- BULK COLLECT: SELECT statements that retrieve multiple rows with a single fetch, improving the speed of data retrieval
- FORALL: INSERTs, UPDATEs, and DELETEs that use collections to change multiple rows of data very quickly
Answer to the ChallengeDECLARE
l_names DBMS_UTILITY.maxname_array;
BEGIN
l_names (1) := ‘Strawberry’;
l_names (10) := ‘Blackberry’;
l_names (2) := ‘Raspberry’;
DECLARE
indx PLS_INTEGER := l_names.FIRST;
BEGIN
WHILE (indx IS NOT NULL)
LOOP
DBMS_OUTPUT.put_line (l_names (indx));
indx := l_names.NEXT (indx);
END LOOP;
END;
END;
/
|
Context Switches and Performance
Almost every program PL/SQL developers write includes both PL/SQL and SQL statements. PL/SQL statements are run by the PL/SQL statement executor; SQL statements are run by the SQL statement executor. When the PL/SQL runtime engine encounters a SQL statement, it stops and passes the SQL statement over to the SQL engine. The SQL engine executes the SQL statement and returns information back to the PL/SQL engine (see Figure 1). This transfer of control is called a context switch, and each one of these switches incurs overhead that slows down the overall performance of your programs. Figure 1: Switching between PL/SQL and SQL engines |
Let’s look at a concrete example to explore context switches more thoroughly and identify the reason that FORALL and BULK COLLECT can have such a dramatic impact on performance.
Suppose my manager asked me to write a procedure that accepts a department ID and a salary percentage increase and gives everyone in that department a raise by the specified percentage. Taking advantage of PL/SQL’s elegant cursor FOR loop and the ability to call SQL statements natively in PL/SQL, I come up with the code in Listing 1.
Code Listing 1: increase_salary procedure with FOR loop
PROCEDURE increase_salary (
department_id_in IN employees.department_id%TYPE,
increase_pct_in IN NUMBER)
IS
BEGIN
FOR employee_rec
IN (SELECT employee_id
FROM employees
WHERE department_id =
increase_salary.department_id_in)
LOOP
UPDATE employees emp
SET emp.salary = emp.salary +
emp.salary * increase_salary.increase_pct_in
WHERE emp.employee_id = employee_rec.employee_id;
END LOOP;
END increase_salary;
Suppose there are 100 employees in department 15. When I execute this block,
BEGIN increase_salary (15, .10); END;
the PL/SQL engine will “switch” over to the SQL engine 100 times, once for each row being updated. Tom Kyte, of AskTom (asktom.oracle.com), refers to row-by-row switching like this as “slow-by-slow processing,” and it is definitely something to be avoided.
I will show you how you can use PL/SQL’s bulk processing features to escape from “slow-by-slow processing.” First, however, you should always check to see if it is possible to avoid the context switching between PL/SQL and SQL by doing as much of the work as possible within SQL.
Take another look at the increase_salary procedure. The SELECT statement identifies all the employees in a department. The UPDATE statement executes for each of those employees, applying the same percentage increase to all. In such a simple scenario, a cursor FOR loop is not needed at all. I can simplify this procedure to nothing more than the code in Listing 2.
Code Listing 2: Simplified increase_salary procedure without FOR loop
PROCEDURE increase_salary (
department_id_in IN employees.department_id%TYPE,
increase_pct_in IN NUMBER)
IS
BEGIN
UPDATE employees emp
SET emp.salary =
emp.salary
+ emp.salary * increase_salary.increase_pct_in
WHERE emp.department_id =
increase_salary.department_id_in;
END increase_salary;
Now there is just a single context switch to execute one UPDATE statement. All the work is done in the SQL engine.
Of course, in most real-world scenarios, life—and code—is not so simple. We often need to perform other steps prior to execution of our data manipulation language (DML) statements. Suppose that, for example, in the case of the increase_salary procedure, I need to check employees for eligibility for the increase in salary and if they are ineligible, send an e-mail notification. My procedure might then look like the version in Listing 3.
Code Listing 3: increase_salary procedure with eligibility checking added
PROCEDURE increase_salary (
department_id_in IN employees.department_id%TYPE,
increase_pct_in IN NUMBER)
IS
l_eligible BOOLEAN;
BEGIN
FOR employee_rec
IN (SELECT employee_id
FROM employees
WHERE department_id =
increase_salary.department_id_in)
LOOP
check_eligibility (employee_rec.employee_id,
increase_pct_in,
l_eligible);
IF l_eligible
THEN
UPDATE employees emp
SET emp.salary =
emp.salary
+ emp.salary
* increase_salary.increase_pct_in
WHERE emp.employee_id = employee_rec.employee_id;
END IF;
END LOOP;
END increase_salary;
I can no longer do everything in SQL, so am I then resigned to the fate of “slow-by-slow processing”? Not with BULK COLLECT and FORALL in PL/SQL.
