
A query against the base tables uses both indexes:
SELECT o_orderdate, o_custkey, l_partkey, l_quantity, l_extendedprice FROM lineitem, order WHERE l_orderkey = o_orderkey AND l_shipdate = '1997-09-18';
An EXPLAIN shows that the RETRIEVE step reads from the order_join_line join index table by way of the shipidx index:
"order_join_line.l_shipdate = 970918"