Error Trapping Statements

By default, any error occurring in a PL/SQL function aborts execution of the function, and indeed of the surrounding transaction as well. You can trap errors and restore from them by using a BEGIN block with an EXCEPTION clause. The syntax is an extension of the normal syntax for a BEGIN block:

    
         [<<label>>]
[DECLARE
    declarations]
BEGIN
    statements
EXCEPTION
    WHEN condition [OR condition ...] THEN
        handler_statements
    [WHEN condition [OR condition ...] THEN
        handler_statements
    ...]
END;

If no error occurs, this form of enclosing block simply executes all the statements, and then control passes to the next statement after END. But if an error occurs within the statements, further processing of the statements is abandoned, and control passes to the EXCEPTION list. The list is searched for the first condition matching the error that occurred. If a match is found, the corresponding handler_statements are executed, and then control passes to the next statement after END. If no match is found, the error propagates out as though the EXCEPTION clause were not there at all: Error codes can be used to catch other error codes of the same type.

The error can be caught by an enclosing block with EXCEPTION, or if there is none it aborts processing of the function.

The condition names can be any of those shown in SQL standard error codes. The special condition name OTHERS matches every error type except QUERY_CANCELED.

If a new error occurs within the selected handler_statements, it cannot be caught by this EXCEPTION clause, but is propagated out. A surrounding EXCEPTION clause could catch it.

When an error is caught by an EXCEPTION clause, the local variables of the PL/SQL function remain as they were when the error occurred, but all changes to persistent database state within the block are rolled back.

Example:

   
    
      
      
        gaussdb=# CREATE TABLE mytab(id INT,firstname VARCHAR(20),lastname VARCHAR(20)) ;
CREATE TABLE

gaussdb=# INSERT INTO mytab(firstname, lastname) VALUES('Tom', 'Jones');
INSERT 0 1

gaussdb=# CREATE FUNCTION fun_exp() RETURNS INT
AS $$
DECLARE
    x INT :=0;
    y INT;
BEGIN
    UPDATE mytab SET firstname = 'Joe' WHERE lastname = 'Jones';
    x := x + 1;
    y := x / 0;
EXCEPTION
    WHEN division_by_zero THEN
        RAISE NOTICE 'caught division_by_zero';
        RETURN x;
END;$$
LANGUAGE plpgsql;
CREATE FUNCTION

gaussdb=# call fun_exp();
NOTICE:  caught division_by_zero
 fun_exp 
---------
       1
(1 row)

gaussdb=# select * from mytab;
 id | firstname | lastname 
----+-----------+----------
    | Tom       | Jones
(1 row)

gaussdb=# DROP FUNCTION fun_exp();
DROP FUNCTION

gaussdb=# DROP TABLE mytab;
DROP TABLE

       

     

   
  

When control reaches the assignment to y, it will fail with a division_by_zero error. This will be caught by the EXCEPTION clause. The value returned in the RETURN statement will be the incremented value of x.

A block containing an EXCEPTION clause is more expensive to enter and exit than a block without one. Therefore, do not use EXCEPTION without need.
In the following scenarios, the processing exception cannot be captured and the entire stored procedure is rolled back: 1. The node is faulty. 2. The thread of the node involved in the stored procedure exits due to a network fault. In addition, the structure of the source data is inconsistent with that of the target table in the COPY FROM operation. 3. Exceptions occur when EXCEPTION clears subtransactions. (For a stored procedure that contains the EXCEPTION statement, an implicit subtransaction is started when the statement is executed. After the statement is executed, the subtransaction is automatically cleared. During the clearing, exceptions may occur due to memory control.)

Example: Exceptions with UPDATE/INSERT

This example uses exception handling to perform either UPDATE or INSERT, as appropriate:

   
    
      
      
        gaussdb=# CREATE TABLE db (a INT, b TEXT);
CREATE TABLE

gaussdb=# CREATE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS
$$
BEGIN
    LOOP

-- First try to update the key
        UPDATE db SET b = data WHERE a = key;
        IF found THEN
            RETURN;
        END IF;
-- The key does not exist. Therefore, try to insert one. If someone else inserts the same key concurrently, the unique key may fail to be obtained.
        BEGIN
            INSERT INTO db(a,b) VALUES (key, data);
            RETURN;
        EXCEPTION WHEN unique_violation THEN
        -- Do nothing, and loop to try the UPDATE again.
        END;
     END LOOP;
END;
$$
LANGUAGE plpgsql;
CREATE FUNCTION

gaussdb=# SELECT merge_db(1, 'david');
 merge_db 
----------

(1 row)

gaussdb=# SELECT merge_db(1, 'dennis');
 merge_db 
----------

(1 row)

--Delete FUNCTION and TABLE:
gaussdb=# DROP FUNCTION merge_db;
DROP FUNCTION

gaussdb=# DROP TABLE db;
DROP TABLE

       

     

   
  