Re: Emitting JSON to file using COPY TO

On 2026-03-08 Su 11:48 PM, jian he wrote:
> On Mon, Mar 9, 2026 at 3:44 AM Andrew Dunstan<andrew@dunslane.net> wrote:
>> Hmm. But should we be scribbling on slot->tts_tupleDescriptor like that?
>> How about something like this?:
>>
>> -        * Full table or query without column list.  Ensure the slot uses
>> -        * cstate->tupDesc so that the datum is stamped with the right type;
>> -        * for queries output type is RECORDOID this must be the blessed
>> -        * descriptor so that composite_to_json can look it up via
>> -        * lookup_rowtype_tupdesc.
>> +        * Full table or query without column list.  For queries, the slot's
>> +        * TupleDesc may carry RECORDOID, which is not registered in the
>> type
>> +        * cache and would cause composite_to_json's lookup_rowtype_tupdesc
>> +        * call to fail.  Build a HeapTuple stamped with the blessed
>> +        * descriptor so the type can be looked up correctly.
>>            */
>>           if (!cstate->rel && slot->tts_tupleDescriptor->tdtypeid ==
>> RECORDOID)
>> -           slot->tts_tupleDescriptor = cstate->queryDesc->tupDesc;
>> +       {
>> +           HeapTuple   tup;
>>
>> -       rowdata = ExecFetchSlotHeapTupleDatum(slot);
>> +           tup = heap_form_tuple(cstate->tupDesc,
>> +                                 slot->tts_values,
>> +                                 slot->tts_isnull);
>> +           rowdata = HeapTupleGetDatum(tup);
>> +       }
>> +       else
>> +       {
>> +           rowdata = ExecFetchSlotHeapTupleDatum(slot);
>> +       }
>>
> This is better. I've tried to get rid of json_projvalues and json_projnulls.
> Just using heap_form_tuple, but it won't work.
>
> I incorporated the v28-0004 COPY column list into v9-0002.
> With this patch set, we added four fields to the struct CopyToStateData.
>
> +    StringInfo    json_buf;        /* reusable buffer for JSON output,
> +                                 * initialized in BeginCopyTo */
> +    TupleDesc    tupDesc;        /* Descriptor for JSON output; for a column
> +                                 * list this is a projected descriptor */
> +    Datum       *json_projvalues;    /* pre-allocated projection values, or
> +                                     * NULL */
> +    bool       *json_projnulls; /* pre-allocated projection nulls, or NULL */
>
> Using the script in
> https://www.postgresql.org/message-id/CACJufxFFZqxC3p4WjpTEi4riaJm%3DpADX%2Bpy0yQ0%3DRWTn5cqK3Q%40mail.gmail.com
> I tested it again on macOS and Linux, and there are no regressions for
> COPY TO with the TEXT and CSV formats.
>

OK, I think we're really close now. Here is a tiny fixup patch that 
fixes an error message and a comment, and adds a missing test case.


cheers


andrew

--
Andrew Dunstan
EDB:https://www.enterprisedb.com