Enforce RETURNING typmod on SQL/JSON DEFAULT behavior expressions

transformJsonBehavior() coerced an ON EMPTY / ON ERROR DEFAULT
expression only when its type differed from the RETURNING type's OID.
When the base type matched but the RETURNING type carried a type
modifier (e.g. numeric(4,1) or varchar(3)), the coercion that enforces
the typmod was skipped, so the DEFAULT value could violate the
declared type:

    SELECT JSON_VALUE(jsonb '{}', '$.a'
                      RETURNING numeric(4,1) DEFAULT 99999.999 ON EMPTY);

returned 99999.999, which 99999.999::numeric(4,1) would reject; the
value could even be stored into a numeric(4,1) column, as later
coercions trust its already-correct type label.

Fix by also coercing when the RETURNING type has a typmod, except for
a NULL constant.  coerce_to_target_type() is a no-op when the typmod
already matches.  The matching-OID short-circuit dates to 74c96699be.

Reported-by: Ewan Young <kdbase.hack@gmail.com>
Author: Ewan Young <kdbase.hack@gmail.com>
Discussion: https://postgr.es/m/CAON2xHPO9f4cAmyGn1mQ=VqoS7wN5rz4yOiqudxX78zninZpCw@mail.gmail.com
Backpatch-through: 17

src/backend/parser/parse_expr.c

@@ -4931,8 +4931,17 @@ transformJsonBehavior(ParseState *pstate, JsonExpr *jsexpr,
 	 *
 	 * For other non-NULL expressions, try to find a cast and error out if one
 	 * is not found.
+	 *
+	 * The DEFAULT expression's base type may already match the RETURNING type
+	 * yet still need coercion: when the RETURNING type carries a type
+	 * modifier (e.g. numeric(4,1)), the cast below is what enforces it, so
+	 * skipping it here would let the DEFAULT yield a value that violates its
+	 * declared RETURNING type.  A NULL constant needs no such enforcement.
 	 */
-	if (expr && exprType(expr) != returning->typid)
+	if (expr &&
+		(exprType(expr) != returning->typid ||
+		 (returning->typmod >= 0 &&
+		  !(IsA(expr, Const) && ((Const *) expr)->constisnull))))
 	{
 		bool		isnull = (IsA(expr, Const) && ((Const *) expr)->constisnull);
 

src/test/regress/expected/sqljson_jsontable.out

@@ -250,6 +250,22 @@ SELECT * FROM JSON_TABLE(jsonb '{"d1": "foo"}', '$'
  {1}
 (1 row)
 
+-- A DEFAULT expression whose base type matches the column type must still be
+-- coerced to the column's typmod.
+SELECT * FROM JSON_TABLE(jsonb '{}', '$'
+    COLUMNS (c numeric(4,1) PATH '$.x' DEFAULT 99999.999 ON EMPTY));
+ERROR:  numeric field overflow
+DETAIL:  A field with precision 4, scale 1 must round to an absolute value less than 10^3.
+SELECT * FROM JSON_TABLE(jsonb '{}', '$'
+    COLUMNS (c bit(3) PATH '$.x' DEFAULT b'10101' ON EMPTY));
+ERROR:  bit string length 5 does not match type bit(3)
+SELECT * FROM JSON_TABLE(jsonb '{}', '$'
+    COLUMNS (c numeric(4,1) PATH '$.x' DEFAULT abs(NULL::numeric) ON EMPTY));
+ c 
+---
+  
+(1 row)
+
 -- JSON_TABLE: Test backward parsing
 CREATE VIEW jsonb_table_view2 AS
 SELECT * FROM

