By definition, the first column in the select list of an aggregate join index definition must be either a COUNT(non_nullable_expression) or COUNT(*) expression. When you create an aggregate join index that does not specify one of these COUNT expressions as the first column in its select list, then Teradata Database automatically moves the first such expression you specify to the first column in the definition. This also occurs whenever Teradata Database must add a COUNT(*) expression to the aggregate join index definition if you specify only a SUM operator.
This means that the column positions in the output of a SHOW JOIN INDEX request for an aggregate join index might not be identical to the column positions in the index definition that you originally typed.