11 Tuning PL/SQL Applications for Performance

This chapter shows you how to write efficient PL/SQL code, and speed up existing code.

This chapter contains these topics:

See Also:

Initialization Parameters for PL/SQL Compilation

There are several Oracle initialization parameters that are used in the compilation of PL/SQL units. These designated PL/SQL compiler parameters include PLSQL_CCFLAGS, PLSQL_CODE_TYPE, PLSQL_DEBUG, PLSQL_NATIVE_LIBRARY_DIR, PLSQL_NATIVE_LIBRARY_SUBDIR_COUNT, PLSQL_OPTIMIZE_LEVEL, PLSQL_WARNINGS, and NLS_LENGTH_SEMANTICS. For information about these initialization parameters, see Oracle Database Reference.

The values at the time of compilation of the PLSQL_CCFLAGS, PLSQL_CODE_TYPE, PLSQL_DEBUG, PLSQL_OPTIMIZE_LEVEL, PLSQL_WARNINGS, and NLS_LENGTH_SEMANTICS initialization parameters are stored with the unit's metadata. You can view information about the settings of these parameters with the ALL_PLSQL_OBJECT_SETTINGS view. For information, see Oracle Database Reference.

You can specify REUSE SETTINGS with the SQL ALTER statement to preserve the values stored in a unit's metadata. For information of the use of the PL/SQL compiler parameters with the ALTER statement, see the ALTER FUNCTION, ALTER PACKAGE, ALTER PROCEDURE, and ALTER SESSION statements in the Oracle Database SQL Reference.

How PL/SQL Optimizes Your Programs

In Oracle releases prior to 10g, the PL/SQL compiler translated your code to machine code without applying many changes for performance. Now, PL/SQL uses an optimizing compiler that can rearrange code for better performance.

You do not need to do anything to get the benefits of this new optimizer. It is enabled by default. In rare cases, if the overhead of the optimizer makes compilation of very large applications take too long, you might lower the optimization by setting the initialization parameter PLSQL_OPTIMIZE_LEVEL=1 instead of its default value 2. In even rarer cases, you might see a change in exception behavior, either an exception that is not raised at all, or one that is raised earlier than expected. Setting PLSQL_OPTIMIZE_LEVEL=0 prevents the code from being rearranged at all. For information on the PLSQL_OPTIMIZE_LEVEL initialization parameter, see Oracle Database Reference.

You can view information about the optimization level and other PLSQL compiler settings for your environment with the ALL_PLSQL_OBJECT_SETTINGS view. For information, see Oracle Database Reference.

When to Tune PL/SQL Code

The information in this chapter is especially valuable if you are responsible for:

  • Programs that do a lot of mathematical calculations. You will want to investigate the datatypes PLS_INTEGER, BINARY_FLOAT, and BINARY_DOUBLE.

  • Functions that are called from PL/SQL queries, where the functions might be executed millions of times. You will want to look at all performance features to make the function as efficient as possible, and perhaps a function-based index to precompute the results for each row and save on query time.

  • Programs that spend a lot of time processing INSERT, UPDATE, or DELETE statements, or looping through query results. You will want to investigate the FORALL statement for issuing DML, and the BULK COLLECT INTO and RETURNING BULK COLLECT INTO clauses for queries.

  • Older code that does not take advantage of recent PL/SQL language features. With the many performance improvements in Oracle Database 10g, any code from earlier releases is a candidate for tuning.

  • Any program that spends a lot of time doing PL/SQL processing, as opposed to issuing DDL statements like CREATE TABLE that are just passed directly to SQL. You will want to investigate native compilation. Because many built-in database features use PL/SQL, you can apply this tuning feature to an entire database to improve performance in many areas, not just your own code.

Before starting any tuning effort, benchmark the current system and measure how long particular subprograms take. PL/SQL in Oracle Database 10g includes many automatic optimizations, so you might see performance improvements without doing any tuning.

Guidelines for Avoiding PL/SQL Performance Problems

When a PL/SQL-based application performs poorly, it is often due to badly written SQL statements, poor programming practices, inattention to PL/SQL basics, or misuse of shared memory.

Avoiding CPU Overhead in PL/SQL Code

This sections discusses how you can avoid excessive CPU overhead in the PL/SQL code.

Make SQL Statements as Efficient as Possible

PL/SQL programs look relatively simple because most of the work is done by SQL statements. Slow SQL statements are the main reason for slow execution.

If SQL statements are slowing down your program:

  • Make sure you have appropriate indexes. There are different kinds of indexes for different situations. Your index strategy might be different depending on the sizes of various tables in a query, the distribution of data in each query, and the columns used in the WHERE clauses.

  • Make sure you have up-to-date statistics on all the tables, using the subprograms in the DBMS_STATS package.

  • Analyze the execution plans and performance of the SQL statements, using:

    • EXPLAIN PLAN statement

    • SQL Trace facility with TKPROF utility

  • Rewrite the SQL statements if necessary. For example, query hints can avoid problems such as unnecessary full-table scans.

For more information about these methods, see Oracle Database Performance Tuning Guide.

Some PL/SQL features also help improve the performance of SQL statements:

  • If you are running SQL statements inside a PL/SQL loop, look at the FORALL statement as a way to replace loops of INSERT, UPDATE, and DELETE statements.

  • If you are looping through the result set of a query, look at the BULK COLLECT clause of the SELECT INTO statement as a way to bring the entire result set into memory in a single operation.

Make Function Calls as Efficient as Possible

Badly written subprograms (for example, a slow sort or search function) can harm performance. Avoid unnecessary calls to subprograms, and optimize their code:

  • If a function is called within a SQL query, you can cache the function value for each row by creating a function-based index on the table in the query. The CREATE INDEX statement might take a while, but queries can be much faster.

  • If a column is passed to a function within an SQL query, the query cannot use regular indexes on that column, and the function might be called for every row in a (potentially very large) table. Consider nesting the query so that the inner query filters the results to a small number of rows, and the outer query calls the function only a few times as shown in Example 11-1.

Example 11-1 Nesting a Query to Improve Performance

-- Inefficient, calls function for every row
   FOR item IN (SELECT DISTINCT(SQRT(department_id)) col_alias FROM employees)
-- Efficient, only calls function once for each distinct value.
   FOR item IN
   ( SELECT SQRT(department_id) col_alias FROM
     ( SELECT DISTINCT department_id FROM employees)

If you use OUT or IN OUT parameters, PL/SQL adds some performance overhead to ensure correct behavior in case of exceptions (assigning a value to the OUT parameter, then exiting the subprogram because of an unhandled exception, so that the OUT parameter keeps its original value).

If your program does not depend on OUT parameters keeping their values in such situations, you can add the NOCOPY keyword to the parameter declarations, so the parameters are declared OUT NOCOPY or IN OUT NOCOPY.

This technique can give significant speedup if you are passing back large amounts of data in OUT parameters, such as collections, big VARCHAR2 values, or LOBs.

This technique also applies to member methods of object types. If these methods modify attributes of the object type, all the attributes are copied when the method ends. To avoid this overhead, you can explicitly declare the first parameter of the member method as SELF IN OUT NOCOPY, instead of relying on PL/SQL's implicit declaration SELF IN OUT. For information about design considerations for object methods, see Oracle Database Application Developer's Guide - Object-Relational Features.

Make Loops as Efficient as Possible

Because PL/SQL applications are often built around loops, it is important to optimize the loop itself and the code inside the loop:

  • To issue a series of DML statements, replace loop constructs with FORALL statements.

  • To loop through a result set and store the values, use the BULK COLLECT clause on the query to bring the query results into memory in one operation.

  • If you have to loop through a result set more than once, or issue other queries as you loop through a result set, you can probably enhance the original query to give you exactly the results you want. Some query operators to explore include UNION, INTERSECT, MINUS, and CONNECT BY.

  • You can also nest one query inside another (known as a subselect) to do the filtering and sorting in multiple stages. For example, instead of calling a PL/SQL function in the inner WHERE clause (which might call the function once for each row of the table), you can filter the result set to a small set of rows in the inner query, and call the function in the outer query.

Do Not Duplicate Built-in String Functions

PL/SQL provides many highly optimized string functions such as REPLACE, TRANSLATE, SUBSTR, INSTR, RPAD, and LTRIM. The built-in functions use low-level code that is more efficient than regular PL/SQL.

If you use PL/SQL string functions to search for regular expressions, consider using the built-in regular expression functions, such as REGEXP_SUBSTR.

  • You can search for regular expressions using the SQL operator REGEXP_LIKE. See Example 6-10.

  • You can test or manipulate strings using the built-in functions REGEXP_INSTR, REGEXP_REPLACE, and REGEXP_SUBSTR.

Oracle's regular expression features use characters like '.', '*', '^', and '$' that you might be familiar with from UNIX or Perl programming. For multi-language programming, there are also extensions such as '[:lower:]' to match a lowercase letter, instead of '[a-z]' which does not match lowercase accented letters.

See Also:

Oracle By Example - Using Regular Expressions on the Oracle Technology Network (OTN):


Reorder Conditional Tests to Put the Least Expensive First

PL/SQL stops evaluating a logical expression as soon as the result can be determined. This functionality is known as short-circuit evaluation. See "Short-Circuit Evaluation".

When evaluating multiple conditions separated by AND or OR, put the least expensive ones first. For example, check the values of PL/SQL variables before testing function return values, because PL/SQL might be able to skip calling the functions.

Minimize Datatype Conversions

At run time, PL/SQL converts between different datatypes automatically. For example, assigning a PLS_INTEGER variable to a NUMBER variable results in a conversion because their internal representations are different.

Whenever possible, choose datatypes carefully to minimize implicit conversions. Use literals of the appropriate types, such as character literals in character expressions and decimal numbers in number expressions.

Minimizing conversions might mean changing the types of your variables, or even working backward and designing your tables with different datatypes. Or, you might convert data once, such as from an INTEGER column to a PLS_INTEGER variable, and use the PL/SQL type consistently after that. Note that the conversion from an INTEGER to a PLS_INTEGER datatype could actually improve performance because of the use of more efficient hardware arithmetic. See "Use PLS_INTEGER for Integer Arithmetic".

Use PLS_INTEGER for Integer Arithmetic

When you need to declare a local integer variable, use the datatype PLS_INTEGER, which is the most efficient integer type. PLS_INTEGER values require less storage than INTEGER or NUMBER values, and PLS_INTEGER operations use hardware arithmetic. Note that the BINARY_INTEGER datatype is identical to PLS_INTEGER.