Bulk Processing in PL/SQL
The bulk processing features of PL/SQL are designed specifically to reduce the number of context switches required to communicate from the PL/SQL engine to the SQL engine.Use the BULK COLLECT clause to fetch multiple rows into one or more collections with a single context switch.
Use the FORALL statement when you need to execute the same DML statement repeatedly for different bind variable values. The UPDATE statement in the increase_salary procedure fits this scenario; the only thing that changes with each new execution of the statement is the employee ID.
I will use the code in Listing 4 to explain how these features affect context switches and how you will need to change your code to take advantage of them.
Code Listing 4: Bulk processing for the increase_salary procedure
1 CREATE OR REPLACE PROCEDURE increase_salary ( 2 department_id_in IN employees.department_id%TYPE, 3 increase_pct_in IN NUMBER) 4 IS 5 TYPE employee_ids_t IS TABLE OF employees.employee_id%TYPE 6 INDEX BY PLS_INTEGER; 7 l_employee_ids employee_ids_t; 8 l_eligible_ids employee_ids_t; 9 10 l_eligible BOOLEAN; 11 BEGIN 12 SELECT employee_id 13 BULK COLLECT INTO l_employee_ids 14 FROM employees 15 WHERE department_id = increase_salary.department_id_in; 16 17 FOR indx IN 1 .. l_employee_ids.COUNT 18 LOOP 19 check_eligibility (l_employee_ids (indx), 20 increase_pct_in, 21 l_eligible); 22 23 IF l_eligible 24 THEN 25 l_eligible_ids (l_eligible_ids.COUNT + 1) := 26 l_employee_ids (indx); 27 END IF; 28 END LOOP; 29 30 FORALL indx IN 1 .. l_eligible_ids.COUNT 31 UPDATE employees emp 32 SET emp.salary = 33 emp.salary 34 + emp.salary * increase_salary.increase_pct_in 35 WHERE emp.employee_id = l_eligible_ids (indx); 36 END increase_salary;
| Lines | Description |
| 5–8 | Declare a new nested table type and two collection variables based on this type. One variable, l_employee_ids, will hold the IDs of all employees in the department. The other, l_eligible_ids, will hold the IDs of all those employees who are eligible for the salary increase. |
| 12–15 | Use BULK COLLECT to fetch all the IDs of employees in the specified department into the l_employee_ids collection. |
| 17–28 | Check for salary increase eligibility: If ineligible, an e-mail is sent. (Note: Implementation of check_eligibility is not included in this article.) If eligible, add the ID to the l_eligible_ids collection. |
| 30–35 | Use a FORALL statement to update all the rows identified by employee IDs in the l_eligible_ids collection. |
- Fetch rows with BULK COLLECT into one or more collections. A single context switch is needed for this step.
- Modify the contents of collections as required (in this case, remove ineligible employees).
- Change the table with FORALL using the modified collections.
I will first explore BULK COLLECT in more detail, and then cover FORALL.
About BULK COLLECT
To take advantage of bulk processing for queries, you simply put BULK COLLECT before the INTO keyword and then provide one or more collections after the INTO keyword. Here are some things to know about how BULK COLLECT works:- It can be used with all three types of collections: associative arrays, nested tables, and VARRAYs.
- You can fetch into individual collections (one for each expression in the SELECT list) or a single collection of records.
- The collection is always populated densely, starting from index value 1.
- If no rows are fetched, then the collection is emptied of all elements.
Code Listing 5: Fetching values for two columns into a collection
DECLARE
TYPE two_cols_rt IS RECORD
(
employee_id employees.employee_id%TYPE,
salary employees.salary%TYPE
);
TYPE employee_info_t IS TABLE OF two_cols_rt;
l_employees employee_info_t;
BEGIN
SELECT employee_id, salary
BULK COLLECT INTO l_employees
FROM employees
WHERE department_id = 10;
END;
If you are fetching lots of rows, the collection that is being filled could consume too much session memory and raise an error. To help you avoid such errors, Oracle Database offers a LIMIT clause for BULK COLLECT. Suppose that, for example, there could be tens of thousands of employees in a single department and my session does not have enough memory available to store 20,000 employee IDs in a collection.
