From: Dirkjan Bussink <d.bussink@gmail.com>
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Subject: JSON_SERIALIZE for JSONB returns parts of the internal JSONB
 representation
Message-Id: <03583FE7-A5EE-407C-907D-67865F3F8BFC@gmail.com>
Date: Tue, 3 Mar 2026 16:02:08 +0100
To: pgsql-bugs@lists.postgresql.org
Archived-At: <https://www.postgresql.org/message-id/03583FE7-A5EE-407C-907D-67865F3F8BFC%40gmail.com>
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Hi,

When running a query like this (tested against PostgreSQL 18.3), it =
shows internals from the JSONB storage:

postgres=3D# select JSON_SERIALIZE('[1,2,4]'::jsonb);
json_serialize
----------------
\x03
(1 row)

I think this qualifies as a bug? I would expect JSON_SERIALIZE to either =
work with JSONB and here output the serialized text, so '[1,2,4]', or to =
error that JSONB is not supported for this function (or that this =
function can=E2=80=99t even be found for a JSONB argument).=20

What happens here is that json_out treats the internal bytes from the =
JSONB representation as a C string.=20

=46rom src/include/utils/jsonb.h:

* ... For that purpose, both an array and an object begin with a uint32
* header field, which contains an JB_FOBJECT or JB_FARRAY flag.

This is part of the header:

typedef struct JsonbContainer
{
uint32 header; /* number of elements or key/value pairs, and
* flags */
JEntry children[FLEXIBLE_ARRAY_MEMBER];

/* the data for each child node follows. */
} JsonbContainer;

The rest of the header contains the number of elements, in this case a =
JSON array of 3 elements. So the total header looks like=20

uint32(JB_FARRAY | 3) =3D=3D 0x40000003

So this also explains the \x03 output above. What is seen there is the =
first byte of the header there as little endian, since that is =
interpreted as a C string with the a 0 byte that happens to be after =
there.=20

--=20
Cheers,

Dirkjan=