The datatype NUMBER and its subtypes are represented in a special internal format, designed for portability and arbitrary scale and precision, not performance. Even the subtype INTEGER is treated as a floating-point number with nothing after the decimal point. Operations on NUMBER or INTEGER variables require calls to library routines.

Avoid constrained subtypes such as INTEGER, NATURAL, NATURALN, POSITIVE, POSITIVEN, and SIGNTYPE in performance-critical code. Variables of these types require extra checking at run time, each time they are used in a calculation.

Use BINARY_FLOAT and BINARY_DOUBLE for Floating-Point Arithmetic

The datatype NUMBER and its subtypes are represented in a special internal format, designed for portability and arbitrary scale and precision, not performance. Operations on NUMBER or INTEGER variables require calls to library routines.

The BINARY_FLOAT and BINARY_DOUBLE types can use native hardware arithmetic instructions, and are more efficient for number-crunching applications such as scientific processing. They also require less space in the database.

These types do not always represent fractional values precisely, and handle rounding differently than the NUMBER types. These types are less suitable for financial code where accuracy is critical.

Avoiding Memory Overhead in PL/SQL Code

This sections discusses how you can avoid excessive memory overhead in the PL/SQL code.

Be Generous When Declaring Sizes for VARCHAR2 Variables

You might need to allocate large VARCHAR2 variables when you are not sure how big an expression result will be. You can actually conserve memory by declaring VARCHAR2 variables with large sizes, such as 32000, rather than estimating just a little on the high side, such as by specifying 256 or 1000. PL/SQL has an optimization that makes it easy to avoid overflow problems and still conserve memory. Specify a size of more than 4000 characters for the VARCHAR2 variable; PL/SQL waits until you assign the variable, then only allocates as much storage as needed.

Group Related Subprograms into Packages

When you call a packaged subprogram for the first time, the whole package is loaded into the shared memory pool. Subsequent calls to related subprograms in the package require no disk I/O, and your code executes faster. If the package is aged out of memory, it must be reloaded if you reference it again.

You can improve performance by sizing the shared memory pool correctly. Make sure it is large enough to hold all frequently used packages but not so large that memory is wasted.

Pin Packages in the Shared Memory Pool

You can pin frequently accessed packages in the shared memory pool, using the supplied package DBMS_SHARED_POOL. When a package is pinned, it is not aged out by the least recently used (LRU) algorithm that Oracle normally uses. The package remains in memory no matter how full the pool gets or how frequently you access the package.

For more information on the DBMS_SHARED_POOL package, see Oracle Database PL/SQL Packages and Types Reference.

Improve Your Code to Avoid Compiler Warnings

The PL/SQL compiler issues warnings about things that do not make a program incorrect, but might lead to poor performance. If you receive such a warning, and the performance of this code is important, follow the suggestions in the warning and change the code to be more efficient.

Profiling and Tracing PL/SQL Programs

As you develop larger and larger PL/SQL applications, it becomes more difficult to isolate performance problems. PL/SQL provides a Profiler API to profile run-time behavior and to help you identify performance bottlenecks. PL/SQL also provides a Trace API for tracing the execution of programs on the server. You can use Trace to trace the execution by subprogram or exception.

Using The Profiler API: Package DBMS_PROFILER

The Profiler API is implemented as PL/SQL package DBMS_PROFILER, which provides services for gathering and saving run-time statistics. The information is stored in database tables, which you can query later. For example, you can learn how much time was spent executing each PL/SQL line and subprogram.

To use the Profiler, you start the profiling session, run your application long enough to get adequate code coverage, flush the collected data to the database, then stop the profiling session.

The Profiler traces the execution of your program, computing the time spent at each line and in each subprogram. You can use the collected data to improve performance. For instance, you might focus on subprograms that run slowly. For information about the DBMS_PROFILER subprograms, see Oracle Database PL/SQL Packages and Types Reference.

After you have collected data with the Profiler, you can do the following:

  • Analyze the Collected Performance Data

    Determine why more time was spent executing certain code segments or accessing certain data structures. Find the problem areas by querying the performance data. Focus on the subprograms and packages that use up the most execution time, inspecting possible performance bottlenecks such as SQL statements, loops, and recursive functions.

  • Use Trace Data to Improve Performance

    Use the results of your analysis to rework slow algorithms. For example, due to an exponential growth in data, you might need to replace a linear search with a binary search. Also, look for inefficiencies caused by inappropriate data structures, and, if necessary, replace those data structures.

Using The Trace API: Package DBMS_TRACE

With large, complex applications, it becomes difficult to keep track of calls between subprograms. By tracing your code with the Trace API, you can see the order in which subprograms execute. The Trace API is implemented as PL/SQL package DBMS_TRACE, which provides services for tracing execution by subprogram or exception.

To use Trace, you start the tracing session, run your application, then stop the tracing session. As the program executes, trace data is collected and stored in database tables.

For information about the DBMS_TRACE subprograms, see Oracle Database PL/SQL Packages and Types Reference.

Controlling the Trace

Tracing large applications can produce huge amounts of data that are difficult to manage. Before starting Trace, you can optionally limit the volume of data collected by selecting specific subprograms for trace data collection.

In addition, you can choose a tracing level. For example, you can choose to trace all subprograms and exceptions, or you can choose to trace selected subprograms and exceptions.

Reducing Loop Overhead for DML Statements and Queries with Bulk SQL

PL/SQL sends SQL statements such as DML and queries to the SQL engine for execution, and SQL returns the result data to PL/SQL. You can minimize the performance overhead of this communication between PL/SQL and SQL by using the PL/SQL language features known collectively as bulk SQL. The FORALL statement sends INSERT, UPDATE, or DELETE statements in batches, rather than one at a time. The BULK COLLECT clause brings back batches of results from SQL. If the DML statement affects four or more database rows, the use of bulk SQL can improve performance considerably.

The assigning of values to PL/SQL variables in SQL statements is called binding. PL/SQL binding operations fall into three categories:

  • in-bind: When a PL/SQL variable or host variable is stored in the database by an INSERT or UPDATE statement.

  • out-bind: When a database value is assigned to a PL/SQL variable or a host variable by the RETURNING clause of an INSERT, UPDATE, or DELETE statement.

  • define: When a database value is assigned to a PL/SQL variable or a host variable by a SELECT or FETCH statement.

Bulk SQL uses PL/SQL collections, such as varrays or nested tables, to pass large amounts of data back and forth in a single operation. This process is known as bulk binding. If the collection has 20 elements, bulk binding lets you perform the equivalent of 20 SELECT, INSERT, UPDATE, or DELETE statements using a single operation. Queries can pass back any number of results, without requiring a FETCH statement for each row.

To speed up INSERT, UPDATE, and DELETE statements, enclose the SQL statement within a PL/SQL FORALL statement instead of a loop construct.

To speed up SELECT statements, include the BULK COLLECT INTO clause in the SELECT statement instead of using INTO.

For full details of the syntax and restrictions for these statements, see "FORALL Statement" and "SELECT INTO Statement".

Using the FORALL Statement

The keyword FORALL lets you run multiple DML statements very efficiently. It can only repeat a single DML statement, unlike a general-purpose FOR loop. For full syntax and restrictions, see "FORALL Statement".

The SQL statement can reference more than one collection, but FORALL only improves performance where the index value is used as a subscript.

Usually, the bounds specify a range of consecutive index numbers. If the index numbers are not consecutive, such as after you delete collection elements, you can use the INDICES OF or VALUES OF clause to iterate over just those index values that really exist.

The INDICES OF clause iterates over all of the index values in the specified collection, or only those between a lower and upper bound.

The VALUES OF clause refers to a collection that is indexed by BINARY_INTEGER or PLS_INTEGER and whose elements are of type BINARY_INTEGER or PLS_INTEGER. The FORALL statement iterates over the index values specified by the elements of this collection.

The FORALL statement in Example 11-2 sends all three DELETE statements to the SQL engine at once.

Example 11-2 Issuing DELETE Statements in a Loop

CREATE TABLE employees_temp AS SELECT * FROM employees;
   depts NumList := NumList(10, 30, 70);  -- department numbers
   FORALL i IN depts.FIRST..depts.LAST
      DELETE FROM employees_temp WHERE department_id = depts(i);

Example 11-3 loads some data into PL/SQL collections. Then it inserts the collection elements into a database table twice: first using a FOR loop, then using a FORALL statement. The FORALL version is much faster.

Example 11-3 Issuing INSERT Statements in a Loop

CREATE TABLE parts1 (pnum INTEGER, pname VARCHAR2(15));
CREATE TABLE parts2 (pnum INTEGER, pname VARCHAR2(15));
  pnums  NumTab;
  pnames NameTab;
  iterations CONSTANT PLS_INTEGER := 500;
  FOR j IN 1..iterations LOOP  -- load index-by tables
     pnums(j) := j;
     pnames(j) := 'Part No. ' || TO_CHAR(j);
  t1 := DBMS_UTILITY.get_time;
  FOR i IN 1..iterations LOOP  -- use FOR loop
     INSERT INTO parts1 VALUES (pnums(i), pnames(i));
  t2 := DBMS_UTILITY.get_time;
  FORALL i IN 1..iterations  -- use FORALL statement
     INSERT INTO parts2 VALUES (pnums(i), pnames(i));
  t3 := DBMS_UTILITY.get_time;
  DBMS_OUTPUT.PUT_LINE('Execution Time (secs)');
  DBMS_OUTPUT.PUT_LINE('FOR loop: ' || TO_CHAR((t2 - t1)/100));
  DBMS_OUTPUT.PUT_LINE('FORALL:   ' || TO_CHAR((t3 - t2)/100));

Executing this block should show that the loop using FORALL is much faster.

The bounds of the FORALL loop can apply to part of a collection, not necessarily all the elements, as shown in Example 11-4.

Example 11-4 Using FORALL with Part of a Collection

CREATE TABLE employees_temp AS SELECT * FROM employees;
   depts NumList := NumList(5,10,20,30,50,55,57,60,70,75);
   FORALL j IN 4..7  -- use only part of varray
      DELETE FROM employees_temp WHERE department_id = depts(j);

You might need to delete some elements from a collection before using the collection in a FORALL statement. The INDICES OF clause processes sparse collections by iterating through only the remaining elements.

