Estimated size of an index

SQL> exec dbms_stats.gather_table_stats(USER,'FIN_EDATT_REJECTED_STD_CALC');
SQL> variable used_bytes number
SQL> variable alloc_bytes number
SQL> SET SERVEROUTPUT ON
SQL> exec dbms_space.create_index_cost( 'create index cust_TEMP_IDS on FIN_EDATT_REJECTED_STD_CALC(CMP_NUM)', :used_byte
s, :alloc_bytes );

PL/SQL procedure successfully completed.

SQL> print :used_bytes

USED_BYTES
----------
   1751718

SQL> print :alloc_bytes

ALLOC_BYTES
-----------
   14680064

The used_bytes variable gives you an estimate of how much room is required for the index data. The
alloc_bytes variable provides an estimate of how much space will be allocated within the tablespace.