src/test/regress/expected/sqljson_queryfuncs.out

@@ -433,6 +433,27 @@ SELECT JSON_VALUE(jsonb '["1"]', '$[*]' RETURNING int FORMAT JSON); -- RETURNING
 ERROR:  cannot specify FORMAT JSON in RETURNING clause of JSON_VALUE()
 LINE 1: ...CT JSON_VALUE(jsonb '["1"]', '$[*]' RETURNING int FORMAT JSO...
                                                              ^
+-- A DEFAULT expression must be coerced to the RETURNING type's typmod even
+-- when its base type already matches, but a matching NULL needs no coercion.
+SELECT JSON_VALUE(jsonb '{}', '$.a' RETURNING numeric(4,1) DEFAULT 99999.999 ON EMPTY);
+ERROR:  numeric field overflow
+DETAIL:  A field with precision 4, scale 1 must round to an absolute value less than 10^3.
+SELECT JSON_VALUE(jsonb '{}', '$.a' RETURNING varchar(3) DEFAULT 'toolong'::varchar(10) ON EMPTY);
+ERROR:  value too long for type character varying(3)
+SELECT JSON_VALUE(jsonb '{}', '$.a' RETURNING numeric(4,1) DEFAULT NULL::numeric ON EMPTY);
+ json_value 
+------------
+           
+(1 row)
+
+SELECT JSON_VALUE(jsonb '{}', '$.a' RETURNING bit(3) DEFAULT b'10101' ON EMPTY);
+ERROR:  bit string length 5 does not match type bit(3)
+SELECT JSON_VALUE(jsonb '{}', '$.a' RETURNING numeric(4,1) DEFAULT abs(NULL::numeric) ON EMPTY);
+ json_value 
+------------
+           
+(1 row)
+
 -- RETUGNING pseudo-types not allowed
 SELECT JSON_VALUE(jsonb '["1"]', '$[*]' RETURNING record);
 ERROR:  returning pseudo-types is not supported in SQL/JSON functions

src/test/regress/sql/sqljson_jsontable.sql

@@ -132,6 +132,15 @@ SELECT * FROM JSON_TABLE(jsonb '{"d1": "foo"}', '$'
 SELECT * FROM JSON_TABLE(jsonb '{"d1": "foo"}', '$'
     COLUMNS (js1 oid[] PATH '$.d2' DEFAULT '{1}'::int[]::oid[] ON EMPTY));
 
+-- A DEFAULT expression whose base type matches the column type must still be
+-- coerced to the column's typmod.
+SELECT * FROM JSON_TABLE(jsonb '{}', '$'
+    COLUMNS (c numeric(4,1) PATH '$.x' DEFAULT 99999.999 ON EMPTY));
+SELECT * FROM JSON_TABLE(jsonb '{}', '$'
+    COLUMNS (c bit(3) PATH '$.x' DEFAULT b'10101' ON EMPTY));
+SELECT * FROM JSON_TABLE(jsonb '{}', '$'
+    COLUMNS (c numeric(4,1) PATH '$.x' DEFAULT abs(NULL::numeric) ON EMPTY));
+
 -- JSON_TABLE: Test backward parsing
 
 CREATE VIEW jsonb_table_view2 AS

src/test/regress/sql/sqljson_queryfuncs.sql

@@ -105,6 +105,14 @@ SELECT JSON_VALUE(jsonb '[" "]', '$[*]' RETURNING int DEFAULT 2 + 3 ON ERROR);
 SELECT JSON_VALUE(jsonb '["1"]', '$[*]' RETURNING int DEFAULT 2 + 3 ON ERROR);
 SELECT JSON_VALUE(jsonb '["1"]', '$[*]' RETURNING int FORMAT JSON); -- RETURNING FORMAT not allowed
 
+-- A DEFAULT expression must be coerced to the RETURNING type's typmod even
+-- when its base type already matches, but a matching NULL needs no coercion.
+SELECT JSON_VALUE(jsonb '{}', '$.a' RETURNING numeric(4,1) DEFAULT 99999.999 ON EMPTY);
+SELECT JSON_VALUE(jsonb '{}', '$.a' RETURNING varchar(3) DEFAULT 'toolong'::varchar(10) ON EMPTY);
+SELECT JSON_VALUE(jsonb '{}', '$.a' RETURNING numeric(4,1) DEFAULT NULL::numeric ON EMPTY);
+SELECT JSON_VALUE(jsonb '{}', '$.a' RETURNING bit(3) DEFAULT b'10101' ON EMPTY);
+SELECT JSON_VALUE(jsonb '{}', '$.a' RETURNING numeric(4,1) DEFAULT abs(NULL::numeric) ON EMPTY);
+
 -- RETUGNING pseudo-types not allowed
 SELECT JSON_VALUE(jsonb '["1"]', '$[*]' RETURNING record);