Transactions not working OK in a PostgreSQL 11 stored procedure - postgresql

Question

Postgres version:
PostgreSQL 11.0 on x86_64-pc-linux-gnu,
compiled by gcc (GCC) 4.8.3 20140911 (Red Hat 4.8.3-9), 64-bit
I have this stored procedure as shown below. This is just a test.
In that procedure I have 2 transactions:
The first one is supposed to finish OK (i.e. I have written the code
so that it does not encounter any errors and so it will reach the COMMIT statement).
The second transaction is supposed to fail as I intentionally
introduce an error in it (either via this cast there, or via an INSERT which causes PK violation).
Also, yb.print_now is a simple function which is just logging (inserting) messages to another table.
When I run this stored procedure I am expecting the updates and the logging of messages done by
the first transaction to be persisted in the database, even though the 2nd transaction failed.
But this is not what happens, both transactions seem to be rolled back.
Why is this? Am I doing something wrong?
And 2 more questions which are very important to me. :
When an error occurs (say like on line marked ***) and when control reaches/jumps to the EXCEPTION block, I have the feeling that the transaction I was in is already rolled back before I even reach the EXCEPTION block.
So in the exception block I cannot do ROLLBACK or COMMIT or anything
related to the transaction. Is that feeling correct?
Say I want to commit all the stuff done, despite of the error, is there a way I can do that?
That's exactly what I want here. The error is an error... OK, but I want everything
which happened before I got the error to get committed.
How do I do this in Postgres 11?
CREATE OR REPLACE PROCEDURE yb.test123()
LANGUAGE plpgsql
AS $procedure$
DECLARE
var_cnt int;
c int;
BEGIN
START TRANSACTION; --- 1 ---
raise notice '001.';
PERFORM yb.print_now('===> 0010.');
var_cnt = 0;
update yb.mbb
set the_price = the_price + 1
where
the_id = 23164;
raise notice '002.';
PERFORM yb.print_now('===> 0020.');
raise notice '003.';
PERFORM yb.print_now('===> 0030.');
update yb.mbb
set the_price = the_price + 1
where
the_id = 23164;
COMMIT; --- 1 ---
START TRANSACTION; --- 2 ---
c = cast('###a1e3Z' as int); --- *** ---
raise notice '004.';
PERFORM yb.print_now('===> 0040.');
update yb.mbb
set the_price = the_price + 1
where
the_id = 23164;
-- insert into yb.mbb(the_id)
-- values (23164); -- this will throw duplicate PK error
raise notice '005.';
PERFORM yb.print_now('===> 0050.');
COMMIT; --- 2 ---
EXCEPTION
WHEN OTHERS THEN
raise notice 'We are in the exception block now.';
-- ROLLBACK;
-- COMMIT;
RETURN;
END
$procedure$;

Answer 1

The error happens right at the start of your procedure, in the statement
START TRANSACTION;
As the documentation says:
A new transaction is started automatically after a transaction is ended using these commands, so there is no separate START TRANSACTION command.
That should answer your first question.
As to the second, when you are in the exception branch, you have effectively rolled back the subtransaction that started with the BEGIN that belongs to the EXCEPTION clause (or after the last COMMIT). You are still in the transaction though, so you can issue COMMIT and ROLLBACK.
To your third question: No, there is no way to commit “everything up to the last exception”. You could only have that by wrapping every statement in a BEGIN ... EXCEPTION ... END block, but that would seriously hurt your performance (apart from making your code unreadable).
Use BEGIN ... EXCEPTION ... END blocks judiciously whenever you expect that a statement could fail.