12.5 Chaining Pipelined Table Functions for Multiple Transformations

Chaining pipelined table functions is an efficient way to perform multiple transformations on data.

Note:

You cannot run a pipelined table function over a database link. The reason is that the return type of a pipelined table function is a SQL user-defined type, which can be used only in a single database (as explained in Oracle Database Object-Relational Developer's Guide). Although the return type of a pipelined table function might appear to be a PL/SQL type, the database actually converts that PL/SQL type to a corresponding SQL user-defined type.

Topics

12.5.1 Overview of Table Functions

A table function is a user-defined PL/SQL function that returns a collection of rows (an associative array, nested table or varray).

You can select from this collection as if it were a database table by invoking the table function inside the TABLE clause in a SELECT statement. The TABLE operator is optional.

For example:

SELECT * FROM TABLE(table_function_name(parameter_list))

Alternatively, the same query can be written without the TABLE operator as follow:

SELECT * FROM table_function_name(parameter_list)

A table function can take a collection of rows as input (that is, it can have an input parameter that is a nested table, varray, or cursor variable). Therefore, output from table function tf1 can be input to table function tf2, and output from tf2 can be input to table function tf3, and so on.

To improve the performance of a table function, you can:

  • Enable the function for parallel execution, with the PARALLEL_ENABLE option.

    Functions enabled for parallel execution can run concurrently.

  • Stream the function results directly to the next process, with Oracle Streams.

    Streaming eliminates intermediate staging between processes.

  • Pipeline the function results, with the PIPELINED option.

    A pipelined table function returns a row to its invoker immediately after processing that row and continues to process rows. Response time improves because the entire collection need not be constructed and returned to the server before the query can return a single result row. (Also, the function needs less memory, because the object cache need not materialize the entire collection.)

    Caution:

    A pipelined table function always references the current state of the data. If the data in the collection changes after the cursor opens for the collection, then the cursor reflects the changes. PL/SQL variables are private to a session and are not transactional. Therefore, read consistency, well known for its applicability to table data, does not apply to PL/SQL collection variables.

See Also:

12.5.2 Creating Pipelined Table Functions

A pipelined table function must be either a standalone function or a package function.

PIPELINED Option (Required)

For a standalone function, specify the PIPELINED option in the CREATE FUNCTION statement (for syntax, see "CREATE FUNCTION Statement"). For a package function, specify the PIPELINED option in both the function declaration and function definition (for syntax, see "Function Declaration and Definition").

PARALLEL_ENABLE Option (Recommended)

To improve its performance, enable the pipelined table function for parallel execution by specifying the PARALLEL_ENABLE option.

AUTONOMOUS_TRANSACTION Pragma

If the pipelined table function runs DML statements, then make it autonomous, with the AUTONOMOUS_TRANSACTION pragma (described in "AUTONOMOUS_TRANSACTION Pragma"). Then, during parallel execution, each instance of the function creates an independent transaction.

DETERMINISTIC Option (Recommended)

Multiple invocations of a pipelined table function, in either the same query or separate queries, cause multiple executions of the underlying implementation. If the function is deterministic, specify the DETERMINISTIC option, described in "DETERMINISTIC Clause".

Parameters

Typically, a pipelined table function has one or more cursor variable parameters. For information about cursor variables as function parameters, see "Cursor Variables as Subprogram Parameters".

See Also:

RETURN Data Type

The data type of the value that a pipelined table function returns must be a collection type defined either at schema level or inside a package (therefore, it cannot be an associative array type). The elements of the collection type must be SQL data types, not data types supported only by PL/SQL (such as PLS_INTEGER and BOOLEAN). For information about collection types, see "Collection Types". For information about SQL data types, see Oracle Database SQL Language Reference.

You can use SQL data types ANYTYPE, ANYDATA, and ANYDATASET 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 types to create unnamed types, including anonymous collection types. For information about these types, see Oracle Database PL/SQL Packages and Types Reference.

PIPE ROW Statement

Inside a pipelined table function, use the PIPE ROW statement to return a collection element to the invoker without returning control to the invoker. See "PIPE ROW Statement" for its syntax and semantics.

RETURN Statement

As in every function, every execution path in a pipelined table function must lead to a RETURN statement, which returns control to the invoker. However, in a pipelined table function, a RETURN statement need not return a value to the invoker. See "RETURN Statement" for its syntax and semantics.

Example

Example 12-29 Creating and Invoking Pipelined Table Function

This example creates a package that includes a pipelined table function, f1, and then selects from the collection of rows that f1 returns. 