You might also want to leave the original collection alone, but process only some elements, process the elements in a different order, or process some elements more than once. Instead of copying the entire elements into new collections, which might use up substantial amounts of memory, the VALUES OF clause lets you set up simple collections whose elements serve as pointers to elements in the original collection.

Example 11-5 creates a collection holding some arbitrary data, a set of table names. Deleting some of the elements makes it a sparse collection that would not work in a default FORALL statement. The program uses a FORALL statement with the INDICES OF clause to insert the data into a table. It then sets up two more collections, pointing to certain elements from the original collection. The program stores each set of names in a different database table using FORALL statements with the VALUES OF clause.

Example 11-5 Using FORALL with Non-Consecutive Index Values

-- Create empty tables to hold order details
CREATE TABLE valid_orders (cust_name VARCHAR2(32), amount NUMBER(10,2));
CREATE TABLE big_orders AS SELECT * FROM valid_orders WHERE 1 = 0;
CREATE TABLE rejected_orders AS SELECT * FROM valid_orders WHERE 1 = 0;
-- Make collections to hold a set of customer names and order amounts.
   SUBTYPE cust_name IS valid_orders.cust_name%TYPE;
   TYPE cust_typ IS TABLe OF cust_name;
   cust_tab cust_typ;
   SUBTYPE order_amount IS valid_orders.amount%TYPE;
   TYPE amount_typ IS TABLE OF NUMBER;
   amount_tab amount_typ;
-- Make other collections to point into the CUST_TAB collection.
   TYPE index_pointer_t IS TABLE OF PLS_INTEGER;
   big_order_tab index_pointer_t := index_pointer_t();
   rejected_order_tab index_pointer_t := index_pointer_t();
   PROCEDURE setup_data IS BEGIN
 -- Set up sample order data, including some invalid orders and some 'big' orders.
     cust_tab := cust_typ('Company1','Company2','Company3','Company4','Company5');
     amount_tab := amount_typ(5000.01, 0, 150.25, 4000.00, NULL);
   DBMS_OUTPUT.PUT_LINE('--- Original order data ---');
   FOR i IN 1..cust_tab.LAST LOOP
     DBMS_OUTPUT.PUT_LINE('Customer #' || i || ', ' || cust_tab(i) || ': $' ||
-- Delete invalid orders (where amount is null or 0).
   FOR i IN 1..cust_tab.LAST LOOP
     IF amount_tab(i) is null or amount_tab(i) = 0 THEN
     END IF;
   DBMS_OUTPUT.PUT_LINE('--- Data with invalid orders deleted ---');
   FOR i IN 1..cust_tab.LAST LOOP
     IF cust_tab.EXISTS(i) THEN
       DBMS_OUTPUT.PUT_LINE('Customer #' || i || ', ' || cust_tab(i) || ': $' ||
      END IF;
-- Because the subscripts of the collections are not consecutive, use
-- FORALL...INDICES OF to iterate through the actual subscripts, 
-- rather than 1..COUNT
   FORALL i IN INDICES OF cust_tab
     INSERT INTO valid_orders(cust_name, amount) 
        VALUES(cust_tab(i), amount_tab(i));
-- Now process the order data differently
-- Extract 2 subsets and store each subset in a different table
   setup_data(); -- Initialize the CUST_TAB and AMOUNT_TAB collections again.
   FOR i IN cust_tab.FIRST .. cust_tab.LAST LOOP
     IF amount_tab(i) IS NULL OR amount_tab(i) = 0 THEN
       rejected_order_tab.EXTEND; -- Add a new element to this collection
-- Record the subscript from the original collection
       rejected_order_tab(rejected_order_tab.LAST) := i; 
     END IF;
     IF amount_tab(i) > 2000 THEN
        big_order_tab.EXTEND; -- Add a new element to this collection
-- Record the subscript from the original collection
        big_order_tab(big_order_tab.LAST) := i;
     END IF;
-- Now it's easy to run one DML statement on one subset of elements, 
-- and another DML statement on a different subset.
   FORALL i IN VALUES OF rejected_order_tab
     INSERT INTO rejected_orders VALUES (cust_tab(i), amount_tab(i));
   FORALL i IN VALUES OF big_order_tab
     INSERT INTO big_orders VALUES (cust_tab(i), amount_tab(i));
-- Verify that the correct order details were stored
SELECT cust_name "Customer", amount "Valid order amount" FROM valid_orders;
SELECT cust_name "Customer", amount "Big order amount" FROM big_orders;
SELECT cust_name "Customer", amount "Rejected order amount" FROM rejected_orders;

How FORALL Affects Rollbacks

In a FORALL statement, if any execution of the SQL statement raises an unhandled exception, all database changes made during previous executions are rolled back. However, if a raised exception is caught and handled, changes are rolled back to an implicit savepoint marked before each execution of the SQL statement. Changes made during previous executions are not rolled back. For example, suppose you create a database table that stores department numbers and job titles, as shown in Example 11-6. Then, you change the job titles so that they are longer. The second UPDATE fails because the new value is too long for the column. Because we handle the exception, the first UPDATE is not rolled back and we can commit that change.

Example 11-6 Using Rollbacks With FORALL

CREATE TABLE emp_temp (deptno NUMBER(2), job VARCHAR2(18));
   depts NumList := NumList(10, 20, 30);
  INSERT INTO emp_temp VALUES(10, 'Clerk');
-- Lengthening this job title causes an exception
  INSERT INTO emp_temp VALUES(20, 'Bookkeeper');
  INSERT INTO emp_temp VALUES(30, 'Analyst');
  FORALL j IN depts.FIRST..depts.LAST -- Run 3 UPDATE statements.
    UPDATE emp_temp SET job = job || ' (Senior)' WHERE deptno = depts(j);
-- raises a "value too large" exception
    DBMS_OUTPUT.PUT_LINE('Problem in the FORALL statement.');
    COMMIT; -- Commit results of successful updates.

Counting Rows Affected by FORALL with the %BULK_ROWCOUNT Attribute

The cursor attributes SQL%FOUND, SQL%ISOPEN, SQL%NOTFOUND, and SQL%ROWCOUNT, return useful information about the most recently executed DML statement. For additional description of cursor attributes, see "Implicit Cursors".

The SQL cursor has one composite attribute, %BULK_ROWCOUNT, for use with the FORALL statement. This attribute works like an associative array: SQL%BULK_ROWCOUNT(i) stores the number of rows processed by the ith execution of an INSERT, UPDATE or DELETE statement. For example:

Example 11-7 Using %BULK_ROWCOUNT With the FORALL Statement

CREATE TABLE emp_temp AS SELECT * FROM employees;
   depts NumList := NumList(30, 50, 60);
   FORALL j IN depts.FIRST..depts.LAST
      DELETE FROM emp_temp WHERE department_id = depts(j);
-- How many rows were affected by each DELETE statement?
   FOR i IN depts.FIRST..depts.LAST
      DBMS_OUTPUT.PUT_LINE('Iteration #' || i || ' deleted ' ||
         SQL%BULK_ROWCOUNT(i) || ' rows.');

The FORALL statement and %BULK_ROWCOUNT attribute use the same subscripts. For example, if FORALL uses the range 5..10, so does %BULK_ROWCOUNT. If the FORALL statement uses the INDICES OF clause to process a sparse collection, %BULK_ROWCOUNT has corresponding sparse subscripts. If the FORALL statement uses the VALUES OF clause to process a subset of elements, %BULK_ROWCOUNT has subscripts corresponding to the values of the elements in the index collection. If the index collection contains duplicate elements, so that some DML statements are issued multiple times using the same subscript, then the corresponding elements of %BULK_ROWCOUNT represent the sum of all rows affected by the DML statement using that subscript. For examples on how to interpret %BULK_ROWCOUNT when using the INDICES OF and VALUES OF clauses, see the PL/SQL sample programs at http://www.oracle.com/technology/sample_code/tech/pl_sql/.

%BULK_ROWCOUNT is usually equal to 1 for inserts, because a typical insert operation affects only a single row. For the INSERT ... SELECT construct, %BULK_ROWCOUNT might be greater than 1. For example, the FORALL statement in Example 11-8 inserts an arbitrary number of rows for each iteration. After each iteration, %BULK_ROWCOUNT returns the number of items inserted.

Example 11-8 Counting Rows Affected by FORALL With %BULK_ROWCOUNT

CREATE TABLE emp_by_dept AS SELECT employee_id, department_id
   FROM employees WHERE 1 = 0;
  TYPE dept_tab IS TABLE OF departments.department_id%TYPE;
  deptnums dept_tab;
  SELECT department_id BULK COLLECT INTO deptnums FROM departments;
  FORALL i IN 1..deptnums.COUNT
     INSERT INTO emp_by_dept
        SELECT employee_id, department_id FROM employees
           WHERE department_id = deptnums(i);
  FOR i IN 1..deptnums.COUNT LOOP
-- Count how many rows were inserted for each department; that is,
-- how many employees are in each department.
   DBMS_OUTPUT.PUT_LINE('Dept '||deptnums(i)||': inserted '||
                          SQL%BULK_ROWCOUNT(i)||' records');
  DBMS_OUTPUT.PUT_LINE('Total records inserted: ' || SQL%ROWCOUNT);

You can also use the scalar attributes %FOUND, %NOTFOUND, and %ROWCOUNT after running a FORALL statement. For example, %ROWCOUNT returns the total number of rows processed by all executions of the SQL statement.

%FOUND and %NOTFOUND refer only to the last execution of the SQL statement. You can use %BULK_ROWCOUNT to infer their values for individual executions. For example, when %BULK_ROWCOUNT(i) is zero, %FOUND and %NOTFOUND are FALSE and TRUE, respectively.

Handling FORALL Exceptions with the %BULK_EXCEPTIONS Attribute

PL/SQL provides a mechanism to handle exceptions raised during the execution of a FORALL statement. This mechanism enables a bulk-bind operation to save information about exceptions and continue processing.

To have a bulk bind complete despite errors, add the keywords SAVE EXCEPTIONS to your FORALL statement after the bounds, before the DML statement. You should also provide an exception handler to track the exceptions that occurred during the bulk operation.

Example 11-9 shows how you can perform a number of DML operations, without stopping if some operations encounter errors. In the example, EXCEPTION_INIT is used to associate the dml_errors exception with the ORA-24381 error. The ORA-24381 error is raised if any exceptions are caught and saved after a bulk operation.

