SQL JSON functions

This Patch introduces three SQL standard JSON functions:

JSON() (incorrectly mentioned in my commit message for f4fb45d15c)
JSON_SCALAR()
JSON_SERIALIZE()

JSON() produces json values from text, bytea, json or jsonb values, and
has facilitites for handling duplicate keys.
JSON_SCALAR() produces a json value from any scalar sql value, including
json and jsonb.
JSON_SERIALIZE() produces text or bytea from input which containis or
represents json or jsonb;

For the most part these functions don't add any significant new
capabilities, but they will be of use to users wanting standard
compliant JSON handling.

Nikita Glukhov

Reviewers have included (in no particular order) Andres Freund, Alexander
Korotkov, Pavel Stehule, Andrew Alsup, Erik Rijkers, Zihong Yu,
Himanshu Upadhyaya, Daniel Gustafsson, Justin Pryzby.

Discussion: https://postgr.es/m/cd0bb935-0158-78a7-08b5-904886deac4b@postgrespro.ru

doc/src/sgml/keywords/sql2016-02-reserved.txt

@@ -156,12 +156,15 @@ INTERVAL
 INTO
 IS
 JOIN
+JSON
 JSON_ARRAY
 JSON_ARRAYAGG
 JSON_EXISTS
 JSON_OBJECT
 JSON_OBJECTAGG
 JSON_QUERY
+JSON_SCALAR
+JSON_SERIALIZE
 JSON_TABLE
 JSON_TABLE_PRIMITIVE
 JSON_VALUE

src/backend/executor/execExpr.c

@@ -47,6 +47,8 @@
 #include "utils/array.h"
 #include "utils/builtins.h"
 #include "utils/datum.h"
+#include "utils/json.h"
+#include "utils/jsonb.h"
 #include "utils/jsonpath.h"
 #include "utils/lsyscache.h"
 #include "utils/typcache.h"
@@ -2460,6 +2462,12 @@ ExecInitExprRec(Expr *node, ExprState *state,
 				{
 					ExecInitExprRec(ctor->func, state, resv, resnull);
 				}
+				else if ((ctor->type == JSCTOR_JSON_PARSE && !ctor->unique) ||
+						 ctor->type == JSCTOR_JSON_SERIALIZE)
+				{
+					/* Use the value of the first argument as a result */
+					ExecInitExprRec(linitial(args), state, resv, resnull);
+				}
 				else
 				{
 					scratch.opcode = EEOP_JSON_CONSTRUCTOR;
@@ -2492,6 +2500,43 @@ ExecInitExprRec(Expr *node, ExprState *state,
 						argno++;
 					}
 
+					/* prepare type cache for datum_to_json[b]() */
+					if (ctor->type == JSCTOR_JSON_SCALAR)
+					{
+						bool		is_jsonb =
+							ctor->returning->format->format_type == JS_FORMAT_JSONB;
+
+						scratch.d.json_constructor.arg_type_cache =
+							palloc(sizeof(*scratch.d.json_constructor.arg_type_cache) * nargs);
+
+						for (int i = 0; i < nargs; i++)
+						{
+							int			category;
+							Oid			outfuncid;
+							Oid			typid = scratch.d.json_constructor.arg_types[i];
+
+							if (is_jsonb)
+							{
+								JsonbTypeCategory jbcat;
+
+								jsonb_categorize_type(typid, &jbcat, &outfuncid);
+
+								category = (int) jbcat;
+							}
+							else
+							{
+								JsonTypeCategory jscat;
+
+								json_categorize_type(typid, &jscat, &outfuncid);
+
+								category = (int) jscat;
+							}
+
+							scratch.d.json_constructor.arg_type_cache[i].outfuncid = outfuncid;
+							scratch.d.json_constructor.arg_type_cache[i].category = category;
+						}
+					}
+
 					ExprEvalPushStep(state, &scratch);
 				}
 

