By Steven Feuerstein 
Best practices for knowing your LIMIT and kicking %NOTFOUND
I have started using BULK COLLECT whenever I
need to fetch large volumes of data. This has caused me some trouble
with my DBA, however. He is complaining that although my programs might
be running much faster, they are also consuming way too much memory. He
refuses to approve them for a production rollout. What's a programmer
to do?
The most important thing to remember when you
learn about and start to take advantage of features such as BULK
COLLECT is that there is no free lunch. There is almost always a
trade-off to be made somewhere. The tradeoff with BULK COLLECT, like so
many other performance-enhancing features, is "run faster but consume
more memory."
Specifically, memory for collections is stored
in the program global area (PGA), not the system global area (SGA). SGA
memory is shared by all sessions connected to Oracle Database, but PGA
memory is allocated for
each
session. Thus, if a program requires 5MB of memory to populate a
collection and there are 100 simultaneous connections, that program
causes the consumption of 500MB of PGA memory, in addition to the
memory allocated to the SGA.
Fortunately, PL/SQL makes it easy for developers
to control the amount of memory used in a BULK COLLECT operation by
using the LIMIT clause.
Suppose I need to retrieve all the rows from the employees table and then perform some compensation analysis
on each row. I can use BULK COLLECT as follows:
PROCEDURE process_all_rows
IS
TYPE employees_aat
IS TABLE OF employees%ROWTYPE
INDEX BY PLS_INTEGER;
l_employees employees_aat;
BEGIN
SELECT *
BULK COLLECT INTO l_employees
FROM employees;
FOR indx IN 1 .. l_employees.COUNT
LOOP
analyze_compensation
(l_employees(indx));
END LOOP;
END process_all_rows;
Very concise, elegant, and efficient code. If,
however, my employees table contains tens of thousands of rows, each of
which contains hundreds of columns, this program can cause excessive
PGA memory consumption.
Consequently, you should avoid this sort of
"unlimited" use of BULK COLLECT. Instead, move the SELECT statement
into an explicit cursor declaration and then use a simple loop to fetch
many, but not all, rows from the table with each execution of the loop
body, as shown in Listing 1.
Code Listing 1:
Using BULK COLLECT with LIMIT clause
PROCEDURE process_all_rows (limit_in IN PLS_INTEGER DEFAULT 100)
IS
CURSOR employees_cur
IS
SELECT * FROM employees;
TYPE employees_aat IS TABLE OF employees_cur%ROWTYPE
INDEX BY PLS_INTEGER;
l_employees employees_aat;
BEGIN
OPEN employees_cur;
LOOP
FETCH employees_cur
BULK COLLECT INTO l_employees LIMIT limit_in;
FOR indx IN 1 .. l_employees.COUNT
LOOP
analyze_compensation (l_employees(indx));
END LOOP;
EXIT WHEN l_employees.COUNT < limit_in;
END LOOP;
CLOSE employees_cur;
END process_all_rows;
The process_all_rows procedure in Listing 1
requests that up to the value of limit_in rows be fetched at a time.
PL/SQL will reuse the same limit_in elements in the collection each
time the data is fetched and thus also reuse the same memory. Even if
my table grows in size, the PGA consumption will remain stable.
How do you decide what number to use in the
LIMIT clause? Theoretically, you will want to figure out how much
memory you can afford to consume in the PGA and then adjust the limit
to be as close to that amount as possible.
