About matching record indexes for join sources

In addition to a join key, you must also configure a record index for each data source that feeds a join. A record index is a key that indicates to the record assembler how it can identify records from that source.

A source's record index key must match its join key. In other words, the key that tells the record assembler how to find a source's records must be the same as the key that the record assembler uses to compare records from that source.