Specific exception handling and messages

Transactions and Error Handling in PostgreSQL

Jason Myers

Principal Engineer

Catching a specific type of exception

DO $$
BEGIN
    UPDATE inventory SET quantity = quantity - 1 WHERE name in ('flour', 'sugar');
EXCEPTION
    WHEN check_violation THEN
           INSERT INTO errors (msg) VALUES ('Quantity can not be less than 0.');
           RAISE INFO 'Quantity can not be less than 0.';
END; 
$$ language 'plpgsql';

Output of our exception handler

INFO:  Quantity can not be less than 0.
DO
postgres=# select * from errors;
 error_id | state |               msg                | detail | context
^---------+-------+----------------------------------+--------+---------
        1 |       | Quantity can not be less than 0. |        |
(1 row)

Common types of exception conditions

Condition Name	Example
unique_violation	Insert two of the same value in a unique column
not_null_violation	Insert null into a field that doesn't allow nulls
check_violation	Failing a check constraint such as being higher than 11 in deliciousness
division_by_zero	Dividing by 0

So many more at the link in the citation below

¹ https://www.postgresql.org/docs/9.4/errcodes-appendix.html

Catching multiple exceptions

DO $$
BEGIN
    UPDATE inventory SET quantity = quantity - 6, cost = null 
    WHERE name='oatmeal dark chocolate';

Catching multiple exception types individually

-- Add check_violation exception
EXCEPTION
  WHEN check_violation THEN
     INSERT INTO errors (msg) VALUES ('Quantity can not be less than 0.');
     RAISE INFO 'Quantity can not be less than 0.';

-- Add non-null exception
  WHEN not_null_violation THEN
     INSERT INTO errors (msg) VALUES ('Cost can not be null.');
     RAISE INFO 'Cost can not be null.';
END; $$ language 'plpgsql';

Catching multiple exceptions output

INFO:  Cost can not be null.
DO
postgres=# select * from errors;
 error_id | state |               msg                | detail | context
^---------+-------+----------------------------------+--------+---------
        2 |       | Cost can not be null.            |        |
(1 row)

Time to apply it!

Transactions and Error Handling in PostgreSQL