Formula for estimating the extent size of an attached index

For an attached index, the database server uses the ratio of the index key size to the row size to assign an appropriate extent size for the index.

The following formula shows how the database server uses the ratio of the index key size to the row size:

Index extent size = (index_key_size /
table_row_size) *
   table_extent_size

In this formula:

index_key_size is the total widths of the indexed column or columns plus 5 for a key descriptor.
table_row_size is the sum of all the columns in the row.
table_extent_size is the value that you specify in the EXTENT SIZE keyword of the CREATE TABLE statement.

If the index is not unique, then the extent size is reduced by 20 percent.

The database server also uses this same ratio for the next-extent size for the index:

Index next extent size =
(index_key_size/table_row_size)* 
   table_next_extent_size