Re: Emitting JSON to file using COPY TO

On Thu, 7 Dec 2023 at 08:47, David G. Johnston <david(dot)g(dot)johnston(at)gmail(dot)com>
wrote:

> On Thursday, December 7, 2023, Daniel Verite <daniel(at)manitou-mail(dot)org>
> wrote:
>
>> Joe Conway wrote:
>>
>> > The attached should fix the CopyOut response to say one column. I.e. it
>> > ought to look something like:
>>
>> Spending more time with the doc I came to the opinion that in this bit
>> of the protocol, in CopyOutResponse (B)
>> ...
>> Int16
>> The number of columns in the data to be copied (denoted N below).
>> ...
>>
>> this number must be the number of columns in the source.
>> That is for COPY table(a,b,c) the number is 3, independently
>> on whether the result is formatted in text, cvs, json or binary.
>>
>> I think that changing it for json can reasonably be interpreted
>> as a protocol break and we should not do it.
>>
>> The fact that this value does not help parsing the CopyData
>> messages that come next is not a new issue. A reader that
>> doesn't know the field separator and whether it's text or csv
>> cannot parse these messages into fields anyway.
>> But just knowing how much columns there are in the original
>> data might be useful by itself and we don't want to break that.
>
>
> This argument for leaving 3 as the column count makes sense to me. I
> agree this content is not meant to facilitate interpreting the contents at
> a protocol level.
>

I'd disagree. From my POV if the data comes back as a JSON Array this is
one object and this should be reflected in the column count.

>
>
>>
>>
>> The other question for me is, in the CopyData message, this
>> bit:
>> " Messages sent from the backend will always correspond to single data
>> rows"
>>
>> ISTM that considering that the "[" starting the json array is a
>> "data row" is a stretch.
>> That might be interpreted as a protocol break, depending
>> on how strict the interpretation is.
>>
>
Well technically it is a single row if you send an array.

Regardless, I expect Euler's comment above that JSON lines format is going
to be the preferred format as the client doesn't have to wait for the
entire object before starting to parse.

Dave

>