All exceptions raised during the execution are saved in the cursor attribute %BULK_EXCEPTIONS, which stores a collection of records. Each record has two fields:

  • %BULK_EXCEPTIONS(i).ERROR_INDEX holds the iteration of the FORALL statement during which the exception was raised.

  • %BULK_EXCEPTIONS(i).ERROR_CODE holds the corresponding Oracle error code.

The values stored by %BULK_EXCEPTIONS always refer to the most recently executed FORALL statement. The number of exceptions is saved in %BULK_EXCEPTIONS.COUNT. Its subscripts range from 1 to COUNT.

The individual error messages, or any substitution arguments, are not saved, but the error message text can looked up using ERROR_CODE with SQLERRM as shown in Example 11-9.

You might need to work backward to determine which collection element was used in the iteration that caused an exception. For example, if you use the INDICES OF clause to process a sparse collection, you must step through the elements one by one to find the one corresponding to %BULK_EXCEPTIONS(i).ERROR_INDEX. If you use the VALUES OF clause to process a subset of elements, you must find the element in the index collection whose subscript matches %BULK_EXCEPTIONS(i).ERROR_INDEX, and then use that element's value as the subscript to find the erroneous element in the original collection. For examples showing how to find the erroneous elements when using the INDICES OF and VALUES OF clauses, see the PL/SQL sample programs at http://www.oracle.com/technology/tech/pl_sql/.

If you omit the keywords SAVE EXCEPTIONS, execution of the FORALL statement stops when an exception is raised. In that case, SQL%BULK_EXCEPTIONS.COUNT returns 1, and SQL%BULK_EXCEPTIONS contains just one record. If no exception is raised during execution, SQL%BULK_EXCEPTIONS.COUNT returns 0.

Example 11-9 Bulk Operation That Continues Despite Exceptions

-- create a temporary table for this example
CREATE TABLE emp_temp AS SELECT * FROM employees;

   TYPE empid_tab IS TABLE OF employees.employee_id%TYPE;
   emp_sr empid_tab;
-- create an exception handler for ORA-24381
   errors NUMBER;
   dml_errors EXCEPTION;
   PRAGMA EXCEPTION_INIT(dml_errors, -24381);
   SELECT employee_id BULK COLLECT INTO emp_sr FROM emp_temp 
         WHERE hire_date < '30-DEC-94';
-- add '_SR' to the job_id of the most senior employees
       UPDATE emp_temp SET job_id = job_id || '_SR' 
          WHERE emp_sr(i) = emp_temp.employee_id;
-- If any errors occurred during the FORALL SAVE EXCEPTIONS,
-- a single exception is raised when the statement completes.

  WHEN dml_errors THEN -- Now we figure out what failed and why.
   DBMS_OUTPUT.PUT_LINE('Number of statements that failed: ' || errors);
   FOR i IN 1..errors LOOP
      DBMS_OUTPUT.PUT_LINE('Error #' || i || ' occurred during '|| 
         'iteration #' || SQL%BULK_EXCEPTIONS(i).ERROR_INDEX); 
          DBMS_OUTPUT.PUT_LINE('Error message is ' ||
DROP TABLE emp_temp;

The output from the example is similar to:

Number of statements that failed: 2
Error #1 occurred during iteration #7
Error message is ORA-12899: value too large for column
Error #2 occurred during iteration #13
Error message is ORA-12899: value too large for column

In Example 11-9, PL/SQL raises predefined exceptions because updated values were too large to insert into the job_id column. After the FORALL statement, SQL%BULK_EXCEPTIONS.COUNT returned 2, and the contents of SQL%BULK_EXCEPTIONS were (7,12899) and (13,12899).

To get the Oracle error message (which includes the code), the value of SQL%BULK_EXCEPTIONS(i).ERROR_CODE was negated and then passed to the error-reporting function SQLERRM, which expects a negative number.

Retrieving Query Results into Collections with the BULK COLLECT Clause

Using the keywords BULK COLLECT with a query is a very efficient way to retrieve the result set. Instead of looping through each row, you store the results in one or more collections, in a single operation. You can use these keywords in the SELECT INTO and FETCH INTO statements, and the RETURNING INTO clause.

With the BULK COLLECT clause, all the variables in the INTO list must be collections. The table columns can hold scalar or composite values, including object types. Example 11-10 loads two entire database columns into nested tables:

Example 11-10 Retrieving Query Results With BULK COLLECT

   TYPE NumTab IS TABLE OF employees.employee_id%TYPE;
   TYPE NameTab IS TABLE OF employees.last_name%TYPE;
   enums NumTab;   -- No need to initialize the collections.
   names NameTab;  -- Values will be filled in by the SELECT INTO.
   PROCEDURE print_results IS
     IF enums.COUNT = 0 THEN 
       DBMS_OUTPUT.PUT_LINE('No results!');
       FOR i IN enums.FIRST .. enums.LAST
         DBMS_OUTPUT.PUT_LINE('  Employee #' || enums(i) || ': ' || names(i));
       END LOOP;
     END IF;
 -- Retrieve data for employees with Ids greater than 1000
   SELECT employee_id, last_name
     BULK COLLECT INTO enums, names FROM employees WHERE employee_id > 1000;
-- The data has all been brought into memory by BULK COLLECT
-- No need to FETCH each row from the result set
 -- Retrieve approximately 20% of all rows
   SELECT employee_id, last_name
      BULK COLLECT INTO enums, names FROM employees SAMPLE (20);

The collections are initialized automatically. Nested tables and associative arrays are extended to hold as many elements as needed. If you use varrays, all the return values must fit in the varray's declared size. Elements are inserted starting at index 1, overwriting any existing elements.

Because the processing of the BULK COLLECT INTO clause is similar to a FETCH loop, it does not raise a NO_DATA_FOUND exception if no rows match the query. You must check whether the resulting nested table or varray is null, or if the resulting associative array has no elements, as shown in Example 11-10.

To prevent the resulting collections from expanding without limit, you can use the LIMIT clause to or pseudocolumn ROWNUM to limit the number of rows processed. You can also use the SAMPLE clause to retrieve a random sample of rows.

Example 11-11 Using the Pseudocolumn ROWNUM to Limit Query Results

   TYPE SalList IS TABLE OF employees.salary%TYPE;
   sals SalList;
-- Limit the number of rows to 50
   SELECT salary BULK COLLECT INTO sals FROM employees
      WHERE ROWNUM <= 50;
-- Retrieve 10% (approximately) of the rows in the table
   SELECT salary BULK COLLECT INTO sals FROM employees SAMPLE (10);

You can process very large result sets by fetching a specified number of rows at a time from a cursor, as shown in the following sections.

Examples of Bulk-Fetching from a Cursor

You can fetch from a cursor into one or more collections as shown in Example 11-12.

Example 11-12 Bulk-Fetching from a Cursor Into One or More Collections

  TYPE NameList IS TABLE OF employees.last_name%TYPE;
  TYPE SalList IS TABLE OF employees.salary%TYPE;
  CURSOR c1 IS SELECT last_name, salary FROM employees WHERE salary > 10000;
  names NameList;
  sals  SalList;
  recs RecList;
  v_limit PLS_INTEGER := 10;
  PROCEDURE print_results IS
    IF names IS NULL OR names.COUNT = 0 THEN  -- check if collections are empty
       DBMS_OUTPUT.PUT_LINE('No results!');
      DBMS_OUTPUT.PUT_LINE('Results: ');
      FOR i IN names.FIRST .. names.LAST
        DBMS_OUTPUT.PUT_LINE('  Employee ' || names(i) || ': $' || sals(i));
      END LOOP;
    END IF;
  DBMS_OUTPUT.PUT_LINE('--- Processing all results at once ---');
  OPEN c1;
  FETCH c1 BULK COLLECT INTO names, sals;
  CLOSE c1;
  DBMS_OUTPUT.PUT_LINE('--- Processing ' || v_limit || ' rows at a time ---');
  OPEN c1;
    FETCH c1 BULK COLLECT INTO names, sals LIMIT v_limit;
    EXIT WHEN names.COUNT = 0;
  CLOSE c1;
  DBMS_OUTPUT.PUT_LINE('--- Fetching records rather than columns ---');
  OPEN c1;
  FOR i IN recs.FIRST .. recs.LAST
-- Now all the columns from the result set come from a single record
    DBMS_OUTPUT.PUT_LINE('  Employee ' || recs(i).last_name || ': $'
         || recs(i).salary);

Example 11-13 shows how you can fetch from a cursor into a collection of records.

Example 11-13 Bulk-Fetching from a Cursor Into a Collection of Records

  TYPE DeptRecTab IS TABLE OF departments%ROWTYPE;
  dept_recs DeptRecTab;
    SELECT department_id, department_name, manager_id, location_id 
      FROM departments WHERE department_id > 70;
  OPEN c1;

Limiting the Rows for a Bulk FETCH Operation with the LIMIT Clause

The optional LIMIT clause, allowed only in bulk FETCH statements, limits the number of rows fetched from the database. In Example 11-14, with each iteration of the loop, the FETCH statement fetches ten rows (or less) into index-by table empids. The previous values are overwritten. Note the use of empids.COUNT to determine when to exit the loop.

Example 11-14 Using LIMIT to Control the Number of Rows In a BULK COLLECT

   CURSOR c1 IS SELECT employee_id FROM employees WHERE department_id = 80;
   empids    numtab;
   rows      PLS_INTEGER := 10;
  OPEN c1;
  LOOP -- the following statement fetches 10 rows or less in each iteration
    EXIT WHEN empids.COUNT = 0;
--  EXIT WHEN c1%NOTFOUND; -- incorrect, can omit some data
    DBMS_OUTPUT.PUT_LINE('------- Results from Each Bulk Fetch --------');
    FOR i IN 1..empids.COUNT LOOP
      DBMS_OUTPUT.PUT_LINE( 'Employee Id: ' || empids(i));
  CLOSE c1;

Retrieving DML Results into a Collection with the RETURNING INTO Clause

You can use the BULK COLLECT clause in the RETURNING INTO clause of an INSERT, UPDATE, or DELETE statement:

Example 11-15 Using BULK COLLECT With the RETURNING INTO Clause

CREATE TABLE emp_temp AS SELECT * FROM employees;
   TYPE NumList IS TABLE OF employees.employee_id%TYPE;
   enums NumList;
   TYPE NameList IS TABLE OF employees.last_name%TYPE;
   names NameList;
   DELETE FROM emp_temp WHERE department_id = 30
      RETURNING employee_id, last_name BULK COLLECT INTO enums, names;
   DBMS_OUTPUT.PUT_LINE('Deleted ' || SQL%ROWCOUNT || ' rows:');
   FOR i IN enums.FIRST .. enums.LAST
      DBMS_OUTPUT.PUT_LINE('Employee #' || enums(i) || ': ' || names(i));

Using FORALL and BULK COLLECT Together

You can combine the BULK COLLECT clause with a FORALL statement. The output collections are built up as the FORALL statement iterates.

