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From: Dirkjan Bussink <[email protected]>
To: [email protected]
Subject: JSON_SERIALIZE for JSONB returns parts of the internal JSONB representation
Date: Tue, 3 Mar 2026 16:02:08 +0100
Message-ID: <[email protected]> (raw)

Hi,

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

postgres=# 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’t even be found for a JSONB argument). 

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

From 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 

uint32(JB_FARRAY | 3) == 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. 

-- 
Cheers,

Dirkjan

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