Обсуждение: procedures and transactions

Hi,

I’m trying to understand how procedures work with transactions. I tried the code below - it’s a simple procedure to
printsome notices and commit a transaction. If I call it from psql after a `begin`, then it gives an error. What does
thaterror mean? Are procedures not allowed to commit/rollback if they are called within in an outer transaction? 

Also, I tried putting a `start transaction` command in the procedure. I got another error: `unsupported transaction
commandin PL/pgSQL`. Are procedures not allowed to start transactions? Or is there another command? 

thanks,
Rob

create or replace procedure t_test(n integer)
as $$
begin
    raise notice 'current isolation level: %', (select current_setting('transaction_isolation'));
    raise notice 'current txid: %', (select txid_current());
    raise notice '---';
    commit;
    raise notice 'current isolation level: %', (select current_setting('transaction_isolation'));
    raise notice 'current txid: %', (select txid_current());
end;
$$ language plpgsql;

psql> begin;
psql> call t_test(1);

NOTICE:  current isolation level: read committed
NOTICE:  current txid: 111490
NOTICE:  ---
ERROR:  invalid transaction termination

On Tuesday, February 19, 2019, Rob Nikander <rob.nikander@gmail.com> wrote:

 Are procedures not allowed to commit/rollback if they are called within in an outer transaction?

Also, I tried putting a `start transaction` command in the procedure. I got another error: `unsupported transaction command in PL/pgSQL`. Are procedures not allowed to start transactions? Or is there another command?

On 2/19/19 12:31 PM, Rob Nikander wrote:
> Hi,
> 
> I’m trying to understand how procedures work with transactions. I tried the code below - it’s a simple procedure to
printsome notices and commit a transaction. If I call it from psql after a `begin`, then it gives an error. What does
thaterror mean? Are procedures not allowed to commit/rollback if they are called within in an outer transaction?
 
> 
> Also, I tried putting a `start transaction` command in the procedure. I got another error: `unsupported transaction
commandin PL/pgSQL`. Are procedures not allowed to start transactions? Or is there another command?
 
> 
> thanks,
> Rob
> 
> create or replace procedure t_test(n integer)
> as $$
> begin
>      raise notice 'current isolation level: %', (select current_setting('transaction_isolation'));
>      raise notice 'current txid: %', (select txid_current());
>      raise notice '---';
>      commit;
>      raise notice 'current isolation level: %', (select current_setting('transaction_isolation'));
>      raise notice 'current txid: %', (select txid_current());
> end;
> $$ language plpgsql;
> 
> psql> begin;
> psql> call t_test(1);

Don't use the begin;

call t_test(1);
NOTICE:  current isolation level: read committed
NOTICE:  current txid: 592
NOTICE:  ---
NOTICE:  current isolation level: read committed
NOTICE:  current txid: 593
CALL


A function already starts in a transaction.

> 
> NOTICE:  current isolation level: read committed
> NOTICE:  current txid: 111490
> NOTICE:  ---
> ERROR:  invalid transaction termination
> 
> 
> 
> 
> 


-- 
Adrian Klaver
adrian.klaver@aklaver.com

I thought if I had some application logic that needed a certain kind of transaction (maybe a non-default isolation level), I could hide that fact in a procedure. App code (Java/Python/whatever) could remain unaware of transactions (except maybe needing to retry after a failure) and simply send `call foo(?, ?)` to the DB. But maybe that kind of design is not supported, and application code needs to start transactions and set isolation levels. Is that accurate?  I supposed a procedure could throw an exception if it doesn’t like the value in `current_setting('transaction_isolation’)`.

On 2/19/19 1:48 PM, Rob Nikander wrote:
> I thought if I had some application logic that needed a certain kind of 
> transaction (maybe a non-default isolation level), I could hide that 
> fact in a procedure. App code (Java/Python/whatever) could remain 
> unaware of transactions (except maybe needing to retry after a failure) 
> and simply send `call foo(?, ?)` to the DB. But maybe that kind of 
> design is not supported, and application code needs to start 
> transactions and set isolation levels. Is that accurate?  I supposed a 
> procedure could throw an exception if it doesn’t like the value in 
> `current_setting('transaction_isolation’)`.

Per the docs in the link David posted:

https://www.postgresql.org/docs/11/sql-call.html

"If CALL is executed in a transaction block, then the called procedure 
cannot execute transaction control statements. Transaction control 
statements are only allowed if CALL is executed in its own transaction."

So:

psql> begin;
psql> call t_test(1);


will not work.

> 
> Rob
> 
>> On Feb 19, 2019, at 2:38 PM, David G. Johnston 
>> <david.g.johnston@gmail.com <mailto:david.g.johnston@gmail.com>> wrote:
>>
>> On Tuesday, February 19, 2019, Rob Nikander <rob.nikander@gmail.com 
>> <mailto:rob.nikander@gmail.com>> wrote:
>>
>>      Are procedures not allowed to commit/rollback if they are called
>>     within in an outer transaction?
>>
>>
>> https://www.postgresql.org/docs/11/sql-call.html
>>
>>     Also, I tried putting a `start transaction` command in the
>>     procedure. I got another error: `unsupported transaction command
>>     in PL/pgSQL`. Are procedures not allowed to start transactions? Or
>>     is there another command?
>>
>>
>> https://www.postgresql.org/docs/11/plpgsql-transactions.html
>>
>> David J.
>>
> 


-- 
Adrian Klaver
adrian.klaver@aklaver.com

On 2019-02-19 22:48, Rob Nikander wrote:
> I thought if I had some application logic that needed a certain kind of
> transaction (maybe a non-default isolation level), I could hide that
> fact in a procedure. App code (Java/Python/whatever) could remain
> unaware of transactions (except maybe needing to retry after a failure)
> and simply send `call foo(?, ?)` to the DB.

You can run SET TRANSACTION ISOLATION LEVEL in a procedure.

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

> On Feb 20, 2019, at 10:07 AM, Peter Eisentraut <peter.eisentraut@2ndquadrant.com> wrote:
>
> You can run SET TRANSACTION ISOLATION LEVEL in a procedure.

I tried that before but I get this error:

    create or replace procedure t_test(n integer)
    as $$
    begin
        set transaction isolation level serializable;
        raise notice 'current isolation level: %', (select current_setting('transaction_isolation'));
        raise notice 'current txid: %', (select txid_current());
    end;
    $$ language plpgsql;

mydb=# call t_test(1);
ERROR:  SET TRANSACTION ISOLATION LEVEL must be called before any query
CONTEXT:  SQL statement "SET transaction isolation level serializable"

On 2019-02-20 17:45, Rob Nikander wrote:
>> On Feb 20, 2019, at 10:07 AM, Peter Eisentraut <peter.eisentraut@2ndquadrant.com> wrote:
>>
>> You can run SET TRANSACTION ISOLATION LEVEL in a procedure.
> 
> I tried that before but I get this error:
> 
>     create or replace procedure t_test(n integer)
>     as $$
>     begin

You need to commit or rollback the preceding transaction here.  Yeah I
know it's a bit weird.

>         set transaction isolation level serializable;
>         raise notice 'current isolation level: %', (select current_setting('transaction_isolation'));
>         raise notice 'current txid: %', (select txid_current());
>     end;
>     $$ language plpgsql;
> 
> mydb=# call t_test(1);
> ERROR:  SET TRANSACTION ISOLATION LEVEL must be called before any query
> CONTEXT:  SQL statement "SET transaction isolation level serializable"

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services