Skip to main content

Why isn't my exception section catching my error?

I got an interesting email today from a reader of one of my PL/SQL 101" articles for Oracle Magazine, Building with Blocks.

Q. had taken the code from the article, made some changes, tried to run them, and got very confused. He wrote:

When I run this code, I see "Hello World".
DECLARE
  l_message VARCHAR2(100) := 'Hello World!';
BEGIN
  DBMS_OUTPUT.put_line (l_message);
EXCEPTION
  WHEN OTHERS
  THEN
    DBMS_OUTPUT.put_line ('Error='||SQLERRM);
END;
/

Hello World!
When I change the block to make the l_message variable too small for its string, I see the VALUE_ERROR error message.
DECLARE
  l_message VARCHAR2(10);
BEGIN
  l_message := 'Hello World!';
  DBMS_OUTPUT.put_line (l_message);
EXCEPTION
  WHEN OTHERS
  THEN
    DBMS_OUTPUT.put_line ('Error='||SQLERRM);
END;
/

Error=ORA-06502: PL/SQL: numeric or value error: character string buffer too small 
But when I change the name of the variables inside the call to DBMS_OUTPUT.PUT_LINE to "l_message1", the exception handler is not displaying the error message, instead I see the following:
When I change the block to make the l_message variable too small for its string, I see the VALUE_ERROR error message.
DECLARE
  l_message VARCHAR2(10) := 'Hello World!';
BEGIN
  DBMS_OUTPUT.put_line (l_message1);
EXCEPTION
  WHEN OTHERS
  THEN
    DBMS_OUTPUT.put_line ('Error='||SQLERRM);
END;
/

ORA-06550: line 5, column 25:
PLS-00201: identifier 'L_MESSAGE1' must be declared 
Why am I seeing this inconsistent behavior?  

I was confused about the source of his confusion, but after a couple of back-and-forth emails (what you read above is the cleaned-up version of that back-and-forth), the light bulb lit in my brain. Which enabled me to clear up his confusion, and inspire me to write a blog post, in case the same confusion was confusing anyone else.

Here's the most important thing to remember:
Exception sections handle exceptions raised when the block is executed. But first that block must be compiled!
When you are writing code within stored program units, like procedures and functions, this is clearly a two stage process:
  1. Compile
  2. Execute
But when you are working with an anonymous block, all you do is execute the block. So you can certainly be forgiven for thinking that anything that goes wrong will be caught in the exception handler.

The reality is different however. When you run an anonymous block, the PL/SQL engine will first of all parse and compile the block. If all goes well, then the PL/SQL runtime engine will execute the compiled code.

If, however, your code fails to compile, well....you will get a compile error, rather than an runtime exception.

How do you know you've gotten a compile error? Check the prefix. If it is "PLS", something went kablooey at compile time. Your code cannot be executed. If the prefix is "PLW", that's a compile-time warning, which means that the code compiled, but the PL/SQL engine has suggestions for improving it. And if the prefix is "ORA", ah, then you've got a runtime error - an exception.

Only in that last case with the exception handler, if present, be able to trap the exception.

I hope that makes thing nice and clear. Any comments or questions? :-)


Comments

  1. „Error=Hello World„ Shoudn‘t this be „Hello World!“? :-)
    Trank you for all the great and inspiring blog posts. They are a great help!

    ReplyDelete
    Replies
    1. Jan, thanks for your kind words and for catching that mistake in my output. I have fixed it!

      Delete
  2. Whilst I understand the point you're trying to make, unfortunately the examples don't seem to do that. In particular, whilst the compiler could (in this case) detect that a VARCHAR2(10) is too small, it doesn't, so you get a runtime error. But an error in a declaration isn't handled within the exception handler for that block, but in the calling block. So you won't see the Error= prefix. I've been caught by this on several occasions where an error message misleadingly comes from the error handler of the calling block, not of the block where the error actually is because it's in the declaration section.

    ReplyDelete
  3. Thanks for pointing that out, M P.

    That was sloppy of me. I have changed the code in that block to move the assignment to the executable section.

    Which reminds me of a rule I tell myself to follow but then at least sometimes do not follow:

    TEST ALL YOUR CODE BEFORE PUBLISHING IT!

    :-)

    Regards, Steven

    ReplyDelete

Post a Comment

Popular posts from this blog

Get rid of mutating table trigger errors with the compound trigger

When something mutates, it is changing. Something that is changing is hard to analyze and to quantify. A mutating table error (ORA-04091) occurs when a row-level trigger tries to examine or change a table that is already undergoing change (via an INSERT, UPDATE, or DELETE statement). In particular, this error occurs when a row-level trigger attempts to read or write the table from which the trigger was fired. Fortunately, the same restriction does not apply in statement-level triggers.

In this post, I demonstrate the kind of scenario that will result in an ORA-04091 errors. I then show the "traditional" solution, using a collection defined in a package. Then I demonstrate how to use the compound trigger, added in Oracle Database 11g Release1,  to solve the problem much more simply.

All the code shown in this example may be found in this LiveSQL script.
How to Get a Mutating Table ErrorI need to implement this rule on my employees table:
Your new salary cannot be more than 25x th…

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 perspective…

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 of this seri…