Re: Emitting JSON to file using COPY TO

On 2026-03-16 Mo 2:24 PM, Masahiko Sawada wrote:
> On Sun, Mar 8, 2026 at 8:49 PM jian he<jian.universality@gmail.com> 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.
> I've reviewed the patch and have some comments:
>
> ---
> I got a SEGV in the following scenario:
>
> postgres(1:1197708)=# create table test (a int, b text, c jsonb);
> CREATE TABLE
> postgres(1:1197708)=# copy test(a, b) to stdout with (format 'json' );
> TRAP: failed Assert("tupdesc->firstNonCachedOffsetAttr >= 0"), File:
> "execTuples.c", Line: 2328, PID: 1197708
> postgres: masahiko postgres [local] COPY(ExceptionalCondition+0x9e) [0xbebe48]
> postgres: masahiko postgres [local] COPY(BlessTupleDesc+0x2b) [0x729b50]
> postgres: masahiko postgres [local] COPY(BeginCopyTo+0xc94) [0x637bdf]
> postgres: masahiko postgres [local] COPY(DoCopy+0xb68) [0x62afbc]
> postgres: masahiko postgres [local]
> COPY(standard_ProcessUtility+0xa22) [0xa0ba48]
> postgres: masahiko postgres [local] COPY(ProcessUtility+0x10e) [0xa0b01f]
> postgres: masahiko postgres [local] COPY() [0xa09872]
> postgres: masahiko postgres [local] COPY() [0xa09acf]
> postgres: masahiko postgres [local] COPY(PortalRun+0x2c8) [0xa0901d]
> postgres: masahiko postgres [local] COPY() [0xa02055]
> postgres: masahiko postgres [local] COPY(PostgresMain+0xaf1) [0xa0724e]
> postgres: masahiko postgres [local] COPY() [0x9fdab9]
> postgres: masahiko postgres [local]
> COPY(postmaster_child_launch+0x165) [0x905378]
> postgres: masahiko postgres [local] COPY() [0x90b600]
> postgres: masahiko postgres [local] COPY() [0x908e6a]
> postgres: masahiko postgres [local] COPY(PostmasterMain+0x14fe) [0x90880c]
> postgres: masahiko postgres [local] COPY(main+0x340) [0x7a1f9c]
>
> It seems to forget to call TupleDescFinalize(). And I think we need
> some regression tests for this case.
>
> ---
> +       if (cstate->opts.format == COPY_FORMAT_JSON)
> +       {
> +               /*
> +                * If FORCE_ARRAY has been specified, send the opening bracket.
> +                */
> +               if (cstate->opts.force_array)
> +               {
> +                       CopySendChar(cstate, '[');
> +                       CopySendTextLikeEndOfRow(cstate);
> +               }
> +       }
>
> We can conjunct the two if statement conditions.
>

Here's a v30 set that I hope fixes these issues.


cheers


andrew

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