Re: Parallel query execution with SPI

On 31.03.2017 13:48, Robert Haas wrote:
> On Fri, Mar 31, 2017 at 3:33 AM, Konstantin Knizhnik
> <k.knizhnik@postgrespro.ru> wrote:
>> It is possible to execute query concurrently using SPI?
>> If so, how it can be enforced?
>> I tried to open cursor with CURSOR_OPT_PARALLEL_OK flag but it doesn't help:
>> query is executed by single backend while the same query been launched at
>> top level uses parallel plan:
>>
>>      fsstate->portal = SPI_cursor_open_with_args(NULL, fsstate->query,
>> fsstate->numParams, argtypes, values, nulls, true, CURSOR_OPT_PARALLEL_OK);
>>      ...
>>      SPI_cursor_fetch(fsstate->portal, true, 1);
> Parallel execution isn't possible if you are using a cursor-type
> interface, because a parallel query can't be suspended and resumed
> like a non-parallel query.  If you use a function that executes the
> query to completion in one go, like SPI_execute_plan, then it's cool.
> See also commit 61c2e1a95f94bb904953a6281ce17a18ac38ee6d.
>
Thank you very much for explanation.
In case of using SPI_execute the query is really executed concurrently.
But it means that when I am executing some query using SPI, I need to 
somehow predict number of returned tuples.
If it is not so much, then it is better to use SPI_execute to allow 
concurrent execution of the query.
But if it is large enough, then SPI_execute without limit can cause 
memory overflow.
Certainly I can specify some reasonable limit and it if is reached, then 
use cursor instead.
But it is neither convenient, neither efficient.

I wonder if somebody can suggest better solution?

-- 
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company