Horton

Re: Parallel execution and prepared statements

Albe Laurenz <laurenz.albe@wien.gv.at>

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From: Albe Laurenz <laurenz.albe@wien.gv.at>
To: "'Robert Haas *EXTERN*'" <robertmhaas@gmail.com>
Cc: "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>, "t.bussmann@gmx.net" <t.bussmann@gmx.net>
Date: 2016-11-18T13:21:27Z
Lists: pgsql-hackers

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  1. Enable parallelism for prepared statements and extended query protocol.

Robert Haas wrote:
> On Tue, Nov 15, 2016 at 10:41 AM, Albe Laurenz <laurenz.albe@wien.gv.at> wrote:
>> Tobias Bussmann has discovered an oddity with prepared statements.
>>
>> Parallel scan is used with prepared statements, but only if they have
>> been created with protocol V3 "Parse".
>> If a prepared statement has been prepared with the SQL statement PREPARE,
>> it will never use a parallel scan.
>>
>> I guess that is an oversight in commit 57a6a72b, right?
>> PrepareQuery in commands/prepare.c should call CompleteCachedPlan
>> with cursor options CURSOR_OPT_PARALLEL_OK, just like
>> exec_prepare_message in tcop/postgres.c does.
>>
>> The attached patch fixes the problem for me.
> 
> Actually, commit 57a6a72b made this change, and then 7bea19d0 backed
> it out again because it turned out to break things.

I didn't notice the CREATE TABLE ... AS EXECUTE problem.

But then the change to use CURSOR_OPT_PARALLEL_OK in tcop/postgres.c should
be reverted as well, because it breaks things just as bad:

   /* creates a parallel-enabled plan */
   PQprepare(conn, "pstmt", "SELECT id FROM l WHERE val = $1", 1, NULL);
   /* blows up with "cannot insert tuples during a parallel operation" */
   PQexec(conn, "CREATE TABLE bad(id) AS EXECUTE pstmt('abcd')");

With Tobias' patch, this does not fail.

I think we should either apply something like that patch or disable parallel
execution for prepared statements altogether and document that.

Yours,
Laurenz Albe