In Example 11-16, the employee_id value of each deleted row is stored in the collection e_ids. The collection depts has 3 elements, so the FORALL statement iterates 3 times. If each DELETE issued by the FORALL statement deletes 5 rows, then the collection e_ids, which stores values from the deleted rows, has 15 elements when the statement completes:

Example 11-16 Using FORALL With BULK COLLECT

CREATE TABLE emp_temp AS SELECT * FROM employees;
   depts NumList := NumList(10,20,30);
   TYPE enum_t IS TABLE OF employees.employee_id%TYPE;
   TYPE dept_t IS TABLE OF employees.department_id%TYPE;
   e_ids enum_t;
   d_ids dept_t;
  FORALL j IN depts.FIRST..depts.LAST
    DELETE FROM emp_temp WHERE department_id = depts(j)
       RETURNING employee_id, department_id BULK COLLECT INTO e_ids, d_ids;
  DBMS_OUTPUT.PUT_LINE('Deleted ' || SQL%ROWCOUNT || ' rows:');
  FOR i IN e_ids.FIRST .. e_ids.LAST
    DBMS_OUTPUT.PUT_LINE('Employee #' || e_ids(i) || ' from dept #' || d_ids(i));

The column values returned by each execution are added to the values returned previously. If you use a FOR loop instead of the FORALL statement, the set of returned values is overwritten by each DELETE statement.

You cannot use the SELECT ... BULK COLLECT statement in a FORALL statement.

Using Host Arrays with Bulk Binds

Client-side programs can use anonymous PL/SQL blocks to bulk-bind input and output host arrays. This is the most efficient way to pass collections to and from the database server.

Host arrays are declared in a host environment such as an OCI or a Pro*C program and must be prefixed with a colon to distinguish them from PL/SQL collections. In the following example, an input host array is used in a DELETE statement. At run time, the anonymous PL/SQL block is sent to the database server for execution.

  -- assume that values were assigned to the host array
  -- and host variables in the host environment
  FORALL i IN :lower..:upper
    DELETE FROM employees WHERE department_id = :depts(i);

Writing Computation-Intensive Programs in PL/SQL

The BINARY_FLOAT and BINARY_DOUBLE datatypes make it practical to write PL/SQL programs to do number-crunching, for scientific applications involving floating-point calculations. These datatypes behave much like the native floating-point types on many hardware systems, with semantics derived from the IEEE-754 floating-point standard.

The way these datatypes represent decimal data make them less suitable for financial applications, where precise representation of fractional amounts is more important than pure performance.

The PLS_INTEGER and BINARY_INTEGER datatypes are PL/SQL-only datatypes that are more efficient than the SQL datatypes NUMBER or INTEGER for integer arithmetic. You can use PLS_INTEGER or BINARY_INTEGER to write pure PL/SQL code for integer arithmetic, or convert NUMBER or INTEGER values to PLS_INTEGER or BINARY_INTEGER for manipulation by PL/SQL. Note that the BINARY_INTEGER datatype is identical to PLS_INTEGER. For additional considerations, see "Change to the BINARY_INTEGER Datatype".

Within a package, you can write overloaded versions of procedures and functions that accept different numeric parameters. The math routines can be optimized for each kind of parameter (BINARY_FLOAT, BINARY_DOUBLE, NUMBER, PLS_INTEGER), avoiding unnecessary conversions.

The built-in math functions such as SQRT, SIN, COS, and so on already have fast overloaded versions that accept BINARY_FLOAT and BINARY_DOUBLE parameters. You can speed up math-intensive code by passing variables of these types to such functions, and by calling the TO_BINARY_FLOAT or TO_BINARY_DOUBLE functions when passing expressions to such functions.

Tuning Dynamic SQL with EXECUTE IMMEDIATE and Cursor Variables

Some programs (a general-purpose report writer for example) must build and process a variety of SQL statements, where the exact text of the statement is unknown until run time. Such statements probably change from execution to execution. They are called dynamic SQL statements.

Formerly, to execute dynamic SQL statements, you had to use the supplied package DBMS_SQL. Now, within PL/SQL, you can execute any kind of dynamic SQL statement using an interface called native dynamic SQL. The main PL/SQL features involved are the EXECUTE IMMEDIATE statement and cursor variables (also known as REF CURSORs).

Native dynamic SQL code is more compact and much faster than calling the DBMS_SQL package. The following example declares a cursor variable, then associates it with a dynamic SELECT statement:

   emp_cv   EmpCurTyp;
   v_ename VARCHAR2(15);
   v_sal   NUMBER := 1000;
   table_name VARCHAR2(30) := 'employees';
   OPEN emp_cv FOR 'SELECT last_name, salary FROM ' || table_name ||
      ' WHERE salary > :s' USING v_sal;
   CLOSE emp_cv;

For more information, see Chapter 7, "Performing SQL Operations with Native Dynamic SQL".

Tuning PL/SQL Procedure Calls with the NOCOPY Compiler Hint

By default, OUT and IN OUT parameters are passed by value. The values of any IN OUT parameters are copied before the subprogram is executed. During subprogram execution, temporary variables hold the output parameter values. If the subprogram exits normally, these values are copied to the actual parameters. If the subprogram exits with an unhandled exception, the original parameters are unchanged.

When the parameters represent large data structures such as collections, records, and instances of object types, this copying slows down execution and uses up memory. In particular, this overhead applies to each call to an object method: temporary copies are made of all the attributes, so that any changes made by the method are only applied if the method exits normally.

To avoid this overhead, you can specify the NOCOPY hint, which allows the PL/SQL compiler to pass OUT and IN OUT parameters by reference. If the subprogram exits normally, the behavior is the same as normal. If the subprogram exits early with an exception, the values of OUT and IN OUT parameters (or object attributes) might still change. To use this technique, ensure that the subprogram handles all exceptions.

The following example asks the compiler to pass IN OUT parameter v_staff by reference, to avoid copying the varray on entry to and exit from the subprogram:

  TYPE Staff IS VARRAY(200) OF Employee;
  PROCEDURE reorganize (v_staff IN OUT NOCOPY Staff) IS ...

Example 11-17 loads 25,000 records into a local nested table, which is passed to two local procedures that do nothing. A call to the procedure that uses NOCOPY takes much less time.

Example 11-17 Using NOCOPY With Parameters

   TYPE EmpTabTyp IS TABLE OF employees%ROWTYPE;
   emp_tab EmpTabTyp := EmpTabTyp(NULL);  -- initialize
   t1 NUMBER;
   t2 NUMBER;
   t3 NUMBER;
     BEGIN t := DBMS_UTILITY.get_time; END;
   PROCEDURE do_nothing1 (tab IN OUT EmpTabTyp) IS
   PROCEDURE do_nothing2 (tab IN OUT NOCOPY EmpTabTyp) IS
   SELECT * INTO emp_tab(1) FROM employees WHERE employee_id = 100;
   emp_tab.EXTEND(49999, 1);  -- copy element 1 into 2..50000
   do_nothing1(emp_tab);  -- pass IN OUT parameter
   do_nothing2(emp_tab);  -- pass IN OUT NOCOPY parameter
   DBMS_OUTPUT.PUT_LINE('Call Duration (secs)');
   DBMS_OUTPUT.PUT_LINE('Just IN OUT: ' || TO_CHAR((t2 - t1)/100.0));
   DBMS_OUTPUT.PUT_LINE('With NOCOPY: ' || TO_CHAR((t3 - t2))/100.0);

Restrictions on NOCOPY

The use of NOCOPY increases the likelihood of parameter aliasing. For more information, see "Understanding Subprogram Parameter Aliasing".

Remember, NOCOPY is a hint, not a directive. In the following cases, the PL/SQL compiler ignores the NOCOPY hint and uses the by-value parameter-passing method; no error is generated:

  • The actual parameter is an element of an associative array. This restriction does not apply if the parameter is an entire associative array.

  • The actual parameter is constrained, such as by scale or NOT NULL. This restriction does not apply to size-constrained character strings. This restriction does not extend to constrained elements or attributes of composite types.

  • The actual and formal parameters are records, one or both records were declared using %ROWTYPE or %TYPE, and constraints on corresponding fields in the records differ.

  • The actual and formal parameters are records, the actual parameter was declared (implicitly) as the index of a cursor FOR loop, and constraints on corresponding fields in the records differ.

  • Passing the actual parameter requires an implicit datatype conversion.

  • The subprogram is called through a database link or as an external procedure.

Compiling PL/SQL Code for Native Execution

You can speed up PL/SQL procedures by compiling them into native code residing in shared libraries. The procedures are translated into C code, then compiled with your usual C compiler and linked into the Oracle process.

You can use native compilation with both the supplied Oracle packages, and procedures you write yourself. Procedures compiled this way work in all server environments, such as the shared server configuration (formerly known as multi-threaded server) and Oracle Real Application Clusters.

See Also:

Before You Begin

If you are a first-time user of native PL/SQL compilation, try it first with a test database, before proceeding to a production environment.

Always back up your database before configuring the database for PL/SQL native compilation. If you find that the performance benefit is outweighed by extra compilation time, it might be faster to restore from a backup than to recompile everything in interpreted mode.

Some of the setup steps require DBA authority. You must change the values of some initialization parameters, and create a new directory on the database server, preferably near the data files for the instance. The database server also needs a C compiler; on a cluster, the compiler is needed on each node. Even if you can test out these steps yourself on a development machine, you will generally need to consult with a DBA and enlist their help to use native compilation on a production server.


The pre-requirements for using native compiled PL/SQL code are documented for each platform in the individual Oracle database installation guides. Check the software requirements section for the certified compilers for native compilation on your platform to ensure that you use a certified compiler.

