When you create an additional column index, the CREATE INDEX command creates the new index as part of the individual table.
If the existing column indexes in the individual table already contain data, the CREATE INDEX statement also inserts data into the new index from an existing index. This ensures data integrity among all the column indexes for columns within an individual table.
This capability is useful if you discover that a column needs an additional index after you have already inserted data. This allows you to add the index without having to start over.