CREATE OR REPLACE PACKAGE pkg1 AUTHID DEFINER AS
  TYPE numset_t IS TABLE OF NUMBER;
  FUNCTION f1(x NUMBER) RETURN numset_t PIPELINED;
END pkg1;
/

Create a pipelined table function f1 that returns a collection of elements (1,2,3,... x).

CREATE OR REPLACE PACKAGE BODY pkg1 AS
  FUNCTION f1(x NUMBER) RETURN numset_t PIPELINED IS
  BEGIN
    FOR i IN 1..x LOOP
      PIPE ROW(i);
    END LOOP;
    RETURN;
  END f1;
END pkg1;

SELECT * FROM TABLE(pkg1.f1(5));

Result:

COLUMN_VALUE
------------
           1
           2
           3
           4
           5
 
5 rows selected.
SELECT * FROM pkg1.f1(2);

Result:

COLUMN_VALUE
------------
           1
           2

12.5.3 Pipelined Table Functions as Transformation Functions

A pipelined table function with a cursor variable parameter can serve as a transformation function. Using the cursor variable, the function fetches an input row. Using the PIPE ROW statement, the function pipes the transformed row or rows to the invoker. If the FETCH and PIPE ROW statements are inside a LOOP statement, the function can transform multiple input rows.

In Example 12-30, the pipelined table function transforms each selected row of the employees table to two nested table rows, which it pipes to the SELECT statement that invokes it. The actual parameter that corresponds to the formal cursor variable parameter is a CURSOR expression; for information about these, see "Passing CURSOR Expressions to Pipelined Table Functions".

Example 12-30 Pipelined Table Function Transforms Each Row to Two Rows

CREATE OR REPLACE PACKAGE refcur_pkg AUTHID DEFINER IS
  TYPE refcur_t IS REF CURSOR RETURN employees%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 f_trans (p refcur_t) RETURN outrecset PIPELINED;
END refcur_pkg;
/

CREATE OR REPLACE PACKAGE BODY refcur_pkg IS
  FUNCTION f_trans (p refcur_t) RETURN outrecset PIPELINED IS
    out_rec outrec_typ;
    in_rec  p%ROWTYPE;
  BEGIN
    LOOP
      FETCH p INTO in_rec;  -- input row
      EXIT WHEN p%NOTFOUND;

      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);  -- first transformed output row

      out_rec.var_char1 := in_rec.email;
      out_rec.var_char2 := in_rec.phone_number;
      PIPE ROW(out_rec);  -- second transformed output row
    END LOOP;
    CLOSE p;
    RETURN;
  END f_trans;
END refcur_pkg;
/

SELECT * FROM TABLE (
  refcur_pkg.f_trans (
    CURSOR (SELECT * FROM employees WHERE department_id = 60)
  )
);

Result:

   VAR_NUM VAR_CHAR1                      VAR_CHAR2
---------- ------------------------------ ------------------------------
       103 Alexander                      Hunold
       103 AHUNOLD                        590.423.4567
       104 Bruce                          Ernst
       104 BERNST                         590.423.4568
       105 David                          Austin
       105 DAUSTIN                        590.423.4569
       106 Valli                          Pataballa
       106 VPATABAL                       590.423.4560
       107 Diana                          Lorentz
       107 DLORENTZ                       590.423.5567

10 rows selected.

12.5.4 Chaining Pipelined Table Functions

To chain pipelined table functions tf1 and tf2 is to make the output of tf1 the input of tf2. For example: 

SELECT * FROM TABLE(tf2(CURSOR(SELECT * FROM TABLE(tf1()))));

The rows that tf1 pipes out must be compatible actual parameters for the formal input parameters of tf2.

If chained pipelined table functions are enabled for parallel execution, then each function runs in a different process (or set of processes).

See Also:

"Passing CURSOR Expressions to Pipelined Table Functions"

12.5.5 Fetching from Results of Pipelined Table Functions

You can associate a named cursor with a query that invokes a pipelined table function. Such a cursor has no special fetch semantics, and such a cursor variable has no special assignment semantics.

However, the SQL optimizer does not optimize across PL/SQL statements. Therefore, in Example 12-31, the first PL/SQL statement is slower than the second—despite the overhead of running two SQL statements in the second PL/SQL statement, and even if function results are piped between the two SQL statements in the first PL/SQL statement.

In Example 12-31, assume that f and g are pipelined table functions, and that each function accepts a cursor variable parameter. The first PL/SQL statement associates cursor variable r with a query that invokes f, and then passes r to g. The second PL/SQL statement passes CURSOR expressions to both f and g.