Determining Whether to Use PL/SQL Native Compilation

PL/SQL native compilation provides the greatest performance gains for computation-intensive procedural operations. Examples of such operations are data warehouse applications, and applications with extensive server-side transformations of data for display. In such cases, expect speed increases of up to 30%.

Because this technique cannot do much to speed up SQL statements called from PL/SQL, it is most effective for compute-intensive PL/SQL procedures that do not spend most of their time executing SQL. You can test to see how much performance gain you can get by enabling PL/SQL native compilation.

It takes longer to compile program units with native compilation than to use the default interpreted mode. You might turn off native compilation during the busiest parts of the development cycle, where code is being frequently recompiled.

If you have determined that there will be significant performance gains in database operations using PL/SQL native compilation, Oracle Corporation recommends that you compile the entire database using the NATIVE setting. Compiling all the PL/SQL code in the database means you see the speedup in your own code, and in calls to all the built-in PL/SQL packages.

If interpreted compilation is required for your environment, you can compile all the PL/SQL units to INTERPRETED. For example, if you import an entire database that includes NATIVE PL/SQL units into an environment where a C compiler is unavailable.

To convert an entire database to NATIVE or INTERPRETED compilation, see "Modifying the Entire Database for PL/SQL Native or Interpreted Compilation".

How PL/SQL Native Compilation Works

If you do not use native compilation, each PL/SQL program unit is compiled into an intermediate form, machine-readable code (m-code). The m-code is stored in the database dictionary and interpreted at run time. With PL/SQL native compilation, the PL/SQL statements are turned into C code that bypasses all the runtime interpretation, giving faster runtime performance.

PLSQL_ initialization parameters set up the environment for PL/SQL native compilation, as described in "Setting up Initialization Parameters for PL/SQL Native Compilation".

PL/SQL uses the command file $ORACLE_HOME/plsql/spnc_commands, and the supported operating system C compiler and linker, to compile and link the resulting C code into shared libraries. See "The spnc_commands File".

The shared libraries are stored inside the data dictionary, so that they can be backed up automatically and are protected from being deleted. These shared library files are copied to the file system and are loaded and run when the PL/SQL subprogram is invoked. If the files are deleted from the file system while the database is shut down, or if you change the directory that holds the libraries, they are extracted again automatically.

Although PL/SQL program units that just call SQL statements might see little or no speedup, natively compiled PL/SQL is always at least as fast as the corresponding interpreted code. The compiled code makes the same library calls as the interpreted code would, so its behavior is exactly the same.

Dependencies, Invalidation and Revalidation

After the procedures are compiled and turned into shared libraries, they are automatically linked into the Oracle process. You do not need to restart the database, or move the shared libraries to a different location. You can call back and forth between stored procedures, whether they are all interpreted, all compiled for native execution, or a mixture of both.

Recompilation is automatic with invalidated PL/SQL modules. For example, if an object on which a natively compiled PL/SQL subprogram depends changes, the subprogram is invalidated. The next time the same subprogram is called, the database recompiles the subprogram automatically. Because the PLSQL_CODE_TYPE setting is stored inside the library unit for each subprogram, the automatic recompilation uses this stored setting for code type.

The stored settings are only used when recompiling as part of revalidation. If a PL/SQL subprogram is explicitly compiled through the SQL commands CREATE OR REPLACE or ALTER...COMPILE, the current session setting is used. See also "Initialization Parameters for PL/SQL Compilation".

The generated shared libraries are stored in the database, in the SYSTEM tablespace. The first time a natively compiled procedure is executed, the corresponding shared library is copied from the database to the directory specified by the initialization parameter PLSQL_NATIVE_LIBRARY_DIR.

Real Application Clusters and PL/SQL Native Compilation

Because any node might need to compile a PL/SQL subprogram, each node in the cluster needs a C compiler and correct settings and paths in the $ORACLE_HOME/plsql/spnc_commands file.

When you use PLSQL native compilation in a Real Application Clusters (RAC) environment, the original copies of the shared library files are stored in the databases, and these files are automatically propagated to all nodes in the cluster. You do not need to do any copying of libraries for this feature to work.

Check that all nodes of a RAC cluster use the same settings for the initialization parameters that control PL/SQL native compilation. Make sure the path specified in PLSQL_NATIVE_LIBRARY_DIR where the shared libraries are placed is created identically on all nodes that are part of the cluster.

Limitations of Native Compilation

The following are some limitations of native compilation:

  • Debugging tools for PL/SQL do not handle procedures compiled for native execution.

  • When many procedures and packages (typically, over 15000) are compiled for native execution, the large number of shared objects in a single directory might affect system performance. See "Setting Up PL/SQL Native Library Subdirectories" for a workaround.

The spnc_commands File

The spnc_commands file, in the $ORACLE_HOME/plsql directory, contains the templates for the commands to compile and link each program. Some special names such as %(src) are predefined, and are replaced by the corresponding filename. The variable $(ORACLE_HOME) is replaced by the location of the Oracle home directory. Comment lines start with a # character. The file contains comments that explain all the special notation.

The spnc_commands file contains a predefined path for the default C compiler, depending on the particular operating system. A specific compiler is supported on each operating system. Only one compiler can be used to compile the PL/SQL modules; do not compile PL/SQL modules in a database with different compilers.

You can view spnc_commands file to confirm that the command templates are correct. You should not need to change this file unless the system administrator has installed the C compiler in another location or you want to use a different supported C compiler. Additional information can be found in the Oracle database installation guide for your platform and by searching for spnc_commands on Oracle Metalink at http://metalink.oracle.com.

Setting up Initialization Parameters for PL/SQL Native Compilation

This section describes the initialization parameters that are used to set PL/SQL native compilation.




To check the settings of these parameters, enter the following in SQL*Plus:


See also "Initialization Parameters for PL/SQL Compilation".

PLSQL_NATIVE_LIBRARY_DIR Initialization Parameter

This is a required system-level only parameter that specifies the full path and directory name of the location of the shared libraries that contain natively compiled PL/SQL code. The value must be explicit and point to an existing, accessible directory; the path cannot contain a variable such as ORACLE_HOME. Use the ALTER SYSTEM command or update the initialization file to set the parameter value.

For example, if the path to the PL/SQL native library directory is /oracle/oradata/db1/natlib, then the setting in the initialization file is:


In accordance with optimal flexible architecture (OFA) rules, Oracle Corporation recommends that you create the shared library directory as a subdirectory where the data files are located. For security reasons, only the users oracle and root should have write privileges for this directory.

If using a Real Applications Cluster environment, see "Real Application Clusters and PL/SQL Native Compilation". For details about the PLSQL_NATIVE_LIBRARY_DIR initialization parameter, see Oracle Database Reference.


This is an optional system-level only parameter that specifies the number of subdirectories in the directory that is specified by the parameter PLSQL_NATIVE_LIBRARY_DIR. Use the ALTER SYSTEM command or update the initialization file to set the parameter value.

For example, if you want to set the parameter for 1,000 subdirectories, then the setting in the initialization parameter file is:


Set this parameter if the number of natively compiled program units is very large. See "Setting Up PL/SQL Native Library Subdirectories".

For details about the PLSQL_NATIVE_LIBRARY_SUBDIR_COUNT initialization parameter, see Oracle Database Reference.

PLSQL_CODE_TYPE Initialization Parameter

The PLSQL_CODE_TYPE initialization parameter determines whether PL/SQL code is natively compiled or interpreted. The default setting is INTERPRETED. To enable PL/SQL native compilation, set the value of PLSQL_CODE_TYPE to NATIVE.

The parameter can be set at the system, session level, or for a specific PL/SQL module. Use the ALTER SYSTEM command, ALTER SESSION command, or update the initialization file to set the parameter value. If you compile the whole database as NATIVE, Oracle Corporation recommends that you set PLSQL_CODE_TYPE at the system level.

The following SQL*Plus syntax sets the parameter at the session level:


You can also use the PLSQL_CODE_TYPE = NATIVE clause with the ALTER .. COMPILE statement for a specific PL/SQL module, as shown in Example 11-18. This affects only the specified module without changing the initialization parameter for the entire session. Note that a package specification and its body do not need to be compiled with the same setting for native compilation.

For details about the PLSQL_CODE_TYPE initialization parameter, see Oracle Database Reference.

Setting Up PL/SQL Native Library Subdirectories

By default, PL/SQL program units are kept in one directory. However, if the number of program units is very large, then the operating system might have difficulty handling a large of files in one directory. To avoid this problem, Oracle Corporation recommends that you spread the PL/SQL program units in subdirectories under the directory specified by the PLSQL_NATIVE_LIBRARY_DIR initialization parameter.

If you have an existing database that you will migrate to the new installation, or if you have set up a test database, use the following SQL query to determine how many PL/SQL program units you are using:


If you are going to exclude any units, such as packages, use the previous query with:


If you need to set up PL/SQL native library subdirectories, first create subdirectories sequentially in the form of d0, d1, d2, d3...dx, where x is the total number of directories. Oracle Corporation recommends that you use a script for this task. For example, you might run a PL/SQL block like the following, save its output to a file, then run that file as a shell script:

SPOOL make_dirs
  FOR j IN 0..1000 -- change to the number of directories needed
    DBMS_OUTPUT.PUT_LINE ( 'mkdir d' || TO_CHAR(j) );

Next, set the PLSQL_NATIVE_LIBARY_SUBDIR_COUNT initialization parameter to the number of subdirectories you have created. For example, if you created 1000 subdirectories, you can use SQL*Plus to enter the following SQL statement:


See "PLSQL_NATIVE_LIBRARY_SUBDIR_COUNT Initialization Parameter".

Setting Up and Testing PL/SQL Native Compilation

To set up and test one or more subprograms through native compilation:

  1. Set up the necessary initialization parameter. See "Setting up Initialization Parameters for PL/SQL Native Compilation".

  2. Compile one or more subprograms, using one of these methods:

    • Use CREATE OR REPLACE to create or recompile the subprogram.

    • Use the ALTER PROCEDURE, ALTER FUNCTION, or ALTER PACKAGE command with the COMPILE option to recompile a specific subprogram or entire package, as shown in Example 11-18.

    • Drop the subprogram and create it again.

    • Run one of the SQL*Plus scripts that creates a set of Oracle-supplied packages.

    • Create a database using a preconfigured initialization file with PLSQL_CODE_TYPE=NATIVE. During database creation, the utlirp script is run to compile all the Oracle-supplied packages.

