Re: postgres_fdw: Emit message when batch_size is reduced

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.

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
>