Duplicating Effort in SQL functions

Question

Hi, If I have a piece of SQL such:
SELECT
      SUBSTR(really complex nested functions [no PLSQL function / context switching however]) AS COL1,
      LENGTH(SUBSTR(really complex functions [no PLSQL function / context switching however])
AS COL2
FROM mytableThe second column is obviously just the length of COL1
Does SQL engine when processing the function for COL2, execute the "really complex function" again - before getting the LENGTH? Or does it just reuse what it already calculated in COL1?
If not, is there anyway of allowing the "Really complex function" result resued by other COLs?
I do not wish to use a PLSQL function.
Thanks for any help

Answer 1

Well the CBO is pretty clever, it won't do two passes if it realises it's not necessary. Here's a bit of simple obfuscation SQL with and without inline view explained:
EXPLAIN PLAN
SET STATEMENT_ID='ORIG'
FOR
SELECT last_name, '555' || Translate (Substr (phone_number, 4),
                    '0123456789',
                    '5678901234') tel,
          Length ('555' || Translate (Substr (phone_number, 4),
                    '0123456789',
                    '5678901234')) len
FROM employees;
EXPLAIN PLAN
SET STATEMENT_ID='ILV'
FOR
SELECT last_name, tel, Length (tel) len
FROM (
SELECT last_name, '555' || Translate (Substr (phone_number, 4),
                    '0123456789',
                    '5678901234') tel
FROM employees);
SQL> START XPLAIN
Explained.
Explained.
Original
Query Plan
SELECT STATEMENT Cost = 3
TABLE ACCESS FULL EMPLOYEES
Inline View
Query Plan
SELECT STATEMENT Cost = 3
TABLE ACCESS FULL EMPLOYEES
Rollback complete.
SQL>
You might also look at the 11g virtual column feature where you define columns of a table as functions of other columns.