    Example 11-18 illustrates the process of altering and testing a procedure using native compilation. The procedure is immediately available to call, and runs as a shared library directly within the Oracle process. If any errors occur during compilation, you can see them using the USER_ERRORS view or the SHOW ERRORS command in SQL*Plus.

    Example 11-18 Compiling a PL/SQL Procedure for Native Execution

    -- PLSQL_NATIVE_LIBRARY_DIR must be set to an existing, accessible directory
      DBMS_OUTPUT.PUT_LINE('Hello world. Today is ' || TO_CHAR(SYSDATE) || '.');
    END hello_native;
    -- check for a file HELLO_NATIVE... in the PLSQL_NATIVE_LIBRARY_DIR directory
    CALL hello_native();
  3. To be sure that the process worked, you can query the data dictionary to see that a procedure is compiled for native execution. To check whether an existing procedure is compiled for native execution or not, you can query the ALL_PLSQL_OBJECT_SETTINGS view. The PLSQL_CODE_TYPE column has a value of NATIVE for procedures that are compiled for native execution, and INTERPRETED otherwise. For information, see Oracle Database Reference.

    For example, to check the status of the procedure hello_native, use the query in Example 11-19.

Example 11-19 Checking plsql_code_type For a Compiled Procedure


Setting Up a New Database for PL/SQL Native Compilation

Use the procedures in this section to set up an new database for PL/SQL native compilation. The performance benefits apply to all the built-in PL/SQL packages, which are used for many database operations. If using a Real Applications Cluster environment, see "Real Application Clusters and PL/SQL Native Compilation".

  1. Check that spnc_commands file has the correct command templates. Contact your system administrator to ensure that you have the required C compiler on your operating system. See "The spnc_commands File".

  2. Ensure that the PL/SQL native library directory is created for each Oracle database. See "PLSQL_NATIVE_LIBRARY_DIR Initialization Parameter".

    • You must set up PL/SQL libraries for each Oracle database. Shared libraries (.so and .dll files) are logically connected to the database. They cannot be shared between databases. If you set up PL/SQL libraries to be shared, the databases will be corrupted.

    • Create a directory in a secure place, in accordance with OFA rules, to prevent .so and .dll files from unauthorized access.

    • Ensure that the compiler executables used for PL/SQL native compilation are writable only by a properly secured user.

    • Ensure that the original copies of the shared libraries are stored inside the database, so they are backed up automatically with the database.

  3. Set up the necessary initialization parameters. See "Setting up Initialization Parameters for PL/SQL Native Compilation".

    If you use Database Configuration Assistant, use it to set the initialization parameters required for PL/SQL native compilation. Make sure that you set PLSQL_NATIVE_LIBRARY_DIR to an accessible directory and PLSQL_CODE_TYPE to NATIVE.

Modifying the Entire Database for PL/SQL Native or Interpreted Compilation

You can recompile all PL/SQL modules in an existing database to NATIVE or INTERPRETED, using the dbmsupgnv.sql and dbmsupgin.sql scripts respectively during the process described in this section. Before making the conversion, review "Determining Whether to Use PL/SQL Native Compilation".

During the conversion to native compilation, TYPE specifications are not recompiled by dbmsupgnv.sql to NATIVE because these specifications do not contain executable code.

Package specifications seldom contain executable code so the runtime benefits of compiling to NATIVE are not measurable. You can use the TRUE command line parameter with the dbmsupgnv.sql script to exclude package specs from recompilation to NATIVE, saving time in the conversion process.

When converting to interpreted compilation, the dbmsupgin.sql script does not accept any parameters and does not exclude any PL/SQL units.


The following procedure describes the conversion to native compilation. If you need to recompile all PL/SQL modules to interpreted compilation, make these changes in the steps.
  • Skip the first step.

  • Set the PLSQL_CODE_TYPE initialization parameter to INTERPRETED rather than NATIVE.

  • Substitute dbmsupgin.sql for the dbmsupgnv.sql script.

  1. Ensure that following are properly configured to support native compilation:

  2. Shut down application services, the listener, and the database.

    • Shut down all of the Application services including the Forms Processes, Web Servers, Reports Servers, and Concurrent Manager Servers. After shutting down all of the Application services, ensure that all of the connections to the database have been terminated.

    • Shut down the TNS listener of the database to ensure that no new connections are made.

    • Shut down the database in normal or immediate mode as the user SYS. See "Starting Up and Shutting Down" in the Oracle Database Administrator's Guide.

  3. Set PLSQL_CODE_TYPE to NATIVE in the initialization parameter file. If the database is using a server parameter file, then set this after the database has started. See "PLSQL_CODE_TYPE Initialization Parameter".

    The value of PLSQL_CODE_TYPE does not affect the conversion of the PL/SQL units in these steps. However, it does affect all subsequently compiled units and it should be explicitly set to the compilation type that you want.

  4. Start up the database in upgrade mode, using the UPGRADE option. For information on SQL*Plus STARTUP, see the SQL*Plus User's Guide and Reference.

  5. Execute the following code to list the invalid PL/SQL units. You can save the output of the query for future reference with the SQL SPOOL command.

    Example 11-20 Checking for Invalid PL/SQL Units

    REM To save the output of the query to a file: SPOOL pre_update_invalid.log
    REM To stop spooling the output: SPOOL OFF

    If any Oracle supplied units are invalid, try to validate them. For example:


    See ALTER FUNCTION, ALTER PACKAGE, and ALTER PROCEDURE, in the Oracle Database SQL Reference.If the units cannot be validated, save the spooled log for future resolution and continue.

  6. Execute the following query to determine how many objects are compiled NATIVE and INTERPRETED. Use the SQL SPOOL command if you want to save the output.

    Example 11-21 Checking for PLSQL Compilation Type


    Any objects with a NULL plsql_code_type are special internal objects and can be ignored.

  7. Run the $ORACLE_HOME/rdbms/admin/dbmsupgnv.sql script as the user SYS to update the plsql_code_type setting to NATIVE in the dictionary tables for all PL/SQL units. This process also invalidates the units. Use TRUE with the script to exclude package specifications; FALSE to include the package specifications.

    This update must be done when the database is in UPGRADE mode. The script is guaranteed to complete successfully or rollback all the changes.

  8. Shut down the database and restart in NORMAL mode.

  9. Before you run the utlrp.sql script, Oracle recommends that no other sessions are connected to avoid possible problems. You can ensure this with:


  10. Run the $ORACLE_HOME/rdbms/admin/utlrp.sql script as the user SYS. This script recompiles all the PL/SQL modules using a default degree of parellelism. See the comments in the script for information on setting the degree explicitly.

    If for any reason the script is abnormally terminated, rerun the utlrp.sql script to recompile any remaining invalid PL/SQL modules.

  11. After the compilation completes successfully, verify that there are no new invalid PL/SQL units using the query in Example 11-20. You can spool the output of the query to the post_upgrade_invalid.log file and compare the contents with the pre_upgrade_invalid.log file, if it was created previously.

  12. Re-execute the query in Example 11-21. If recompiling with dbmsupgnv.sql, confirm that all PL/SQL units, except TYPE specifications and package specifications if excluded, are NATIVE. If recompiling with dbmsupgin.sql, confirm that all PL/SQL units are INTERPRETED.

  13. Disable the restricted session mode for the database, then start the services that you previously shut down. To disable restricted session mode:


Setting Up Transformations with Pipelined Functions

This section describes how to chain together special kinds of functions known as pipelined table functions. These functions are used in situations such as data warehousing to apply multiple transformations to data.

See Also:

Overview of Pipelined Table Functions

Pipelined table functions are functions that produce a collection of rows (either a nested table or a varray) that can be queried like a physical database table or assigned to a PL/SQL collection variable. You can use a table function in place of the name of a database table in the FROM clause of a query or in place of a column name in the SELECT list of a query.

A table function can take a collection of rows as input. An input collection parameter can be either a collection type (such as a VARRAY or a PL/SQL table) or a REF CURSOR.

Execution of a table function can be parallelized, and returned rows can be streamed directly to the next process without intermediate staging. Rows from a collection returned by a table function can also be pipelined, that is, iteratively returned as they are produced instead of in a batch after all processing of the table function's input is completed.

Streaming, pipelining, and parallel execution of table functions can improve performance:

  • By enabling multi-threaded, concurrent execution of table functions

  • By eliminating intermediate staging between processes

  • By improving query response time: With non-pipelined table functions, the entire collection returned by a table function must be constructed and returned to the server before the query can return a single result row. Pipelining enables rows to be returned iteratively, as they are produced. This also reduces the memory that a table function requires, as the object cache does not need to materialize the entire collection.

  • By iteratively providing result rows from the collection returned by a table function as the rows are produced instead of waiting until the entire collection is staged in tables or memory and then returning the entire collection.

Writing a Pipelined Table Function

You declare a pipelined table function by specifying the PIPELINED keyword. Pipelined functions can be defined at the schema level with CREATE FUNCTION or in a package. The PIPELINED keyword indicates that the function returns rows iteratively. The return type of the pipelined table function must be a supported collection type, such as a nested table or a varray. This collection type can be declared at the schema level or inside a package. Inside the function, you return individual elements of the collection type. The elements of the collection type must be supported SQL datatypes, such as NUMBER and VARCHAR2. PL/SQL datatypes, such as PLS_INTEGER and BOOLEAN, are not supported as collection elements in a pipelined function.

Example 11-22 shows how to assign the result of a pipelined table function to a PL/SQL collection variable and use the function in a SELECT statement.

Example 11-22 Assigning the Result of a Table Function

END pkg1;

-- FUNCTION f1 returns a collection of elements (1,2,3,... x)
    FOR i IN 1..x LOOP
      PIPE ROW(i);
END pkg1;

-- pipelined function is used in FROM clause of SELECT statement
SELECT * FROM TABLE(pkg1.f1(5));

Using Pipelined Table Functions for Transformations

A pipelined table function can accept any argument that regular functions accept. A table function that accepts a REF CURSOR as an argument can serve as a transformation function. That is, it can use the REF CURSOR to fetch the input rows, perform some transformation on them, and then pipeline the results out.

In Example 11-23, the f_trans function converts a row of the employees table into two rows.

