Enhancing exception handling with stacked diagnostics

Transactions and Error Handling in PostgreSQL

Jason Myers

Principal Engineer

Capturing more error information

DO $$
BEGIN
   UPDATE inventory SET cost = 35.0 WHERE name = 'Macaron';
   UPDATE inventory SET cost = 3.50 WHERE name = 'Panellets';
EXCEPTION
   WHEN others THEN
       INSERT INTO errors (msg) VALUES ('Max cost is 10!');
       RAISE INFO 'Max cost is 10!';
END; 
$$ language 'plpgsql';

Using stacked diagnostics

DO $$
DECLARE
   exc_message text;
   exc_detail text;

BEGIN
   UPDATE inventory SET cost = 35.0 WHERE name = 'Macaron';
   UPDATE inventory SET cost = 3.50 WHERE name = 'Panellets';

EXCEPTION
   WHEN others THEN
          GET STACKED DIAGNOSTICS exc_message = MESSAGE_TEXT,
                                  exc_detail = PG_EXCEPTION_DETAIL;
       INSERT INTO errors (msg, detail) VALUES (exc_message, exc_detail);
       RAISE INFO 'Exception Messaage: % | Exception Details: %', exc_message, exc_detail;
END$$;

Example diagnostic output

INFO:  Exception Messaage: new row for relation "inventory" violates check constraint 
"cost_check" | Exception Details: Failing row contains (7, 35, Macaron).
DO

postgres=# \x on
Expanded display is on.
postgres=# select msg, detail from errors;
-[ RECORD 1 ]-------------------------------------------------------------------
msg    | new row for relation "inventory" violates check constraint "cost_check"
detail | Failing row contains (7, 35, Macaron).

So what all can you get?

Name	Description
RETURNED_SQLSTATE	the SQLSTATE error code of the exception
COLUMN_NAME	the name of the column related to exception
CONSTRAINT_NAME	the name of the constraint related to exception
MESSAGE_TEXT	the text of the exception's primary message
PG_EXCEPTION_DETAIL	the text of the exception's detail message, if any

¹ https://www.postgresql.org/docs/12/plpgsql-control-structures.html

More diagnostic datapoints

Name	Description
PG_DATATYPE_NAME	the name of the data type related to exception
TABLE_NAME	the name of the table related to exception
SCHEMA_NAME	the name of the schema related to exception
PG_EXCEPTION_HINT	the text of the exception's hint message, if any
PG_EXCEPTION_CONTEXT	line(s) of text of the call stack at the time of the exception

Let's practice!

Transactions and Error Handling in PostgreSQL