src/backend/executor/execExprInterp.c

@@ -3982,7 +3982,7 @@ ExecEvalJsonIsPredicate(ExprState *state, ExprEvalStep *op)
 		 * JSON text validation.
 		 */
 		if (res && (pred->unique_keys || exprtype == TEXTOID))
-			res = json_validate(json, pred->unique_keys);
+			res = json_validate(json, pred->unique_keys, false);
 	}
 	else if (exprtype == JSONBOID)
 	{
@@ -4527,6 +4527,46 @@ ExecEvalJsonConstructor(ExprState *state, ExprEvalStep *op,
 										 op->d.json_constructor.arg_types,
 										 op->d.json_constructor.constructor->absent_on_null,
 										 op->d.json_constructor.constructor->unique);
+	else if (ctor->type == JSCTOR_JSON_SCALAR)
+	{
+		if (op->d.json_constructor.arg_nulls[0])
+		{
+			res = (Datum) 0;
+			isnull = true;
+		}
+		else
+		{
+			Datum		value = op->d.json_constructor.arg_values[0];
+			int			category = op->d.json_constructor.arg_type_cache[0].category;
+			Oid			outfuncid = op->d.json_constructor.arg_type_cache[0].outfuncid;
+
+			if (is_jsonb)
+				res = to_jsonb_worker(value, category, outfuncid);
+			else
+				res = to_json_worker(value, category, outfuncid);
+		}
+	}
+	else if (ctor->type == JSCTOR_JSON_PARSE)
+	{
+		if (op->d.json_constructor.arg_nulls[0])
+		{
+			res = (Datum) 0;
+			isnull = true;
+		}
+		else
+		{
+			Datum		value = op->d.json_constructor.arg_values[0];
+			text	   *js = DatumGetTextP(value);
+
+			if (is_jsonb)
+				res = jsonb_from_text(js, true);
+			else
+			{
+				(void) json_validate(js, true, true);
+				res = value;
+			}
+		}
+	}
 	else
 	{
 		res = (Datum) 0;

src/backend/nodes/copyfuncs.c

@@ -2345,6 +2345,50 @@ _copyJsonValueExpr(const JsonValueExpr *from)
 	return newnode;
 }
 
+/*
+ * _copyJsonParseExpr
+ */
+static JsonParseExpr *
+_copyJsonParseExpr(const JsonParseExpr *from)
+{
+	JsonParseExpr  *newnode = makeNode(JsonParseExpr);
+
+	COPY_NODE_FIELD(expr);
+	COPY_SCALAR_FIELD(unique_keys);
+	COPY_LOCATION_FIELD(location);
+
+	return newnode;
+}
+
+/*
+ * _copyJsonScalarExpr
+ */
+static JsonScalarExpr *
+_copyJsonScalarExpr(const JsonScalarExpr *from)
+{
+	JsonScalarExpr *newnode = makeNode(JsonScalarExpr);
+
+	COPY_NODE_FIELD(expr);
+	COPY_LOCATION_FIELD(location);
+
+	return newnode;
+}
+
+/*
+ * _copyJsonSerializeExpr
+ */
+static JsonSerializeExpr *
+_copyJsonSerializeExpr(const JsonSerializeExpr *from)
+{
+	JsonSerializeExpr *newnode = makeNode(JsonSerializeExpr);
+
+	COPY_NODE_FIELD(expr);
+	COPY_NODE_FIELD(output);
+	COPY_LOCATION_FIELD(location);
+
+	return newnode;
+}
+
 /*
  * _copyJsonConstructorExpr
  */
@@ -5744,6 +5788,15 @@ copyObjectImpl(const void *from)
 		case T_JsonValueExpr:
 			retval = _copyJsonValueExpr(from);
 			break;
+		case T_JsonParseExpr:
+			retval = _copyJsonParseExpr(from);
+			break;
+		case T_JsonScalarExpr:
+			retval = _copyJsonScalarExpr(from);
+			break;
+		case T_JsonSerializeExpr:
+			retval = _copyJsonSerializeExpr(from);
+			break;
 		case T_JsonKeyValue:
 			retval = _copyJsonKeyValue(from);
 			break;

src/backend/nodes/equalfuncs.c

@@ -871,6 +871,35 @@ _equalJsonValueExpr(const JsonValueExpr *a, const JsonValueExpr *b)
 	return true;
 }
 