From tests I (and others) have performed,
however, it appears that you will see roughly the same performance no
matter what value you choose for the limit, as long as it is at least
25. The test_diff_limits.sql script, included with the sample code for
this column, at otn.oracle.com/oramag/oracle/08-mar/o28plsql.zip
, demonstrates this behavior, using the ALL_SOURCE data dictionary view on an Oracle Database 11g
instance. Here are the results I saw (in hundredths of seconds) when fetching all the rows (a total of 470,000):
Elapsed CPU time for limit of 1 = 1839
Elapsed CPU time for limit of 5 = 716
Elapsed CPU time for limit of 25 = 539
Elapsed CPU time for limit of 50 = 545
Elapsed CPU time for limit of 75 = 489
Elapsed CPU time for limit of 100 = 490
Elapsed CPU time for limit of 1000 = 501
Elapsed CPU time for limit of 10000 = 478
Elapsed CPU time for limit of 100000 = 527
Kicking the %NOTFOUND Habit
I was very happy to learn that Oracle Database 10g
will automatically optimize my cursor FOR loops to perform at speeds
comparable to BULK COLLECT. Unfortunately, my company is still running
on Oracle9i
Database, so I have started converting my cursor FOR
loops to BULK COLLECTs. I have run into a problem: I am using a LIMIT
of 100, and my query retrieves a total of 227 rows, but my program
processes only 200 of them. [The query is shown in Listing 2.] What am
I doing wrong?
Code Listing 2:
BULK COLLECT, %NOTFOUND, and missing rows
PROCEDURE process_all_rows
IS
CURSOR table_with_227_rows_cur
IS
SELECT * FROM table_with_227_rows;
TYPE table_with_227_rows_aat IS
TABLE OF table_with_227_rows_cur%ROWTYPE
INDEX BY PLS_INTEGER;
l_table_with_227_rows table_with_227_rows_aat;
BEGIN
OPEN table_with_227_rows_cur;
LOOP
FETCH table_with_227_rows_cur
BULK COLLECT INTO l_table_with_227_rows LIMIT 100;
EXIT WHEN table_with_227_rows_cur%NOTFOUND; /* cause of missing rows */
FOR indx IN 1 .. l_table_with_227_rows.COUNT
LOOP
analyze_compensation (l_table_with_227_rows(indx));
END LOOP;
END LOOP;
CLOSE table_with_227_rows_cur;
END process_all_rows;
You came
so
close to a completely correct conversion from your cursor FOR loop to
BULK COLLECT! Your only mistake was that you didn't give up the habit
of using the %NOTFOUND cursor attribute in your EXIT WHEN clause.
The statement
EXIT WHEN
table_with_227_rows_cur%NOTFOUND;
makes perfect sense when you are fetching your
data one row at a time. With BULK COLLECT, however, that line of code
can result in incomplete data processing, precisely as you described.
Let's examine what is happening when you run
your program and why those last 27 rows are left out. After opening the
cursor and entering the loop, here is what occurs:
1.
The fetch statement retrieves rows 1 through 100.
2.
table_with_227_rows_cur%NOTFOUND evaluates to FALSE, and the rows are processed.
3.
The fetch statement retrieves rows 101 through 200.
4.
table_with_227_rows_cur%NOTFOUND evaluates to FALSE, and the rows are processed.
5.
The fetch statement retrieves rows 201 through 227.
6.
table_with_227_rows_cur%NOTFOUND evaluates to
TRUE
, and the loop is terminated—with 27 rows left to process!
When you are using BULK COLLECT and collections to fetch data from your cursor, you should
never
rely on the cursor attributes to decide whether to terminate your loop and data processing.
So, to make sure that your query processes all 227 rows, replace this statement:
EXIT WHEN
table_with_227_rows_cur%NOTFOUND;
with
EXIT WHEN
l_table_with_227_rows.COUNT = 0;
Generally, you should keep all of the following in mind when working with BULK COLLECT:
- The collection is always filled sequentially, starting from index value 1.
- It is always safe (that is, you will never raise a NO_DATA_FOUND exception) to iterate through a collection from 1 to
collection
.COUNT when it has been filled with BULK COLLECT. - The collection is empty when no rows are fetched.
- Always check the contents of the collection (with the COUNT method) to see if there are more rows to process.
- Ignore the values returned by the cursor attributes, especially %NOTFOUND.