Instead I use the approach in Listing 6.
Code Listing 6: Fetching up to the number of rows specified
DECLARE
c_limit PLS_INTEGER := 100;
CURSOR employees_cur
IS
SELECT employee_id
FROM employees
WHERE department_id = department_id_in;
TYPE employee_ids_t IS TABLE OF
employees.employee_id%TYPE;
l_employee_ids employee_ids_t;
BEGIN
OPEN employees_cur;
LOOP
FETCH employees_cur
BULK COLLECT INTO l_employee_ids
LIMIT c_limit;
EXIT WHEN l_employee_ids.COUNT = 0;
END LOOP;
END;
With this approach, I open the cursor that identifies all the rows I want to fetch. Then, inside a loop, I use FETCH-BULK COLLECT-INTO to fetch up to the number of rows specified by the c_limit constant (set to 100). Now, no matter how many rows I need to fetch, my session will never consume more memory than that required for those 100 rows, yet I will still benefit from the improvement in performance of bulk querying.
About FORALL
Whenever you execute a DML statement inside of a loop, you should convert that code to use FORALL. The performance improvement will amaze you and please your users.The FORALL statement is not a loop; it is a declarative statement to the PL/SQL engine: “Generate all the DML statements that would have been executed one row at a time, and send them all across to the SQL engine with one context switch.”
As you can see in Listing 4, lines 30 through 35, the “header” of the FORALL statement looks just like a numeric FOR loop, yet there are no LOOP or END LOOP keywords.
Here are some things to know about FORALL:
- Each FORALL statement may contain just a single DML statement. If your loop contains two updates and a delete, then you will need to write three FORALL statements.
- PL/SQL declares the FORALL iterator (indx on line 30 in Listing 4) as an integer, just as it does with a FOR loop. You do not need to—and you should not—declare a variable with this same name.
- In at least one place in the DML statement, you need to reference a collection and use the FORALL iterator as the index value in that collection (see line 35 in Listing 4).
- When using the IN low_value . . . high_value syntax in the FORALL header, the collections referenced inside the FORALL statement must be densely filled. That is, every index value between the low_value and high_value must be defined.
- If your collection is not densely filled, you should use the INDICES OF or VALUES OF syntax in your FORALL header.
FORALL and DML Errors
Suppose that I’ve written a program that is supposed to insert 10,000 rows into a table. After inserting 9,000 of those rows, the 9,001st insert fails with a DUP_VAL_ON_INDEX error (a unique index violation). The SQL engine passes that error back to the PL/SQL engine, and if the FORALL statement is written like the one in Listing 4, PL/SQL will terminate the FORALL statement. The remaining 999 rows will not be inserted.If you want the PL/SQL engine to execute as many of the DML statements as possible, even if errors are raised along the way, add the SAVE EXCEPTIONS clause to the FORALL header. Then, if the SQL engine raises an error, the PL/SQL engine will save that information in a pseudocollection named SQL%BULK_EXCEPTIONS, and continue executing statements. When all statements have been attempted, PL/SQL then raises the ORA-24381 error.
You can—and should—trap that error in the exception section and then iterate through the contents of SQL%BULK_EXCEPTIONS to find out which errors have occurred. You can then write error information to a log table and/or attempt recovery of the DML statement.
Listing 7 contains an example of using SAVE EXCEPTIONS in a FORALL statement; in this case, I simply display on the screen the index in the l_eligible_ids collection on which the error occurred, and the error code that was raised by the SQL engine.
Code Listing 7: Using SAVE EXCEPTIONS with FORALL
BEGIN
FORALL indx IN 1 .. l_eligible_ids.COUNT SAVE EXCEPTIONS
UPDATE employees emp
SET emp.salary =
emp.salary + emp.salary * increase_pct_in
WHERE emp.employee_id = l_eligible_ids (indx);
EXCEPTION
WHEN OTHERS
THEN
IF SQLCODE = -24381
THEN
FOR indx IN 1 .. SQL%BULK_EXCEPTIONS.COUNT
LOOP
DBMS_OUTPUT.put_line (
SQL%BULK_EXCEPTIONS (indx).ERROR_INDEX
|| ‘: ‘
|| SQL%BULK_EXCEPTIONS (indx).ERROR_CODE);
END LOOP;
ELSE
RAISE;
END IF;
END increase_salary;
From SQL to PL/SQL
This article talks mostly about the context switch from the PL/SQL engine to the SQL engine that occurs when a SQL statement is executed from within a PL/SQL block. It is important to remember that a context switch also takes place when a user-defined PL/SQL function is invoked from within an SQL statement.Suppose that I have written a function named betwnstr that returns the string between a start and end point. Here’s the header of the function:
FUNCTION betwnstr ( string_in IN VARCHAR2 , start_in IN INTEGER , end_in IN INTEGER ) RETURN VARCHAR2
I can then call this function as follows:
SELECT betwnstr (last_name, 2, 6) FROM employees WHERE department_id = 10
If the employees table has 100 rows and 20 of those have department_id set to 10, then there will be 20 context switches from SQL to PL/SQL to run this function.