+static bool
+_equalJsonParseExpr(const JsonParseExpr *a, const JsonParseExpr *b)
+{
+	COMPARE_NODE_FIELD(expr);
+	COMPARE_SCALAR_FIELD(unique_keys);
+	COMPARE_LOCATION_FIELD(location);
+
+	return true;
+}
+
+static bool
+_equalJsonScalarExpr(const JsonScalarExpr *a, const JsonScalarExpr *b)
+{
+	COMPARE_NODE_FIELD(expr);
+	COMPARE_LOCATION_FIELD(location);
+
+	return true;
+}
+
+static bool
+_equalJsonSerializeExpr(const JsonSerializeExpr *a, const JsonSerializeExpr *b)
+{
+	COMPARE_NODE_FIELD(expr);
+	COMPARE_NODE_FIELD(output);
+	COMPARE_LOCATION_FIELD(location);
+
+	return true;
+}
+
 static bool
 _equalJsonConstructorExpr(const JsonConstructorExpr *a, const JsonConstructorExpr *b)
 {
@@ -3661,6 +3690,15 @@ equal(const void *a, const void *b)
 		case T_JsonValueExpr:
 			retval = _equalJsonValueExpr(a, b);
 			break;
+		case T_JsonParseExpr:
+			retval = _equalJsonParseExpr(a, b);
+			break;
+		case T_JsonScalarExpr:
+			retval = _equalJsonScalarExpr(a, b);
+			break;
+		case T_JsonSerializeExpr:
+			retval = _equalJsonSerializeExpr(a, b);
+			break;
 		case T_JsonConstructorExpr:
 			retval = _equalJsonConstructorExpr(a, b);
 			break;

src/backend/nodes/nodeFuncs.c

@@ -4363,6 +4363,20 @@ raw_expression_tree_walker(Node *node,
 					return true;
 			}
 			break;
+		case T_JsonParseExpr:
+			return walker(((JsonParseExpr *) node)->expr, context);
+		case T_JsonScalarExpr:
+			return walker(((JsonScalarExpr *) node)->expr, context);
+		case T_JsonSerializeExpr:
+			{
+				JsonSerializeExpr *jse = (JsonSerializeExpr *) node;
+
+				if (walker(jse->expr, context))
+					return true;
+				if (walker(jse->output, context))
+					return true;
+			}
+			break;
 		case T_JsonConstructorExpr:
 			{
 				JsonConstructorExpr *ctor = (JsonConstructorExpr *) node;

src/backend/parser/gram.y

@@ -568,7 +568,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 %type <list>	copy_options
 
 %type <typnam>	Typename SimpleTypename ConstTypename
-				GenericType Numeric opt_float
+				GenericType Numeric opt_float JsonType
 				Character ConstCharacter
 				CharacterWithLength CharacterWithoutLength
 				ConstDatetime ConstInterval
@@ -656,6 +656,9 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 					json_value_func_expr
 					json_query_expr
 					json_exists_predicate
+					json_parse_expr
+					json_scalar_expr
+					json_serialize_expr
 					json_api_common_syntax
 					json_context_item
 					json_argument
@@ -776,7 +779,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 	INTERSECT INTERVAL INTO INVOKER IS ISNULL ISOLATION
 
 	JOIN JSON JSON_ARRAY JSON_ARRAYAGG JSON_EXISTS JSON_OBJECT JSON_OBJECTAGG
-	JSON_QUERY JSON_VALUE
+	JSON_QUERY JSON_SCALAR JSON_SERIALIZE JSON_VALUE
 
 	KEY KEYS KEEP
 
@@ -13345,6 +13348,7 @@ SimpleTypename:
 					$$->typmods = list_make2(makeIntConst(INTERVAL_FULL_RANGE, -1),
 											 makeIntConst($3, @3));
 				}
+			| JsonType								{ $$ = $1; }
 		;
 
