When considering creating a join index, compare the number of rows in the top table to the number of rows in the related table(s).
In general, a join index improves performance for many queries where the ratio of rows in the top table to rows in the related table is less than 10 to 1. Some users may find the join index advantageous with a ratio as high as 100 to 1, but others may find that the join index inhibits query performance if the ratio is as low as 10 to 1. If you are considering using a higher ratio, test to be sure it helps your queries.