Compare Current With Previous Row Until Mismatch

We have employee salary table which will have salary of an employee on daily basis (only working days). Below is the table structure:

CODEemp_salary
----------
emp_id NUMBER(15) NOT NULL
effective_date DATE NOT NULL
salary NUMBER(15) NOT NULL

Primary key - emp_id, effective_date..This table is yearly partitioned...I have to find out how long the salary is not changed for an employee from given date, and last salary. I am using below query to do this:

CODEWITH salary_tab AS
(SELECT effective_date, salary,
(CASE
WHEN (LAG (salary) OVER (PARTITION BY emp_id ORDER BY effective_date ASC) =
salary

[code]....

For emp_id 1, if we ran this query for 10/31/2011, then it has to compare the 10/31 salary with 10/29 and do the same until the salary mismatches. In this case, salary salary mismatch occurs on 10/20, so the stale salary period is from 10/31 to 10/21 which is 7 days.Below query will give that result:

CODE
WITH salary_tab AS
(SELECT effective_date, salary,
(CASE
WHEN (LAG (salary) OVER (PARTITION BY emp_id ORDER BY effective_date ASC) =
salary

[code]...