 /* We have a separate ConstTypename to allow defaulting fixed-length
@@ -13363,6 +13367,7 @@ ConstTypename:
 			| ConstBit								{ $$ = $1; }
 			| ConstCharacter						{ $$ = $1; }
 			| ConstDatetime							{ $$ = $1; }
+			| JsonType								{ $$ = $1; }
 		;
 
 /*
@@ -13731,6 +13736,13 @@ interval_second:
 				}
 		;
 
+JsonType:
+			JSON
+				{
+					$$ = SystemTypeName("json");
+					$$->location = @1;
+				}
+		;
 
 /*****************************************************************************
  *
@@ -15596,8 +15608,42 @@ json_func_expr:
 			| json_value_func_expr
 			| json_query_expr
 			| json_exists_predicate
+			| json_parse_expr
+			| json_scalar_expr
+			| json_serialize_expr
+		;
+
+json_parse_expr:
+			JSON '(' json_value_expr json_key_uniqueness_constraint_opt ')'
+				{
+					JsonParseExpr *n = makeNode(JsonParseExpr);
+					n->expr = (JsonValueExpr *) $3;
+					n->unique_keys = $4;
+					n->location = @1;
+					$$ = (Node *) n;
+				}
+		;
+
+json_scalar_expr:
+			JSON_SCALAR '(' a_expr ')'
+				{
+					JsonScalarExpr *n = makeNode(JsonScalarExpr);
+					n->expr = (Expr *) $3;
+					n->location = @1;
+					$$ = (Node *) n;
+				}
 		;
 
+json_serialize_expr:
+			JSON_SERIALIZE '(' json_value_expr json_output_clause_opt ')'
+				{
+					JsonSerializeExpr *n = makeNode(JsonSerializeExpr);
+					n->expr = (JsonValueExpr *) $3;
+					n->output = (JsonOutput *) $4;
+					n->location = @1;
+					$$ = (Node *) n;
+				}
+		;
 
 json_value_func_expr:
 			JSON_VALUE '('
@@ -16654,7 +16700,6 @@ unreserved_keyword:
 			| INSTEAD
 			| INVOKER
 			| ISOLATION
-			| JSON
 			| KEEP
 			| KEY
 			| KEYS
@@ -16872,12 +16917,15 @@ col_name_keyword:
 			| INT_P
 			| INTEGER
 			| INTERVAL
+			| JSON
 			| JSON_ARRAY
 			| JSON_ARRAYAGG
 			| JSON_EXISTS
 			| JSON_OBJECT
 			| JSON_OBJECTAGG
 			| JSON_QUERY
+			| JSON_SCALAR
+			| JSON_SERIALIZE
 			| JSON_VALUE
 			| LEAST
 			| NATIONAL
@@ -17243,6 +17291,8 @@ bare_label_keyword:
 			| JSON_OBJECT
 			| JSON_OBJECTAGG
 			| JSON_QUERY
+			| JSON_SCALAR
+			| JSON_SERIALIZE
 			| JSON_VALUE
 			| KEEP
 			| KEY

src/backend/parser/parse_expr.c

@@ -88,6 +88,10 @@ static Node *transformJsonArrayAgg(ParseState *pstate, JsonArrayAgg *agg);
 static Node *transformJsonIsPredicate(ParseState *pstate, JsonIsPredicate *p);
 static Node *transformJsonFuncExpr(ParseState *pstate, JsonFuncExpr *p);
 static Node *transformJsonValueExpr(ParseState *pstate, JsonValueExpr *jve);
+static Node *transformJsonParseExpr(ParseState *pstate, JsonParseExpr *expr);
+static Node *transformJsonScalarExpr(ParseState *pstate, JsonScalarExpr *expr);
+static Node *transformJsonSerializeExpr(ParseState *pstate,
+										JsonSerializeExpr *expr);
 static Node *make_row_comparison_op(ParseState *pstate, List *opname,
 									List *largs, List *rargs, int location);
 static Node *make_row_distinct_op(ParseState *pstate, List *opname,
@@ -347,6 +351,18 @@ transformExprRecurse(ParseState *pstate, Node *expr)
 			result = transformJsonValueExpr(pstate, (JsonValueExpr *) expr);
 			break;
 
