Can a Postgres Commit Exist in Procedure that has an Exception Block?

Question

I am having a difficult time understanding transactions in Postgres. I have a procedure that may encounter an exception. There are parts of the procedure where I might want to commit my work so-far so that it won't be rolled back if an exceptions ensues.

I want to have an exception handling block at the end of the procedure where I catch the exception and insert the information from the exception into a logging table.

I have boiled the problem down to a simple procedure, below, which fails on PostgreSQL 11.2 with

2D000 cannot commit while a subtransaction is active
PL/pgSQL function x_transaction_try() line 6 at COMMIT

    drop procedure if exists x_transaction_try;
    create or replace procedure x_transaction_try()
        language plpgsql
    as $$
    declare
    begin
         raise notice 'A';
         -- TODO A: do some insert or update that I want to commit no matter what
         commit;
         raise notice 'B';
         -- TODO B: do something else that might raise an exception, without rolling
         -- back the work that we did in "TODO A".
    exception when others then
      declare
        my_ex_state text;
        my_ex_message text;
        my_ex_detail text;
        my_ex_hint text;
        my_ex_ctx text;
      begin
          raise notice 'C';
          GET STACKED DIAGNOSTICS
            my_ex_state   = RETURNED_SQLSTATE,
            my_ex_message = MESSAGE_TEXT,
            my_ex_detail  = PG_EXCEPTION_DETAIL,
            my_ex_hint    = PG_EXCEPTION_HINT,
            my_ex_ctx     = PG_EXCEPTION_CONTEXT
          ;
          raise notice '% % % % %', my_ex_state, my_ex_message, my_ex_detail, my_ex_hint, my_ex_ctx;
          -- TODO C: insert this exception information in a logging table and commit
      end;
    end;
    $$;

    call x_transaction_try();

Why doesn't this stored procedure work? Why is it that we never see the output of raise notice 'B' and instead we go into the exception block? Is it possible to do what I have described above with a Postgres 11 stored procedure?

Edit: This is a complete code sample. Paste the above complete code sample (including both the create procedure and call statements) into a sql file and run it in a Postgres 11.2 database to repro. The desired output would be for the function to print A then B, but instead it prints A then C along with the exception information.

Also notice that if you comment out all of the exception handling block such that the function does not catch exceptions at all, then the function will output 'A' then 'B' without an exception occurring. This is why I titled the question the way that I did 'Can a Postgres Commit Exist in Procedure that has an Exception Block?'

Answer 1

The semantics of PL/pgSQL's error handling dictate that:

When an error is caught by an EXCEPTION clause ... all changes to persistent database state within the block are rolled back.

This is implemented using subtransactions, which are basically the same as savepoints. In other words, when you run the following PL/pgSQL code:

BEGIN
  PERFORM foo();
EXCEPTION WHEN others THEN
  PERFORM handle_error();
END

...what's actually happening is something like this:

BEGIN
  SAVEPOINT a;
  PERFORM foo();
  RELEASE SAVEPOINT a;
EXCEPTION WHEN others THEN
  ROLLBACK TO SAVEPOINT a;
  PERFORM handle_error();
END

A COMMIT within the block would break this completely; your changes would be made permanent, the savepoint would be discarded, and the exception handler would be left with no way to roll back. As a result, commits are not allowed in this context, and trying to execute a COMMIT will result in a "cannot commit while a subtransaction is active" error.

That's why you see your procedure jump to the exception handler instead of running the raise notice 'B': when it reaches the commit, it throws an error, and the handler catches it.

This is fairly straightforward to work around, though. BEGIN ... END blocks can be nested, and only blocks with EXCEPTION clauses involve setting savepoints, so you can just wrap the commands before and after the commit in their own exception handlers:

create or replace procedure x_transaction_try() language plpgsql
as $$
declare
  my_ex_state text;
  my_ex_message text;
  my_ex_detail text;
  my_ex_hint text;
  my_ex_ctx text;
begin
  begin
    raise notice 'A';
  exception when others then
    raise notice 'C';
    GET STACKED DIAGNOSTICS
      my_ex_state   = RETURNED_SQLSTATE,
      my_ex_message = MESSAGE_TEXT,
      my_ex_detail  = PG_EXCEPTION_DETAIL,
      my_ex_hint    = PG_EXCEPTION_HINT,
      my_ex_ctx     = PG_EXCEPTION_CONTEXT
    ;
    raise notice '% % % % %', my_ex_state, my_ex_message, my_ex_detail, my_ex_hint, my_ex_ctx;
  end;

  commit;

  begin
    raise notice 'B';
  exception when others then
    raise notice 'C';
    GET STACKED DIAGNOSTICS
      my_ex_state   = RETURNED_SQLSTATE,
      my_ex_message = MESSAGE_TEXT,
      my_ex_detail  = PG_EXCEPTION_DETAIL,
      my_ex_hint    = PG_EXCEPTION_HINT,
      my_ex_ctx     = PG_EXCEPTION_CONTEXT
    ;
    raise notice '% % % % %', my_ex_state, my_ex_message, my_ex_detail, my_ex_hint, my_ex_ctx;
  end;      
end;
$$;

Unfortunately, it does lead to a lot of duplication in the error handlers, but I can't think of a nice way to avoid it.

Answer 2

The problem is the EXCEPTION clause.

This is implemented in PL/pgSQL as a subtransaction (the same thing as a SAVEPOINT in SQL), which is rolled back when the exception block is reached.

You cannot COMMIT while a subtransaction is active.

See this comment in src/backend/executor/spi.c:

/*
 * This restriction is required by PLs implemented on top of SPI.  They
 * use subtransactions to establish exception blocks that are supposed to
 * be rolled back together if there is an error.  Terminating the
 * top-level transaction in such a block violates that idea.  A future PL
 * implementation might have different ideas about this, in which case
 * this restriction would have to be refined or the check possibly be
 * moved out of SPI into the PLs.
 */
if (IsSubTransaction())
    ereport(ERROR,
            (errcode(ERRCODE_INVALID_TRANSACTION_TERMINATION),
             errmsg("cannot commit while a subtransaction is active")));