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From: Masahiko Sawada <sawada.mshk@gmail.com>
Date: Mon, 16 Mar 2026 11:24:58 -0700
Message-ID: <CAD21AoASG2uXJXN=23jkSgKWJNVz2TPazQmrOYA7ZvO3GxjVhw@mail.gmail.com>
Subject: Re: Emitting JSON to file using COPY TO
To: jian he <jian.universality@gmail.com>
Cc: Andrew Dunstan <andrew@dunslane.net>, Joe Conway <mail@joeconway.com>, 
	Junwang Zhao <zhjwpku@gmail.com>, Florents Tselai <florents.tselai@gmail.com>, 
	"Andrey M. Borodin" <x4mmm@yandex-team.ru>, Dean Rasheed <dean.a.rasheed@gmail.com>, 
	Daniel Verite <daniel@manitou-mail.org>, Davin Shearer <davin@apache.org>, 
	PostgreSQL-development <pgsql-hackers@postgresql.org>
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On Sun, Mar 8, 2026 at 8:49=E2=80=AFPM jian he <jian.universality@gmail.com=
> wrote:
>
> On Mon, Mar 9, 2026 at 3:44=E2=80=AFAM Andrew Dunstan <andrew@dunslane.ne=
t> 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 us=
es
> > -        * cstate->tupDesc so that the datum is stamped with the right =
type;
> > -        * for queries output type is RECORDOID this must be the blesse=
d
> > -        * descriptor so that composite_to_json can look it up via
> > -        * lookup_rowtype_tupdesc.
> > +        * Full table or query without column list.  For queries, the s=
lot's
> > +        * TupleDesc may carry RECORDOID, which is not registered in th=
e
> > type
> > +        * cache and would cause composite_to_json's lookup_rowtype_tup=
desc
> > +        * 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 =3D=3D
> > RECORDOID)
> > -           slot->tts_tupleDescriptor =3D cstate->queryDesc->tupDesc;
> > +       {
> > +           HeapTuple   tup;
> >
> > -       rowdata =3D ExecFetchSlotHeapTupleDatum(slot);
> > +           tup =3D heap_form_tuple(cstate->tupDesc,
> > +                                 slot->tts_values,
> > +                                 slot->tts_isnull);
> > +           rowdata =3D HeapTupleGetDatum(tup);
> > +       }
> > +       else
> > +       {
> > +           rowdata =3D ExecFetchSlotHeapTupleDatum(slot);
> > +       }
> >
> This is better. I've tried to get rid of json_projvalues and json_projnul=
ls.
> 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 co=
lumn
> +                                 * list this is a projected descriptor *=
/
> +    Datum       *json_projvalues;    /* pre-allocated projection values,=
 or
> +                                     * NULL */
> +    bool       *json_projnulls; /* pre-allocated projection nulls, or NU=
LL */
>
> 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)=3D# create table test (a int, b text, c jsonb);
CREATE TABLE
postgres(1:1197708)=3D# copy test(a, b) to stdout with (format 'json' );
TRAP: failed Assert("tupdesc->firstNonCachedOffsetAttr >=3D 0"), File:
"execTuples.c", Line: 2328, PID: 1197708
postgres: masahiko postgres [local] COPY(ExceptionalCondition+0x9e) [0xbebe=
48]
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 =3D=3D COPY_FORMAT_JSON)
+       {
+               /*
+                * If FORCE_ARRAY has been specified, send the opening brac=
ket.
+                */
+               if (cstate->opts.force_array)
+               {
+                       CopySendChar(cstate, '[');
+                       CopySendTextLikeEndOfRow(cstate);
+               }
+       }

We can conjunct the two if statement conditions.

Regards,

--=20
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com