+		case T_JsonParseExpr:
+			result = transformJsonParseExpr(pstate, (JsonParseExpr *) expr);
+			break;
+
+		case T_JsonScalarExpr:
+			result = transformJsonScalarExpr(pstate, (JsonScalarExpr *) expr);
+			break;
+
+		case T_JsonSerializeExpr:
+			result = transformJsonSerializeExpr(pstate, (JsonSerializeExpr *) expr);
+			break;
+
 		default:
 			/* should not reach here */
 			elog(ERROR, "unrecognized node type: %d", (int) nodeTag(expr));
@@ -3229,7 +3245,8 @@ makeCaseTestExpr(Node *expr)
  */
 static Node *
 transformJsonValueExprExt(ParseState *pstate, JsonValueExpr *ve,
-						  JsonFormatType default_format, bool isarg)
+						  JsonFormatType default_format, bool isarg,
+						  Oid targettype)
 {
 	Node	   *expr = transformExprRecurse(pstate, (Node *) ve->raw_expr);
 	Node	   *rawexpr;
@@ -3303,17 +3320,17 @@ transformJsonValueExprExt(ParseState *pstate, JsonValueExpr *ve,
 	else
 		format = default_format;
 
-	if (format == JS_FORMAT_DEFAULT)
+	if (format == JS_FORMAT_DEFAULT &&
+		(!OidIsValid(targettype) || exprtype == targettype))
 		expr = rawexpr;
 	else
 	{
-		Oid			targettype = format == JS_FORMAT_JSONB ? JSONBOID : JSONOID;
 		Node	   *orig = makeCaseTestExpr(expr);
 		Node	   *coerced;
+		bool		cast_is_needed = OidIsValid(targettype);
 
-		expr = orig;
-
-		if (!isarg && exprtype != BYTEAOID && typcategory != TYPCATEGORY_STRING)
+		if (!isarg && !cast_is_needed &&
+			exprtype != BYTEAOID && typcategory != TYPCATEGORY_STRING)
 			ereport(ERROR,
 					(errcode(ERRCODE_DATATYPE_MISMATCH),
 					 errmsg(ve->format->format_type == JS_FORMAT_DEFAULT ?
@@ -3322,6 +3339,8 @@ transformJsonValueExprExt(ParseState *pstate, JsonValueExpr *ve,
 					 parser_errposition(pstate, ve->format->location >= 0 ?
 										ve->format->location : location)));
 
+		expr = orig;
+
 		/* Convert encoded JSON text from bytea. */
 		if (format == JS_FORMAT_JSON && exprtype == BYTEAOID)
 		{
@@ -3329,6 +3348,9 @@ transformJsonValueExprExt(ParseState *pstate, JsonValueExpr *ve,
 			exprtype = TEXTOID;
 		}
 
+		if (!OidIsValid(targettype))
+			targettype = format == JS_FORMAT_JSONB ? JSONBOID : JSONOID;
+
 		/* Try to coerce to the target type. */
 		coerced = coerce_to_target_type(pstate, expr, exprtype,
 										targettype, -1,
@@ -3339,11 +3361,21 @@ transformJsonValueExprExt(ParseState *pstate, JsonValueExpr *ve,
 		if (!coerced)
 		{
 			/* If coercion failed, use to_json()/to_jsonb() functions. */
-			Oid			fnoid = targettype == JSONOID ? F_TO_JSON : F_TO_JSONB;
-			FuncExpr   *fexpr = makeFuncExpr(fnoid, targettype,
-											 list_make1(expr),
-											 InvalidOid, InvalidOid,
-											 COERCE_EXPLICIT_CALL);
+			FuncExpr   *fexpr;
+			Oid			fnoid;
+
+			if (cast_is_needed)		/* only CAST is allowed */
+				ereport(ERROR,
+						(errcode(ERRCODE_CANNOT_COERCE),
+						 errmsg("cannot cast type %s to %s",
+								format_type_be(exprtype),
+								format_type_be(targettype)),
+								parser_errposition(pstate, location)));
+
+			fnoid = targettype == JSONOID ? F_TO_JSON : F_TO_JSONB;
+			fexpr = makeFuncExpr(fnoid, targettype, list_make1(expr),
+								 InvalidOid, InvalidOid, COERCE_EXPLICIT_CALL);
+
 			fexpr->location = location;
 
