Skip to main content

How many times does my table function execute?

A left correlation join occurs when you pass as an argument to your table function a column value from a table or view referenced to the left in the table clause. This technique is used with XMLTABLE and JSON_TABLE built-in functions, but also applies to your own table functions.

Here's the thing to remember:
The table function will be called for each row in the table/view that is providing the column to the function. 
Clearly, this could cause some performance issues, so be sure that is what you want and need to do.

The following code demonstrates this behavior, for both pipelined and non-pipelined functions.

CREATE TABLE things
(
   thing_id     NUMBER,
   thing_name   VARCHAR2 (100)
)
/

BEGIN
   INSERT INTO things VALUES (1, 'Thing 1');
   INSERT INTO things VALUES (2, 'Thing 2');
   COMMIT;
END;
/

CREATE OR REPLACE TYPE numbers_t IS TABLE OF NUMBER
/

CREATE OR REPLACE FUNCTION more_numbers (id_in IN NUMBER)
   RETURN numbers_t
IS
   l_numbers   numbers_t := numbers_t ();
BEGIN
   l_numbers.EXTEND (id_in * 5);

   FOR indx IN 1 .. id_in * 5
   LOOP
      l_numbers (indx) := indx;
   END LOOP;

   DBMS_OUTPUT.put_line ('more numbers');
   RETURN l_numbers;
END;
/

BEGIN
   FOR rec IN (SELECT th.thing_name, t.COLUMN_VALUE thing_number
                 FROM things th, TABLE (more_numbers (th.thing_id)) t)
   LOOP
      DBMS_OUTPUT.put_line ('more numbers ' || rec.thing_number);
   END LOOP;
END;
/

more numbers
more numbers
more numbers 1
more numbers 2
more numbers 3
more numbers 4
more numbers 5
more numbers 1
more numbers 2
more numbers 3
more numbers 4
more numbers 5
more numbers 6
more numbers 7
more numbers 8
more numbers 9
more numbers 10

CREATE OR REPLACE FUNCTION more_numbers (id_in IN NUMBER)
   RETURN numbers_t PIPELINED
IS
BEGIN
   DBMS_OUTPUT.put_line ('more numbers');
   FOR indx IN 1 .. id_in * 5
   LOOP
      PIPE ROW (indx);
   END LOOP;
   RETURN;
END;
/

BEGIN
   FOR rec IN (SELECT th.thing_name, t.COLUMN_VALUE thing_number
                 FROM things th, TABLE (more_numbers (th.thing_id)) t)
   LOOP
      DBMS_OUTPUT.put_line ('more numbers ' || rec.thing_number);
   END LOOP;
END;
/

more numbers
more numbers
more numbers 1
more numbers 2
more numbers 3
more numbers 4
more numbers 5
more numbers 1
more numbers 2
more numbers 3
more numbers 4
more numbers 5
more numbers 6
more numbers 7
more numbers 8
more numbers 9
more numbers 10


Labels:

Comments

Post a Comment

Popular posts from this blog

Why DBMS_OUTPUT.PUT_LINE should not be in your application code

A database developer recently came across my  Bulletproof PL/SQL  presentation, which includes this slide. That first item in the list caught his attention: Never put calls to DBMS_OUTPUT.PUT_LINE in your application code. So he sent me an email asking why I would say that. Well, I suppose that is the problem with publishing slide decks. All the explanatory verbiage is missing. I suppose maybe I should do a video. :-) But in the meantime, allow me to explain. First, what does DBMS_OUTPUT.PUT_LINE do? It writes text out to a buffer, and when your current PL/SQL block terminates, the buffer is displayed on your screen. [Note: there can be more to it than that. For example, you could in your own code call DBMS_OUTPUT.GET_LINE(S) to get the contents of the buffer and do something with it, but I will keep things simple right now.] Second, if I am telling you not to use this built-in, how could text from your program be displayed on your screen? Not without a lot o...
4 comments
Read more

How to Pick the Limit for BULK COLLECT

This question rolled into my In Box today: In the case of using the LIMIT clause of BULK COLLECT, how do we decide what value to use for the limit? First I give the quick answer, then I provide support for that answer Quick Answer Start with 100. That's the default (and only) setting for cursor FOR loop optimizations. It offers a sweet spot of improved performance over row-by-row and not-too-much PGA memory consumption. Test to see if that's fast enough (likely will be for many cases). If not, try higher values until you reach the performance level you need - and you are not consuming too much PGA memory.  Don't hard-code the limit value: make it a parameter to your subprogram or a constant in a package specification. Don't put anything in the collection you don't need. [from Giulio Dottorini] Remember: each session that runs this code will use that amount of memory. Background When you use BULK COLLECT, you retrieve more than row with each fetch, ...
4 comments
Read more

Table Functions, Part 1: Introduction and Exploration

Please do feel encouraged to read this and my other posts on table functions, but you will learn much more about table functions by taking my Get Started with PL/SQL Table Functions class at the Oracle Dev Gym. Videos, tutorials and quizzes - then print a certificate when you are done! Table functions - functions that can be called in the FROM clause of a query from inside the TABLE operator - are fascinating and incredibly helpful constructs. So I've decided to write a series of blog posts on them: how to build them, how to use them, issues you might run into. Of course, I am not the first to do so. I encourage to check out the  documentation , as well as excellent posts from Adrian Billington (search for "table functions") and Tim Hall . Adrian and Tim mostly focus on pipelined table functions, a specialized variant of table functions designed to improve performance and reduce PGA consumption. I will take a look at pipelined table functions in the latter part...
31 comments
Read more