·
BULK COLLECT: SELECT statements that retrieve multiple rows with a
single fetch, improving the speed of data retrieval
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. 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.
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.
|
Listing 4
also contains an explanation of the code in this new-and-improved
increase_salary procedure. There are three phases of execution:
1. Fetch
rows with BULK COLLECT into one or more collections. A single context switch is
needed for this step.
2.
Modify the contents of collections as required (in this case, remove ineligible
employees).
3. Change the table with FORALL using
the modified collections.
Rather
than move back and forth between the PL/SQL and SQL engines to update each row,
FORALL “bundles up” all the updates and passes them to the SQL engine with a
single context switch. The result is an extraordinary boost in
performance.
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.
Listing 5
demonstrates an example of fetching values for two columns into a collection of
records.
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.
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.
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.
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.
I hope you all have enjoyed reading this article. Comments
are welcome....
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Good... breifly explained. ..keep doing good work
ReplyDeleteGood... breifly explained. ..keep doing good work
ReplyDeleteExcellent article! Great explanation of Bulk collections and FORALL statement. I will bookmark it in my Firefox, for wherever i need a quick reminder of this feature (since , realistically, it is not used very often).
ReplyDelete