 			coerced = (Node *) fexpr;
@@ -3370,7 +3402,8 @@ transformJsonValueExprExt(ParseState *pstate, JsonValueExpr *ve,
 static Node *
 transformJsonValueExpr(ParseState *pstate, JsonValueExpr *jve)
 {
-	return transformJsonValueExprExt(pstate, jve, JS_FORMAT_JSON, false);
+	return transformJsonValueExprExt(pstate, jve, JS_FORMAT_JSON, false,
+									 InvalidOid);
 }
 
 /*
@@ -3379,7 +3412,8 @@ transformJsonValueExpr(ParseState *pstate, JsonValueExpr *jve)
 static Node *
 transformJsonValueExprDefault(ParseState *pstate, JsonValueExpr *jve)
 {
-	return transformJsonValueExprExt(pstate, jve, JS_FORMAT_DEFAULT, false);
+	return transformJsonValueExprExt(pstate, jve, JS_FORMAT_DEFAULT, false,
+									 InvalidOid);
 }
 
 /*
@@ -4022,7 +4056,7 @@ transformJsonPassingArgs(ParseState *pstate, JsonFormatType format, List *args,
 	{
 		JsonArgument *arg = castNode(JsonArgument, lfirst(lc));
 		Node	   *expr = transformJsonValueExprExt(pstate, arg->val,
-													 format, true);
+													 format, true, InvalidOid);
 
 		assign_expr_collations(pstate, expr);
 
@@ -4415,3 +4449,93 @@ transformJsonFuncExpr(ParseState *pstate, JsonFuncExpr *func)
 
 	return (Node *) jsexpr;
 }
+
+/*
+ * Transform a JSON() expression.
+ */
+static Node *
+transformJsonParseExpr(ParseState *pstate, JsonParseExpr *jsexpr)
+{
+	JsonReturning *returning = makeNode(JsonReturning);
+	Node	   *arg;
+
+	returning->format = makeJsonFormat(JS_FORMAT_JSON, JS_ENC_DEFAULT, -1);
+	returning->typid = JSONOID;
+	returning->typmod = -1;
+
+	if (jsexpr->unique_keys)
+	{
+		/*
+		 * Coerce string argument to text and then to json[b] in the executor
+		 * node with key uniqueness check.
+		 */
+		JsonValueExpr *jve = jsexpr->expr;
+		Oid			arg_type;
+
+		arg = transformJsonParseArg(pstate, (Node *) jve->raw_expr, jve->format,
+									&arg_type);
+
+		if (arg_type != TEXTOID)
+			ereport(ERROR,
+					(errcode(ERRCODE_DATATYPE_MISMATCH),
+					 errmsg("cannot use non-string types with WITH UNIQUE KEYS clause"),
+					 parser_errposition(pstate, jsexpr->location)));
+	}
+	else
+	{
+		/*
+		 * Coerce argument to target type using CAST for compatibilty with PG
+		 * function-like CASTs.
+		 */
+		arg = transformJsonValueExprExt(pstate, jsexpr->expr, JS_FORMAT_JSON,
+										false, returning->typid);
+	}
+
+	return makeJsonConstructorExpr(pstate, JSCTOR_JSON_PARSE, list_make1(arg), NULL,
+							returning, jsexpr->unique_keys, false,
+							jsexpr->location);
+}
+
+/*
+ * Transform a JSON_SCALAR() expression.
+ */
+static Node *
+transformJsonScalarExpr(ParseState *pstate, JsonScalarExpr *jsexpr)
+{
+	JsonReturning *returning = makeNode(JsonReturning);
+	Node	   *arg = transformExprRecurse(pstate, (Node *) jsexpr->expr);
+
+	returning->format = makeJsonFormat(JS_FORMAT_JSON, JS_ENC_DEFAULT, -1);
+	returning->typid = JSONOID;
+	returning->typmod = -1;
+
+	if (exprType(arg) == UNKNOWNOID)
+		arg = coerce_to_specific_type(pstate, arg, TEXTOID, "JSON_SCALAR");
+
+	return makeJsonConstructorExpr(pstate, JSCTOR_JSON_SCALAR, list_make1(arg), NULL,
+							returning, false, false, jsexpr->location);
+}
+
+/*
+ * Transform a JSON_SERIALIZE() expression.
+ */
+static Node *
+transformJsonSerializeExpr(ParseState *pstate, JsonSerializeExpr *expr)
+{
+	Node	   *arg = transformJsonValueExpr(pstate, expr->expr);
+	JsonReturning *returning;
+
+	if (expr->output)
+		returning = transformJsonOutput(pstate, expr->output, true);
+	else
+	{
+		/* RETURNING TEXT FORMAT JSON is by default */
+		returning = makeNode(JsonReturning);
+		returning->format = makeJsonFormat(JS_FORMAT_JSON, JS_ENC_DEFAULT, -1);
+		returning->typid = TEXTOID;
+		returning->typmod = -1;
+	}
+
+	return makeJsonConstructorExpr(pstate, JSCTOR_JSON_SERIALIZE, list_make1(arg),
+							NULL, returning, false, false, expr->location);
+}

