The clustering factor is a benchmark that expresses the correlation between the index row order and the table row order.
For example, an
that grows every day, might have an index on the order date and another
one on the customer id. Because orders don't get deleted there are no
holes in the table so that each new order is added to the end. The table
grows chronologically. An index on the order date has a very low
clustering factor because the index order is essentially the same as the
table order. The index on customer id has a higher clustering factor
because the index order is different from the table order; the table row
will be inserted at the end of the table, the corresponding index entry
somewhere in the middle of the index—according to the customer id.