Re: postgres_fdw: Emit message when batch_size is reduced

On Fri, 17 Jul 2026 at 18:39, Nurlan Tulemisov <nurlan(dot)tulemisov(at)gmail(dot)com>
wrote:

> Hi Rafia,
>
> Wouldn't that give duplicate warnings with the changes already in the
>> patch...or you are suggesting to have it only in postgres_fdw_validator() ?
>
> Yes, after thinking about it some more, I think it would be better to emit
> the WARNING only from postgres_fdw_validator() when the option is SET
> (CREATE/ALTER), while still allowing values greater than
> PQ_QUERY_PARAM_MAX_LIMIT to be stored.
>
> This also seems consistent with Corey's earlier suggestion:
>
>> I'm saying that we may want a separate check when the batch_size option is
>> SET (via CREATE/ALTER TABLE/SERVER) to test the value against
>> PQ_QUERY_PARAM_MAX_LIMIT
>> and issue a NOTICE/WARNING that the chosen value will always have to be
>> set
>> at or below $PQ_QUERY_PARAM_MAX_LIMIT - but still let them store the
>> values
>> as-is.
>
>
> The WARNING currently emitted during INSERT seems potentially noisy, since
> it may be repeated every time the foreign table is used.
>
> Nit: since this table is used to verify that no warning is emitted:
>
>> +-- Verify that no WARNING is emitted when batch_size is within the
>> +-- libpq 65535-parameter limit.
>> +CREATE TABLE batch_warn_table ( x int );
>
> Would batch_no_warn_table be a clearer name? The current name may be
> slightly confusing in this context.
>
> Thank you for the review, I have made both the changes in the attached
version.

> On Thu, Jul 16, 2026 at 10:51 AM Rafia Sabih <rafia(dot)pghackers(at)gmail(dot)com>
> wrote:
>
>>
>>
>> On Sat, 11 Jul 2026 at 23:56, Nurlan Tulemisov <
>> nurlan(dot)tulemisov(at)gmail(dot)com> wrote:
>>
>>> Hi,
>>> I reviewed v5, applied it to the current master, and ran the
>>> postgres_fdw regression tests. The patch applies cleanly and the tests pass.
>>>
>> Thank you for the review.
>>
>>>
>>> Would it also make sense to emit a NOTICE or WARNING from
>>> postgres_fdw_validator() in option.c when batch_size is set to a value
>>> greater than PQ_QUERY_PARAM_MAX_LIMIT?
>>>
>>> Wouldn't that give duplicate warnings with the changes already in the
>> patch...or you are suggesting to have it only in postgres_fdw_validator() ?
>>
>>> That would notify the user immediately during CREATE or ALTER, while
>>> still accepting the value and preserving the existing behavior.
>>>
>>> The existing tests cover cases where the messages are emitted, but I
>>> think it may also be useful to cover the exact boundaries and the cases
>>> where no message should be emitted.
>>>
>>> For a two-parameter foreign insert, batch_size = 32767 should not
>>> produce a DEBUG1 message, while batch_size = 32768 should be reduced to
>>> 32767. Similarly, for a single parameter, batch_size = 65535 should not
>>> produce a warning, while 65536 should.
>>>
>> Done.
>> Please find the updated patch attached.
>>
>>> Regards,
>>> Nurlan
>>>
>>> сб, 11 июл. 2026 г. в 21:13, Rafia Sabih <rafia(dot)pghackers(at)gmail(dot)com>:
>>>
>>>>
>>>>
>>>> On Mon, 22 Jun 2026 at 22:40, Corey Huinker <corey(dot)huinker(at)gmail(dot)com>
>>>> wrote:
>>>>
>>>>> On Fri, Jun 19, 2026 at 8:38 AM Rafia Sabih <rafia(dot)pghackers(at)gmail(dot)com>
>>>>> wrote:
>>>>>
>>>>>>
>>>>>>
>>>>>> On Tue, 16 Jun 2026 at 22:20, Corey Huinker <corey(dot)huinker(at)gmail(dot)com>
>>>>>> wrote:
>>>>>>
>>>>>>> On Wed, Jun 10, 2026 at 5:09 AM Rafia Sabih <
>>>>>>> rafia(dot)pghackers(at)gmail(dot)com> wrote:
>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> On Tue, 9 Jun 2026 at 22:22, Corey Huinker <corey(dot)huinker(at)gmail(dot)com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>>
>>>>>>>>>> Thanks for your inputs. Reworked patch is attached.
>>>>>>>>>> --
>>>>>>>>>> Regards,
>>>>>>>>>> Rafia Sabih
>>>>>>>>>> CYBERTEC PostgreSQL International GmbH
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> You've addressed all my concerns, aside from the desire for the
>>>>>>>>> check on the set/update of the value. Do you have a commitfest entry? I
>>>>>>>>> didn't find one.
>>>>>>>>>
>>>>>>>> There is commitfest entry now -->
>>>>>>>> https://commitfest.postgresql.org/patch/6873/
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>> I've added myself as a reviewer. Did you want to try adding the
>>>>>>> check at time of the option being set? If not, I can make an attempt at
>>>>>>> that.
>>>>>>>
>>>>>> Please find the attached file for the patch with the warning message
>>>>>> at the time of batch_size option addition. Looking forward to your inputs.
>>>>>>
>>>>>> --
>>>>>> Regards,
>>>>>> Rafia Sabih
>>>>>> CYBERTEC PostgreSQL International GmbH
>>>>>>
>>>>>
>>>>> Applies clean, passes.
>>>>>
>>>>> I think we need to tweak the elog() below:
>>>>>
>>>>> + if (batch_size > PQ_QUERY_PARAM_MAX_LIMIT)
>>>>> + elog(WARNING, "postgres_fdw: batch_size %d is at or above the libpq "
>>>>> + "%d-parameter limit; the effective per-batch ceiling is "
>>>>> + "limit / number_of_columns and may be lower",
>>>>> + batch_size, PQ_QUERY_PARAM_MAX_LIMIT);
>>>>>
>>>>> I think this should be an ereport() because it's the sort of error
>>>>> we'd want the caller to see, and that means we need the message to conform
>>>>> the guidelines at
>>>>> https://www.postgresql.org/docs/current/error-style-guide.html, and
>>>>> I'm going to suggest this as a starting point:
>>>>>
>>>>> ereport(WARNING,
>>>>> errmsg("%s of %d exceeds protocol limit of %d", "batch_size",
>>>>> batch_size, PQ_QUERY_PARAM_MAX_LIMIT),
>>>>> errdetail("The %s for a query will be reduced to protocol
>>>>> limit divided by the number of columns in the query.", "batch_size"));
>>>>>
>>>>> Done.
>>>>
>>>>> I'd like to hear other people's opinions on what the proper conforming
>>>>> error message would be.
>>>>>
>>>>
>>>>
>>>> --
>>>> Regards,
>>>> Rafia Sabih
>>>> CYBERTEC PostgreSQL International GmbH
>>>>
>>>
>>>
>>> --
>>> Regards,
>>> Nurlan
>>>
>>
>>
>> --
>> Regards,
>> Rafia Sabih
>> CYBERTEC PostgreSQL International GmbH
>>
>

--
Regards,
Rafia Sabih
CYBERTEC PostgreSQL International GmbH