src/backend/parser/parse_target.c

@@ -1958,6 +1958,15 @@ FigureColnameInternal(Node *node, char **name)
 		case T_XmlSerialize:
 			*name = "xmlserialize";
 			return 2;
+		case T_JsonParseExpr:
+			*name = "json";
+			return 2;
+		case T_JsonScalarExpr:
+			*name = "json_scalar";
+			return 2;
+		case T_JsonSerializeExpr:
+			*name = "json_serialize";
+			return 2;
 		case T_JsonObjectConstructor:
 			*name = "json_object";
 			return 2;

src/backend/utils/adt/format_type.c

@@ -294,6 +294,10 @@ format_type_extended(Oid type_oid, int32 typemod, bits16 flags)
 			else
 				buf = pstrdup("character varying");
 			break;
+
+		case JSONOID:
+			buf = pstrdup("json");
+			break;
 	}
 
 	if (buf == NULL)

src/backend/utils/adt/json.c

@@ -30,21 +30,6 @@
 #include "utils/lsyscache.h"
 #include "utils/typcache.h"
 
-typedef enum					/* type categories for datum_to_json */
-{
-	JSONTYPE_NULL,				/* null, so we didn't bother to identify */
-	JSONTYPE_BOOL,				/* boolean (built-in types only) */
-	JSONTYPE_NUMERIC,			/* numeric (ditto) */
-	JSONTYPE_DATE,				/* we use special formatting for datetimes */
-	JSONTYPE_TIMESTAMP,
-	JSONTYPE_TIMESTAMPTZ,
-	JSONTYPE_JSON,				/* JSON itself (and JSONB) */
-	JSONTYPE_ARRAY,				/* array */
-	JSONTYPE_COMPOSITE,			/* composite */
-	JSONTYPE_CAST,				/* something with an explicit cast to JSON */
-	JSONTYPE_OTHER				/* all else */
-} JsonTypeCategory;
-
 /* Common context for key uniqueness check */
 typedef struct HTAB *JsonUniqueCheckState;	/* hash table for key names */
 
