Skip to main content

Emulating a finally clause in PL/SQL

PL/SQL does not support a finally clause, as many other languages do, including Java. Here's a description of the finally block from the Java SE doc:
The finally block always executes when the try block exits. This ensures that the finally block is executed even if an unexpected exception occurs. But finally is useful for more than just exception handling — it allows the programmer to avoid having cleanup code accidentally bypassed by a return, continue, or break. Putting cleanup code in a finally block is always a good practice, even when no exceptions are anticipated.
The first thing to say regarding PL/SQL and finally is that the need for it in PL/SQL is likely less critical than in other languages, precisely because the PL/SQL runtime engine (and the underlying Oracle Database engine) does most of the clean up for you.

Any variables you declare, cursors you open, types you define inside a block are automatically cleaned up (memory released) when that block terminates.

Still, there are exceptions to this rule, including:

>> Changes to tables are not automatically rolled back or committed when a block terminates.

If you include an autonomous transaction pragma in your block, PL/SQL will "insist" (raise an exception at runtime) if you do not  rollback or commit, but that's different.

>> Elements declared at the package level have session scope. They will not be automatically cleaned up when a block in which they are used terminates.

Here's a very simple demonstration of that fact. I declare a cursor at the package level, open it inside a block, "forget" to close it, and then try to open it again in another block:
CREATE OR REPLACE PACKAGE serial_package AUTHID DEFINER
AS
   CURSOR emps_cur
   IS
      SELECT *
        FROM employees;
END serial_package;
/

BEGIN
   OPEN serial_package.emps_cur;
END;
/

BEGIN
   OPEN serial_package.emps_cur;
END;
/

BEGIN
   OPEN serial_package.emps_cur;
END;
/

ORA-06511: PL/SQL: cursor already open
ORA-06512: at "STEVEN.SERIAL_PACKAGE", line 5
ORA-06512: at line 2
Try it out yourself in LiveSQL.

Since there is no finally clause, you have to take care of things yourself. The best way to do this - and I am not claiming it is optimal - is to create a nested cleanup procedure and invoke that as needed.

Here we go - no more error when I attempt to open the cursor the second time.
CREATE OR REPLACE PACKAGE serial_package AUTHID DEFINER
AS
   CURSOR emps_cur
   IS
      SELECT *
        FROM employees;
END serial_package;
/

CREATE OR REPLACE PROCEDURE use_packaged_cursor AUTHID DEFINER
IS
   PROCEDURE cleanup
   IS
   BEGIN
      /* If called from exception section log the error */
      IF SQLCODE <> 0
      THEN
         /* Uses open source Logger utility:
               https://github.com/OraOpenSource/Logger */
         logger.log_error ('use_packaged_cursor');
      END IF;
      
      IF serial_package.emps_cur%ISOPEN
      THEN
         CLOSE serial_package.emps_cur;
      END IF;
   END cleanup;
BEGIN
   OPEN serial_package.emps_cur;
   
   cleanup;
EXCEPTION
   WHEN NO_DATA_FOUND
   THEN
      /* Clean up but do not re-raise (just to show that you might want
         different behaviors for different exceptions). */
      cleanup;
      
   WHEN OTHERS
   THEN
      cleanup;
      RAISE;
END;
/

BEGIN
   use_packaged_cursor;
END;
/

PL/SQL procedure successfully completed.

BEGIN
   use_packaged_cursor;
END;
/

PL/SQL procedure successfully completed.
(also available in LiveSQL)

Now, I am not, repeat NOT, claiming that this is as good as having a finally clause. I am just saying: this is how you can (have to) achieve a similar effect.

Comments

Popular posts from this blog

Quick Guide to User-Defined Types in Oracle PL/SQL

A Twitter follower recently asked for more information on user-defined types in the PL/SQL language, and I figured the best way to answer is to offer up this blog post. PL/SQL is a strongly-typed language . Before you can work with a variable or constant, it must be declared with a type (yes, PL/SQL also supports lots of implicit conversions from one type to another, but still, everything must be declared with a type). PL/SQL offers a wide array of pre-defined data types , both in the language natively (such as VARCHAR2, PLS_INTEGER, BOOLEAN, etc.) and in a variety of supplied packages (e.g., the NUMBER_TABLE collection type in the DBMS_SQL package). Data types in PL/SQL can be scalars, such as strings and numbers, or composite (consisting of one or more scalars), such as record types, collection types and object types. You can't really declare your own "user-defined" scalars, though you can define subtypes  from those scalars, which can be very helpful from the p

The differences between deterministic and result cache features

 EVERY once in a while, a developer gets in touch with a question like this: I am confused about the exact difference between deterministic and result_cache. Do they have different application use cases? I have used deterministic feature in many functions which retrieve data from some lookup tables. Is it essential to replace these 'deterministic' key words with 'result_cache'?  So I thought I'd write a post about the differences between these two features. But first, let's make sure we all understand what it means for a function to be  deterministic. From Wikipedia : In computer science, a deterministic algorithm is an algorithm which, given a particular input, will always produce the same output, with the underlying machine always passing through the same sequence of states.  Another way of putting this is that a deterministic subprogram (procedure or function) has no side-effects. If you pass a certain set of arguments for the parameters, you will always get

My two favorite APEX 5 features: Regional Display Selector and Cards

We (the over-sized development team for the PL/SQL Challenge - myself and my son, Eli) have been busy creating a new website on top of the PLCH platform (tables and packages): The Oracle Dev Gym! In a few short months (and just a part time involvement by yours truly), we have leveraged Oracle Application Express 5 to create what I think is an elegant, easy-to-use site that our users will absolutely love.  We plan to initially make the Dev Gym available only for current users of PL/SQL Challenge, so we can get feedback from our loyal user base. We will make the necessary adjustments and then offer it for general availability later this year. Anyway, more on that as the date approaches (the date being June 27, the APEX Open Mic Night at Kscope16 , where I will present it to a packed room of APEX experts). What I want to talk about today are two features of APEX that are making me so happy these days: Regional Display Selector and Cards. Regional Display Sel