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Subject: BUG #19625: SQL/JSON boolean DEFAULT expression silently replaced
 with 'false'
To: pgsql-bugs@lists.postgresql.org
From: PG Bug reporting form <noreply@postgresql.org>
Cc: zengman@halodbtech.com
Reply-To: zengman@halodbtech.com, pgsql-bugs@lists.postgresql.org
Date: Mon, 17 Aug 2026 14:04:21 +0000
Message-ID: <19625-683b498c92087bc8@postgresql.org>
Auto-Submitted: auto-generated
Archived-At: 
 <https://www.postgresql.org/message-id/19625-683b498c92087bc8%40postgresql.org>
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The following bug has been logged on the website:

Bug reference:      19625
Logged by:          Man Zeng
Email address:      zengman@halodbtech.com
PostgreSQL version: 19beta3
Operating system:   24.04.1-Ubuntu
Description:       =20

Hi all,

A boolean DEFAULT expression in the SQL/JSON functions can be silently
replaced with 'false', discarding the user's expression at parse time.

For example:

```sql
postgres=3D# SELECT JSON_VALUE('{"a":"abc"}', 'strict $.b' DEFAULT true ON
ERROR);  -- expected true
 json_value
------------
 false
(1 row)

postgres=3D# SELECT JSON_VALUE('{"a":"abc"}', 'strict $.b' DEFAULT (1=3D1) =
ON
ERROR); -- expected true
 json_value
------------
 false
(1 row)

postgres=3D# SELECT JSON_QUERY('{"a":"abc"}', 'strict $.b' DEFAULT false ON
ERROR);
 json_query
------------
 false
(1 row)
```
The user asked for true, so the result is wrong.  A boolean
expression such as DEFAULT (1=3D1) is affected in the same way, and so
is JSON_QUERY().  Non-boolean DEFAULT expressions (e.g. DEFAULT 'ok')
return the correct value, and so do boolean ones when the RETURNING
type is boolean or integer.

The problem is in transformJsonBehavior() in parse_expr.c.  When the
DEFAULT expression is boolean-valued and the RETURNING type is neither
boolean nor integer (or a domain over integer), the code guesses the
jsonb value from the behavior type:

```c
char *val =3D btype =3D=3D JSON_BEHAVIOR_TRUE ? "true" : "false";
```

This only works for the canned TRUE/FALSE constants.  A user-supplied
DEFAULT clause has btype =3D=3D JSON_BEHAVIOR_DEFAULT, so the condition is
always false and the expression is replaced with jsonb 'false' without
being evaluated.

I think we can use to_jsonb() to handle all of these cases -- whether a
canned TRUE/FALSE constant or a user-supplied boolean DEFAULT
expression, the value is converted per the expression's actual
evaluation (there is no bool->jsonb cast, hence the explicit function
call).  Below is my change:

```c
diff --git a/src/backend/parser/parse_expr.c
b/src/backend/parser/parse_expr.c
index 30c889f505f..175b3d3ee69 100644
--- a/src/backend/parser/parse_expr.c
+++ b/src/backend/parser/parse_expr.c
@@ -4956,20 +4956,17 @@ transformJsonBehavior(ParseState *pstate, JsonExpr
*jsexpr,
                        coerce_at_runtime =3D true;
=20
                        /*
-                        * json_populate_type() expects to be passed a jsonb
value, so gin
-                        * up a Const containing the appropriate boolean
value represented
-                        * as jsonb, discarding the original Const
containing a plain
-                        * boolean.
+                        * json_populate_type() only takes a jsonb value, so
convert a
+                        * boolean to jsonb by calling to_jsonb() on it.
That way any
+                        * boolean-valued expression -- whether a canned
TRUE/FALSE
+                        * constant or a user-supplied DEFAULT expression --
is converted
+                        * according to the value it actually evaluates to.
                         */
                        if (exprType(expr) =3D=3D BOOLOID)
-                       {
-                               char       *val =3D btype =3D=3D
JSON_BEHAVIOR_TRUE ? "true" : "false";
-
-                               expr =3D (Node *) makeConst(JSONBOID, -1,
InvalidOid, -1,
-
DirectFunctionCall1(jsonb_in,
-
CStringGetDatum(val)),
-
false, false);
-                       }
+                               expr =3D (Node *) makeFuncExpr(F_TO_JSONB,
JSONBOID,
+
list_make1(expr),
+
InvalidOid, InvalidOid,
+
COERCE_EXPLICIT_CALL);
                }
                else
                {
```

Thoughts?

--
Regards,
Man Zeng