index questions

Hi everyone.

I have created a simplified example of a real case, to show you what I'm
tryng to do. I have
a table, like this:

CREATE TABLE sales (
saleId SERIAL,
clientId INTEGER,
branchId INTEGER,
productId INTEGER,
employeeId INTEGER,
saleDate DATE,
price NUMERIC(12, 2),
qty INTEGER,
PRIMARY KEY(saleId)
);
CREATE INDEX sales_k1 ON sales(clientId, branchId, productId,
employeeId, saleDate, price, qty);

This table will grow to *many* rows in the future.

I want to make a function that returns the FIRS saleId of the sale that
matches some conditions. I will
always receive the Client Id, but not always the other arguments (sent
as NULLs).

The fetched resultset shoud prioritize the passed arguments, and after
that, the saleDate, price
and quantity.

/**
* Finds the first sale that matches the conditions received.
* @param $1 Client Id.
* @param $2 Preferred Branch Id.
* @param $3 Preferred Product Id.
* @param $4 Preferred Employee Id.
* @return Sale Id if found, NULL if not.
*/
CREATE OR REPLACE FUNCTION findSale(INTEGER, INTEGER, INTEGER, INTEGER)
RETURNS INTEGER AS '
DECLARE
a_clientId ALIAS FOR $1;
a_branchId ALIAS FOR $1;
a_productId ALIAS FOR $1;
a_employeeId ALIAS FOR $1;
r_result INTEGER;
BEGIN
SELECT
INTO r_result employeeId
FROM
sales
WHERE
clientId=a_clientId AND
branchId=coalesce(a_branchId, branchId) AND /*branchId is null?
anything will be ok*/
productId=coalesce(a_productId, productId) AND /*productId is
null? anything will be ok*/
employeeId=coalesce(a_employeeId, employeeId) /*employeeId is
null? anything will be ok*/
ORDER BY
clientId, branchId, productId, employeeId, saleDate, price, qty
LIMIT 1;

RETURN r_result;
END;
' LANGUAGE 'plpgsql';

Will findSale() in the future, when I have *many* rows still use the
index when only the first couple of
arguments are passed to the function?
If not, should I create more indexes (and functions) for each possible
argument combination? (of course, with
the given order)

The thing here is that I don't understand how postgreSQL solves the
query when the COALESCEs are used... it uses
the index now, with a few thowsand records, but what will happen in a
few months?

Thanks in advance.