Re: Why must AUTOCOMMIT be ON to do txn control in plpgsql procedure?

Thanks, Adrian.

On 07-Aug-2019, at 13:19, Adrian Klaver <adrian.klaver@aklaver.com> wrote:

On 8/7/19 12:54 PM, Bryn Llewellyn wrote:
> Yes, I do believe that I understand this. But there’s no getting away from the fact that the AUTOCOMMIT mode, and
whatthis implies, is a server-side phenomenon—at least as several PostgreSQL experts have assured me. For example, when
youuse client-side Python with the psycopg2 driver, then once you’ve done “my_session = psycopg2.connect(connect_str)”,
youcan then do “my_session.set_session(autocommit=False)”. And then everything we’ve been saying in the psql context
nowapplies in that context—yes? 

The server responds to instructions from the client.

General rule:
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.
Transactioncontrol statements are only allowed if CALL is executed in its own transaction. 
"

> B.t.w., I’m guessing that the “begin” SQL command that you see in the log that I mentioned is actually issued by
(some)clients—at least psql and Python-on-psycopg2—as an explicit call from the client. In other words, it isn’t the
serverthat generates this. Does anyone know for sure how this works? 

psql:
https://doxygen.postgresql.org/bin_2psql_2common_8c.html
Starting ~ line 1355

if (transaction_status == PQTRANS_IDLE &&
        !pset.autocommit &&
        !command_no_begin(query))
    {
        results = PQexec(pset.db, "BEGIN");
 if (PQresultStatus(results) != PGRES_COMMAND_OK)

...

psycopg2:

https://github.com/psycopg/psycopg2/blob/master/psycopg/connection_int.c
~line 1294

> On 07-Aug-2019, at 11:56, Adrian Klaver <adrian.klaver@aklaver.com <mailto:adrian.klaver@aklaver.com>> wrote:
> On 8/7/19 11:46 AM, Bryn Llewellyn wrote:
>> Thanks for your response, Kyotaro. I’m happy, now, to accept the rule that “call proc_that_does_txn_control()” is
legalonly when AUTOCOMMIT is ON. Esp. when I’m told (on twitter, by 2ndQuadrant’s Peter Eisentraut, that this rule is
“animplementation restriction, for the most part.” See HERE
<https://twitter.com/petereisentraut/status/1158802910865756160>.
>> About your “In-procedure transaction control premises that no transaction is active before calling the procedure”…
yes.Nevertheless, as the code that Umair Sahid showed us in the blog post that I referenced in my email that started
thisthread, you can indeed start end end transactions from an executing proc (as long as the session’s AUTOCOMMIT mode
sON). 
> The key is that the AUTOCOMMIT status is just a specific case of the general rule. The general rule being that a
PROCEDUREcannot do transaction ending commands when it it called within an outer transaction. You can run into the same
issuein other situations e.g. ORM's that start a transaction behind the scenes. In other words this is not psql
specific. As long as you understand the general rule then things become clearer. 
> --
> Adrian Klaver
> adrian.klaver@aklaver.com <mailto:adrian.klaver@aklaver.com>


--
Adrian Klaver
adrian.klaver@aklaver.com