See Also:

"Cursor Variables as Subprogram Parameters"

Example 12-31 Fetching from Results of Pipelined Table Functions

DECLARE
  r SYS_REFCURSOR;
  ...
  -- First PL/SQL statement (slower):
BEGIN
  OPEN r FOR SELECT * FROM TABLE(f(CURSOR(SELECT * FROM tab)));
  SELECT * BULK COLLECT INTO rec_tab FROM TABLE(g(r));

  -- NOTE: When g completes, it closes r.
END;

-- Second PL/SQL statement (faster):

SELECT * FROM TABLE(g(CURSOR(SELECT * FROM
  TABLE(f(CURSOR(SELECT * FROM tab))))));
/

12.5.6 Passing CURSOR Expressions to Pipelined Table Functions

As Example 12-31 shows, the actual parameter for the cursor variable parameter of a pipelined table function can be either a cursor variable or a CURSOR expression, and the latter is more efficient.

Note:

When a SQL SELECT statement passes a CURSOR expression to a function, the referenced cursor opens when the function begins to run and closes when the function completes.

See Also:

"CURSOR Expressions" for general information about CURSOR expressions

Example 12-32 creates a package that includes a pipelined table function with two cursor variable parameters and then invokes the function in a SELECT statement, using CURSOR expressions for actual parameters.

Example 12-33 uses a pipelined table function as an aggregate function, which takes a set of input rows and returns a single result. The SELECT statement selects the function result. (For information about the pseudocolumn COLUMN_VALUE, see Oracle Database SQL Language Reference.)

Example 12-32 Pipelined Table Function with Two Cursor Variable Parameters

CREATE OR REPLACE PACKAGE refcur_pkg AUTHID DEFINER IS
  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;
/

CREATE PACKAGE BODY refcur_pkg IS
  FUNCTION g_trans (
    p1 refcur_t1,
    p2 refcur_t2
  ) RETURN outrecset PIPELINED
  IS
    out_rec outrec_typ;
    in_rec1 p1%ROWTYPE;
    in_rec2 p2%ROWTYPE;
  BEGIN
    LOOP
      FETCH p2 INTO in_rec2;
      EXIT WHEN p2%NOTFOUND;
    END LOOP;
    CLOSE p2;
    LOOP
      FETCH p1 INTO in_rec1;
      EXIT WHEN p1%NOTFOUND;
      -- 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);
    END LOOP;
    CLOSE p1;
    RETURN;
  END g_trans;
END refcur_pkg;
/

SELECT * FROM TABLE (
  refcur_pkg.g_trans (
    CURSOR (SELECT * FROM employees WHERE department_id = 60),
    CURSOR (SELECT * FROM departments WHERE department_id = 60)
  )
);

Result:

   VAR_NUM VAR_CHAR1                      VAR_CHAR2
---------- ------------------------------ ------------------------------
       103 Alexander                      Hunold
        60 IT                             1400
       104 Bruce                          Ernst
        60 IT                             1400
       105 David                          Austin
        60 IT                             1400
       106 Valli                          Pataballa
        60 IT                             1400
       107 Diana                          Lorentz
        60 IT                             1400
 
10 rows selected.

Example 12-33 Pipelined Table Function as Aggregate Function

DROP TABLE gradereport;
CREATE TABLE gradereport (
  student VARCHAR2(30),
  subject VARCHAR2(30),
  weight NUMBER,
  grade NUMBER
);

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


CREATE PACKAGE pkg_gpa AUTHID DEFINER IS
  TYPE gpa IS TABLE OF NUMBER;
  FUNCTION weighted_average(input_values SYS_REFCURSOR)
    RETURN gpa PIPELINED;
END pkg_gpa;

CREATE PACKAGE BODY pkg_gpa IS
  FUNCTION weighted_average (input_values SYS_REFCURSOR)
    RETURN gpa PIPELINED
  IS
    grade         NUMBER;
    total         NUMBER := 0;
    total_weight  NUMBER := 0;
    weight        NUMBER := 0;
  BEGIN
    LOOP
      FETCH input_values INTO weight, grade;
      EXIT WHEN input_values%NOTFOUND;
      total_weight := total_weight + weight;  -- Accumulate weighted average
      total := total + grade*weight;
    END LOOP;
    PIPE ROW (total / total_weight);
    RETURN; -- returns single result
  END weighted_average;
END pkg_gpa;

This query shows how the table function can be invoked without the optional TABLE operator.

SELECT w.column_value "weighted result" 
FROM pkg_gpa.weighted_average (
    CURSOR (SELECT weight, grade FROM gradereport)
  ) w;

Result:

weighted result
---------------
            3.5
 
1 row selected.