You should, consequently, play close attention to all invocations of user-defined functions in SQL, especially those that occur in the WHERE clause of the statement. Consider the following query:
SELECT employee_id FROM employees WHERE betwnstr (last_name, 2, 6) = 'MITHY'
In this query, the betwnstr function will be executed 100 times—and there will be 100 context switches.
FORALL with Sparse Collections
If you try to use the IN low_value .. high_value syntax with FORALL and there is an undefined index value within that range, Oracle Database will raise the “ORA-22160: element at index [N] does not exist” error.To avoid this error, you can use the INDICES OF or VALUES OF clauses. To see how these clauses can be used, let’s go back to the code in Listing 4. In this version of increase_salary, I declare a second collection, l_eligible_ids, to hold the IDs of those employees who are eligible for a raise.
Instead of doing that, I can simply remove all ineligible IDs from the l_employee_ids collection, as follows:
FOR indx IN 1 .. l_employee_ids.COUNT
LOOP
check_eligibility (l_employee_ids (indx),
increase_pct_in,
l_eligible);
IF NOT l_eligible
THEN
l_employee_ids.delete (indx);
END IF;
END LOOP;
But now my l_employee_ids collection may have gaps in it: index values that are undefined between 1 and the highest index value populated by the BULK COLLECT.
No worries. I will simply change my FORALL statement to the following:
FORALL indx IN INDICES OF l_employee_ids
UPDATE employees emp
SET emp.salary =
emp.salary
+ emp.salary *
increase_salary.increase_pct_in
WHERE emp.employee_id =
l_employee_ids (indx);
Now I am telling the PL/SQL engine to use only those index values that are defined in l_employee_ids, rather than specifying a fixed range of values. Oracle Database will simply skip any undefined index values, and the ORA-22160 error will not be raised.
This is the simplest application of INDICES OF. Check the documentation for more-complex usages of INDICES OF, as well as when and how to use VALUES OF.
Bulk Up Your Code!
Optimizing the performance of your code can be a difficult and time-consuming task. It can also be a relatively easy and exhilarating experience—if your code has not yet been modified to take advantage of BULK COLLECT and FORALL. In that case, you have some low-hanging fruit to pick!Take the Challenge
I create and populate my employees table as follows:
CREATE TABLE plch_employees ( employee_id INTEGER, last_name VARCHAR2 (100) ) / BEGIN INSERT INTO plch_employees VALUES (100, ‘Picasso’); INSERT INTO plch_employees VALUES (200, ‘Mondrian’); INSERT INTO plch_employees VALUES (300, ‘O’’Keefe’); COMMIT; END; /
Question
Which of these blocks will uppercase the last names of all employees in the table?a.
DECLARE TYPE ids_t IS TABLE OF plch_employees.employee_id%TYPE; l_ids ids_t := ids_t (100, 200, 300); BEGIN FORALL indx IN 1 .. l_ids.COUNT LOOP UPDATE plch_employees SET last_name = UPPER (last_name) WHERE employee_id = l_ids (indx); END LOOP; END; /
b.
DECLARE TYPE ids_t IS TABLE OF plch_employees.employee_id%TYPE; l_ids ids_t := ids_t (100, 200, 300); BEGIN FORALL indx IN 1 .. l_ids.COUNT UPDATE plch_employees SET last_name = UPPER (last_name) WHERE employee_id = l_ids (indx); END; /
c.
BEGIN UPDATE plch_employees SET last_name = UPPER (last_name); END; /d.
DECLARE TYPE ids_t IS TABLE OF plch_employees.employee_id%TYPE; l_ids ids_t := ids_t (100, 200, 300); BEGIN FORALL indx IN INDICES OF l_ids UPDATE plch_employees SET last_name = UPPER (last_name) WHERE employee_id = l_ids (indx); END; /