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:

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

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