12.5.7 DML Statements on Pipelined Table Function Results

The "table" that a pipelined table function returns cannot be the target table of a DELETE, INSERT, UPDATE, or MERGE statement. However, you can create a view of such a table and create INSTEAD OF triggers on the view. For information about INSTEAD OF triggers, see "INSTEAD OF DML Triggers".

See Also:

Oracle Database SQL Language Reference for information about the CREATE VIEW statement

12.5.8 NO_DATA_NEEDED Exception

You must understand the predefined exception NO_DATA_NEEDED in two cases:

  • You include an OTHERS exception handler in a block that includes a PIPE ROW statement

  • Your code that feeds a PIPE ROW statement must be followed by a clean-up procedure

    Typically, the clean-up procedure releases resources that the code no longer needs.

When the invoker of a pipelined table function needs no more rows from the function, the PIPE ROW statement raises NO_DATA_NEEDED. If the pipelined table function does not handle NO_DATA_NEEDED, as in Example 12-34, then the function invocation terminates but the invoking statement does not terminate. If the pipelined table function handles NO_DATA_NEEDED, its exception handler can release the resources that it no longer needs, as in Example 12-35.

In Example 12-34, the pipelined table function pipe_rows does not handle the NO_DATA_NEEDED exception. The SELECT statement that invokes pipe_rows needs only four rows. Therefore, during the fifth invocation of pipe_rows, the PIPE ROW statement raises the exception NO_DATA_NEEDED. The fifth invocation of pipe_rows terminates, but the SELECT statement does not terminate.

If the exception-handling part of a block that includes a PIPE ROW statement includes an OTHERS exception handler to handle unexpected exceptions, then it must also include an exception handler for the expected NO_DATA_NEEDED exception. Otherwise, the OTHERS exception handler handles the NO_DATA_NEEDED exception, treating it as an unexpected error. The following exception handler reraises the NO_DATA_NEEDED exception, instead of treating it as a irrecoverable error: 

EXCEPTION
  WHEN NO_DATA_NEEDED THEN
    RAISE;
  WHEN OTHERS THEN
    -- (Put error-logging code here)
    RAISE_APPLICATION_ERROR(-20000, 'Fatal error.');
END;

In Example 12-35, assume that the package External_Source contains these public items:

  • Procedure Init, which allocates and initializes the resources that Next_Row needs

  • Function Next_Row, which returns some data from a specific external source and raises the user-defined exception Done (which is also a public item in the package) when the external source has no more data

  • Procedure Clean_Up, which releases the resources that Init allocated

The pipelined table function get_external_source_data pipes rows from the external source by invoking External_Source.Next_Row until either:

  • The external source has no more rows.

    In this case, the External_Source.Next_Row function raises the user-defined exception External_Source.Done.

  • get_external_source_data needs no more rows.

    In this case, the PIPE ROW statement in get_external_source_data raises the NO_DATA_NEEDED exception.

In either case, an exception handler in block b in get_external_source_data invokes External_Source.Clean_Up, which releases the resources that Next_Row was using.

Example 12-34 Pipelined Table Function Does Not Handle NO_DATA_NEEDED

CREATE TYPE t IS TABLE OF NUMBER
/
CREATE OR REPLACE FUNCTION pipe_rows RETURN t PIPELINED AUTHID DEFINER IS
  n NUMBER := 0;
BEGIN
  LOOP
    n := n + 1;
    PIPE ROW (n);
  END LOOP;
END pipe_rows;
/
SELECT COLUMN_VALUE
  FROM TABLE(pipe_rows())
  WHERE ROWNUM < 5
/

Result:

COLUMN_VALUE
------------
           1
           2
           3
           4

4 rows selected.

Example 12-35 Pipelined Table Function Handles NO_DATA_NEEDED

CREATE OR REPLACE FUNCTION get_external_source_data
  RETURN t PIPELINED AUTHID DEFINER IS
BEGIN
  External_Source.Init();           -- Initialize.
  <<b>> BEGIN
    LOOP                            -- Pipe rows from external source.
      PIPE ROW (External_Source.Next_Row());
    END LOOP;
  EXCEPTION
    WHEN External_Source.Done THEN  -- When no more rows are available,
      External_Source.Clean_Up();   --  clean up.
    WHEN NO_DATA_NEEDED THEN        -- When no more rows are needed,
      External_Source.Clean_Up();   --  clean up.
      RAISE NO_DATA_NEEDED;           -- Optional, equivalent to RETURN.
  END b;
END get_external_source_data;
/