Catching exceptions

Transactions and Error Handling in PostgreSQL

Jason Myers

Principal Engineer

Statements that error

INSERT INTO sales (name, quantity, cost) 
VALUES 
  ('chocolate chip', 6, null);

ERROR:  null value in column "cost" violates not-null constraint
DETAIL:  Failing row contains 
(1, "chocolate chip", 6, null, 2020-04-28 19:58:55.715886).

Generic exception capture

tryCatch(
  sqrt("a"), 
  error=function(e) 
    print("Boom!")
)

Python

try:
   math.sqrt("a")
except Exception as e:
   print("Boom!")

PL/pgSQL

BEGIN 
SELECT 
  SQRT("a");
EXCEPTION WHEN others THEN RAISE INFO 'Boom!';
END;

Results

     R: Boom!
Python: Boom!
   SQL: INFO: Boom!

PL/pgSQL DO commands (anonymous functions)

DO $$

DECLARE
   some_variable text;

BEGIN
    SELECT text from a table;
END;

$$ language 'plpgsql';

Exception handling function

DO $$
BEGIN
    SELECT SQRT("a");
EXCEPTION
    WHEN others THEN
       INSERT INTO errors (msg) VALUES ('Can not take the square root of a string.');
       RAISE INFO 'Can not take the square root of a string.';
END; 
$$ language 'plpgsql';

Using exception handling wisely

Using an EXCEPTION clause adds significant overhead
Python or R exception handling is more efficient

Don't sacrifice getting the right context to solve the exception
Don't optimize before you understand your exceptions.

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

Changing data sets

patients

column	type
patient_id	integer
a1c	double (float)
glucose	integer
fasting	boolean
created_on	timestamp

errors

column	type
error_id	integer
state	string
msg	string
detail	string
context	string

Let's practice!

Transactions and Error Handling in PostgreSQL