Example 11-23 Using a Pipelined Table Function For a Transformation

-- Define the ref cursor types and function
  TYPE outrec_typ IS RECORD ( 
    var_num    NUMBER(6),
    var_char1  VARCHAR2(30),
    var_char2  VARCHAR2(30));
  TYPE outrecset IS TABLE OF outrec_typ;
 FUNCTION f_trans(p refcur_t) 
      RETURN outrecset PIPELINED;
END refcur_pkg;

  FUNCTION f_trans(p refcur_t) 
    out_rec outrec_typ;
    in_rec  p%ROWTYPE;
    FETCH p INTO in_rec;
    -- first row
    out_rec.var_num := in_rec.employee_id;
    out_rec.var_char1 := in_rec.first_name;
    out_rec.var_char2 := in_rec.last_name;
    PIPE ROW(out_rec);
    -- second row
    out_rec.var_char1 := in_rec.email;
    out_rec.var_char2 := in_rec.phone_number;
    PIPE ROW(out_rec);
  CLOSE p;
END refcur_pkg;
-- SELECT query using the f_transc table function
   refcur_pkg.f_trans(CURSOR(SELECT * FROM employees WHERE department_id = 60)));

In the preceding query, the pipelined table function f_trans fetches rows from the CURSOR subquery SELECT * FROM employees ..., performs the transformation, and pipelines the results back to the user as a table. The function produces two output rows (collection elements) for each input row.

Note that when a CURSOR subquery is passed from SQL to a REF CURSOR function argument as in Example 11-23, the referenced cursor is already open when the function begins executing.

Returning Results from Pipelined Table Functions

In PL/SQL, the PIPE ROW statement causes a pipelined table function to pipe a row and continue processing. The statement enables a PL/SQL table function to return rows as soon as they are produced. For performance, the PL/SQL runtime system provides the rows to the consumer in batches.

In Example 11-23, the PIPE ROW(out_rec) statement pipelines data out of the PL/SQL table function. out_rec is a record, and its type matches the type of an element of the output collection.

The PIPE ROW statement may be used only in the body of pipelined table functions; an error is raised if it is used anywhere else. The PIPE ROW statement can be omitted for a pipelined table function that returns no rows.

A pipelined table function may have a RETURN statement that does not return a value. The RETURN statement transfers the control back to the consumer and ensures that the next fetch gets a NO_DATA_FOUND exception.

Because table functions pass control back and forth to a calling routine as rows are produced, there is a restriction on combining table functions and PRAGMA AUTONOMOUS_TRANSACTION. If a table function is part of an autonomous transaction, it must COMMIT or ROLLBACK before each PIPE ROW statement, to avoid an error in the calling subprogram.

Oracle has three special SQL datatypes that enable you to dynamically encapsulate and access type descriptions, data instances, and sets of data instances of any other SQL type, including object and collection types. You can also use these three special types to create anonymous (that is, unnamed) types, including anonymous collection types. The types are SYS.ANYTYPE, SYS.ANYDATA, and SYS.ANYDATASET. The SYS.ANYDATA type can be useful in some situations as a return value from table functions.

See Also:

Oracle Database PL/SQL Packages and Types Reference for information about the interfaces to the ANYTYPE, ANYDATA, and ANYDATASET types and about the DBMS_TYPES package for use with these types.

Pipelining Data Between PL/SQL Table Functions

With serial execution, results are pipelined from one PL/SQL table function to another using an approach similar to co-routine execution. For example, the following statement pipelines results from function g to function f:


Parallel execution works similarly except that each function executes in a different process (or set of processes).

Optimizing Multiple Calls to Pipelined Table Functions

Multiple invocations of a pipelined table function, either within the same query or in separate queries result in multiple executions of the underlying implementation. By default, there is no buffering or reuse of rows. For example:

SELECT * FROM TABLE(f(...)) t1, TABLE(f(...)) t2
  WHERE t1.id = t2.id;

If the function always produces the same result value for each combination of values passed in, you can declare the function DETERMINISTIC, and Oracle automatically buffers rows for it. If the function is not really deterministic, results are unpredictable.

Fetching from the Results of Pipelined Table Functions

PL/SQL cursors and ref cursors can be defined for queries over table functions. For example:


Cursors over table functions have the same fetch semantics as ordinary cursors. REF CURSOR assignments based on table functions do not have any special semantics.

However, the SQL optimizer will not optimize across PL/SQL statements. For example:


does not execute as well as:


This is so even ignoring the overhead associated with executing two SQL statements and assuming that the results can be pipelined between the two statements.

Passing Data with Cursor Variables

You can pass a set of rows to a PL/SQL function in a REF CURSOR parameter. For example, this function is declared to accept an argument of the predefined weakly typed REF CURSOR type SYS_REFCURSOR:


Results of a subquery can be passed to a function directly:


In the preceding example, the CURSOR keyword is required to indicate that the results of a subquery should be passed as a REF CURSOR parameter.

A predefined weak REF CURSOR type SYS_REFCURSOR is also supported. With SYS_REFCURSOR, you do not need to first create a REF CURSOR type in a package before you can use it.

To use a strong REF CURSOR type, you still must create a PL/SQL package and declare a strong REF CURSOR type in it. Also, if you are using a strong REF CURSOR type as an argument to a table function, then the actual type of the REF CURSOR argument must match the column type, or an error is generated. Weak REF CURSOR arguments to table functions can only be partitioned using the PARTITION BY ANY clause. You cannot use range or hash partitioning for weak REF CURSOR arguments.

PL/SQL functions can accept multiple REF CURSOR input variables as shown in Example 11-24.

Example 11-24 Using Multiple REF CURSOR Input Variables

-- Define the ref cursor types
  TYPE refcur_t1 IS REF CURSOR RETURN employees%ROWTYPE;
  TYPE refcur_t2 IS REF CURSOR RETURN departments%ROWTYPE;  
  TYPE outrec_typ IS RECORD ( 
    var_num    NUMBER(6),
    var_char1  VARCHAR2(30),
    var_char2  VARCHAR2(30));
  TYPE outrecset IS TABLE OF outrec_typ;
  FUNCTION g_trans(p1 refcur_t1, p2 refcur_t2) 
    RETURN outrecset PIPELINED;
END refcur_pkg;

FUNCTION g_trans(p1 refcur_t1, p2 refcur_t2) 
    out_rec outrec_typ;
    in_rec1 p1%ROWTYPE;
    in_rec2 p2%ROWTYPE;
    FETCH p2 INTO in_rec2;
  CLOSE p2;
    FETCH p1 INTO in_rec1;
    -- first row
    out_rec.var_num := in_rec1.employee_id;
    out_rec.var_char1 := in_rec1.first_name;
    out_rec.var_char2 := in_rec1.last_name;
    PIPE ROW(out_rec);
    -- second row
    out_rec.var_num := in_rec2.department_id;
    out_rec.var_char1 := in_rec2.department_name;
    out_rec.var_char2 := TO_CHAR(in_rec2.location_id);
    PIPE ROW(out_rec);
  CLOSE p1;
END refcur_pkg;

-- SELECT query using the g_trans table function
SELECT * FROM TABLE(refcur_pkg.g_trans(
  CURSOR(SELECT * FROM employees WHERE department_id = 60),
  CURSOR(SELECT * FROM departments WHERE department_id = 60)));

You can pass table function return values to other table functions by creating a REF CURSOR that iterates over the returned data:


You can explicitly open a REF CURSOR for a query and pass it as a parameter to a table function:

  rec ...;
  -- Must return a single row result set.

In this case, the table function closes the cursor when it completes, so your program should not explicitly try to close the cursor.

A table function can compute aggregate results using the input ref cursor. Example 11-25 computes a weighted average by iterating over a set of input rows.

Example 11-25 Using a Pipelined Table Function as an Aggregate Function

CREATE TABLE gradereport (student VARCHAR2(30), subject VARCHAR2(30),
                          weight NUMBER, grade NUMBER);
INSERT INTO gradereport VALUES('Mark', 'Physics', 4, 4);
INSERT INTO gradereport VALUES('Mark','Chemistry', 4, 3);
INSERT INTO gradereport VALUES('Mark','Maths', 3, 3);
INSERT INTO gradereport VALUES('Mark','Economics', 3, 4);

  FUNCTION weighted_average(input_values SYS_REFCURSOR)
END pkg_gpa;
FUNCTION weighted_average(input_values SYS_REFCURSOR)
  grade NUMBER;
  total NUMBER := 0;
  total_weight NUMBER := 0;
  weight NUMBER := 0;
-- The function accepts a ref cursor and loops through all the input rows
     FETCH input_values INTO weight, grade;
     EXIT WHEN input_values%NOTFOUND;
-- Accumulate the weighted average
     total_weight := total_weight + weight;
     total := total + grade*weight;
  PIPE ROW (total / total_weight);
  RETURN; -- the function returns a single result
END pkg_gpa;
-- the query result comes back as a nested table with a single row
-- COLUMN_VALUE is a keyword that returns the contents of a nested table
SELECT w.column_value "weighted result" FROM TABLE(
       pkg_gpa.weighted_average(CURSOR(SELECT weight, grade FROM gradereport))) w;

Performing DML Operations Inside Pipelined Table Functions

To execute DML statements, declare a pipelined table function with the AUTONOMOUS_TRANSACTION pragma, which causes the function to execute in a new transaction not shared by other processes:


During parallel execution, each instance of the table function creates an independent transaction.

Performing DML Operations on Pipelined Table Functions

Pipelined table functions cannot be the target table in UPDATE, INSERT, or DELETE statements. For example, the following statements will raise an error:

UPDATE F(CURSOR(SELECT * FROM tab)) SET col = value;
  INSERT INTO f(...) VALUES ('any', 'thing');

However, you can create a view over a table function and use INSTEAD OF triggers to update it. For example:

CREATE VIEW BookTable AS SELECT x.Name, x.Author
  FROM TABLE(GetBooks('data.txt')) x;

The following INSTEAD OF trigger is fired when the user inserts a row into the BookTable view:

CREATE TRIGGER BookTable_insert

INSTEAD OF triggers can be defined for all DML operations on a view built on a table function.

Handling Exceptions in Pipelined Table Functions

Exception handling in pipelined table functions works just as it does with regular functions.

Some languages, such as C and Java, provide a mechanism for user-supplied exception handling. If an exception raised within a table function is handled, the table function executes the exception handler and continues processing. Exiting the exception handler takes control to the enclosing scope. If the exception is cleared, execution proceeds normally.

An unhandled exception in a table function causes the parent transaction to roll back.