Caution when using PL/SQL functions in a SQL statement

Be cautious if you use PL/SQL functions/packages in your SQL statements, especially in the Where-Clause!

Why?

Because the function is probably called more often than you think it is! If you don't want to read the hole article, scroll down and check the APEX impact.


An example

Let's create the following simple function.

CREATE OR REPLACE FUNCTION getValue
( pValue IN NUMBER
)
RETURN NUMBER
IS
BEGIN
  DBMS_Output.put_line
    ( 'called getValue with '||pValue||' at '||
      TO_CHAR(SYSDATE, 'HH24:MI:SS')
    );
  RETURN pValue;
END getValue;

And now look at the following SQL Statement where we call our new function with a constant value.

SELECT COUNT(*)
 FROM EMPLOYEES
WHERE SALARY = getValue(1)
;

How often do you think is the function called?

• One time because it always uses the same value as parameter?
• For each row which is checked by the query engine?
• Some number between one time and for all rows?

Check the DBMS_Output.

If you have bet your money on For each row which is checked by the query engine, then you are the winner! It's called 107 times! As often as we have rows in the table.

If we change the query to

SELECT COUNT(*)
 FROM EMPLOYEES
WHERE EMPLOYEE_ID > getValue(1)
;

How often to you think it is called? You have the same choice as before.

Check the DBMS_Output.

It's just called one time! What is the difference? In our second example the optimizer is using the EMP_EMP_ID_PK index and in that case as Tom Kyte explained me

The optimizer gets the start/stop range (the function might be called twice actually) and then it range scans with the constants.

So what's the deal all about it?

Keep in mind the performance issue when you have a function which is selecting against other tables and you use such a function in a where-clause, where 1.000's of records are processed!

Possible solutions?

You can change the query in the following way. The optimizer then knows that is ok when he executes the query just one time. So called scalar subquery caching, which works with all DB versions. Checkout AskTom about that term.

SELECT COUNT(*)
 FROM EMPLOYEES
WHERE SALARY = (SELECT getValue(1) FROM DUAL)
;

Or you take a look at the DETERMINISTIC clause which can specified when creating a function or a package functions. What does it do? It's a hint for the optimizer to indicate that the function returns the same result value whenever it is called with the same values for its arguments. But, as noted by Tom Kyte

10gr2 supports deterministic as an optimization in SQL for the first time. Prior to 10gr2 - deterministic was all about function based indexes only - the SQL engine ignored it.

So only if you have a 10.2.x database, using DETERMINISTIC will give you a speed up of your query. Use the above solution instead.

But let's change our function and see what we get now.

CREATE OR REPLACE FUNCTION getValue
( pValue IN NUMBER
)
RETURN NUMBER DETERMINISTIC
IS
BEGIN
 DBMS_Output.put_line
   ( 'called getValue with '||pValue||' at '||
     TO_CHAR(SYSDATE, 'HH24:MI:SS')
   );
 RETURN pValue;
END getValue;

Run the first query again and you will see that the function is called just once!
Try the following SQL statement.

SELECT COUNT(*)
FROM EMPLOYEES
WHERE SALARY = getValue(SALARY)
;

How often do you think it is called? It's just called as often as there are different salaries in the table. With the old version of the function is called for each row again.

I know that the Oracle documentation about DETERMINISTIC says

Do not specify this clause to define a function that uses package variables or that accesses the database in any way that might affect the return result of the function.

Which I think is basically to see in relation to the function based index for which this "hint" has been introduced. From my observation, if you have such a function call in several SQL statements which are called after each other (eg in a anonymous pl/sql block), the function is executed again for each SQL statement. It's just a hint for the optimizer during execution of the current statement.

DON'T USE DETERMINISTIC if your function is changing some global variables (eg increase a counter) or doing DML and you expect that the function is called for each row!

And how is that all related to APEX?

Are you using the V or NV function in your SQL statements? In APEX 2.0 the V function isn't wrapped (in 2.2 it is) and I don't see that the DETERMINISTIC is used in the code... It's just memory processing they do in there, but if you call it for 1.000's of records...

Always use bind variables instead! Only use the V function in your packages, but never use them in your report SQL statements.

I have done a posting on OTN about speeding up the V function. Stay tuned.

UPDATE: See my posting Drop in replacement for V and NV function which contains a wrapper for the existing functions.