@@ -99,9 +84,6 @@ static void array_dim_to_json(StringInfo result, int dim, int ndims, int *dims,
 							  bool use_line_feeds);
 static void array_to_json_internal(Datum array, StringInfo result,
 								   bool use_line_feeds);
-static void json_categorize_type(Oid typoid,
-								 JsonTypeCategory *tcategory,
-								 Oid *outfuncoid);
 static void datum_to_json(Datum val, bool is_null, StringInfo result,
 						  JsonTypeCategory tcategory, Oid outfuncoid,
 						  bool key_scalar);
@@ -180,7 +162,7 @@ json_recv(PG_FUNCTION_ARGS)
  * output function OID.  If the returned category is JSONTYPE_CAST, we
  * return the OID of the type->JSON cast function instead.
  */
-static void
+void
 json_categorize_type(Oid typoid,
 					 JsonTypeCategory *tcategory,
 					 Oid *outfuncoid)
@@ -762,6 +744,16 @@ row_to_json_pretty(PG_FUNCTION_ARGS)
 	PG_RETURN_TEXT_P(cstring_to_text_with_len(result->data, result->len));
 }
 
+Datum
+to_json_worker(Datum val, JsonTypeCategory tcategory, Oid outfuncoid)
+{
+	StringInfo	result = makeStringInfo();
+
+	datum_to_json(val, false, result, tcategory, outfuncoid, false);
+
+	return PointerGetDatum(cstring_to_text_with_len(result->data, result->len));
+}
+
 bool
 to_json_is_immutable(Oid typoid)
 {
@@ -802,7 +794,6 @@ to_json(PG_FUNCTION_ARGS)
 {
 	Datum		val = PG_GETARG_DATUM(0);
 	Oid			val_type = get_fn_expr_argtype(fcinfo->flinfo, 0);
-	StringInfo	result;
 	JsonTypeCategory tcategory;
 	Oid			outfuncoid;
 
@@ -814,11 +805,7 @@ to_json(PG_FUNCTION_ARGS)
 	json_categorize_type(val_type,
 						 &tcategory, &outfuncoid);
 
-	result = makeStringInfo();
-
-	datum_to_json(val, false, result, tcategory, outfuncoid, false);
-
-	PG_RETURN_TEXT_P(cstring_to_text_with_len(result->data, result->len));
+	PG_RETURN_DATUM(to_json_worker(val, tcategory, outfuncoid));
 }
 
 /*
@@ -1712,7 +1699,7 @@ json_unique_object_field_start(void *_state, char *field, bool isnull)
 
 /* Validate JSON text and additionally check key uniqueness */
 bool
-json_validate(text *json, bool check_unique_keys)
+json_validate(text *json, bool check_unique_keys, bool throw_error)
 {
 	JsonLexContext *lex = makeJsonLexContext(json, check_unique_keys);
 	JsonSemAction uniqueSemAction = {0};
@@ -1736,10 +1723,22 @@ json_validate(text *json, bool check_unique_keys)
 	result = pg_parse_json(lex, check_unique_keys ? &uniqueSemAction : &nullSemAction);
 
 	if (result != JSON_SUCCESS)
+	{
+		if (throw_error)
+			json_ereport_error(result, lex);
+
 		return false;	/* invalid json */
+	}
 
 	if (check_unique_keys && !state.unique)
+	{
+		if (throw_error)
+			ereport(ERROR,
+					(errcode(ERRCODE_DUPLICATE_JSON_OBJECT_KEY_VALUE),
+					 errmsg("duplicate JSON object key value")));
+
 		return false;	/* not unique keys */
+	}
 
 	return true;	/* ok */
 }

src/backend/utils/adt/jsonb.c

@@ -34,25 +34,9 @@ typedef struct JsonbInState
 {
 	JsonbParseState *parseState;
 	JsonbValue *res;
+	bool		unique_keys;
 } JsonbInState;
 
-/* unlike with json categories, we need to treat json and jsonb differently */
-typedef enum					/* type categories for datum_to_jsonb */
-{
-	JSONBTYPE_NULL,				/* null, so we didn't bother to identify */
-	JSONBTYPE_BOOL,				/* boolean (built-in types only) */
-	JSONBTYPE_NUMERIC,			/* numeric (ditto) */
-	JSONBTYPE_DATE,				/* we use special formatting for datetimes */
-	JSONBTYPE_TIMESTAMP,		/* we use special formatting for timestamp */
-	JSONBTYPE_TIMESTAMPTZ,		/* ... and timestamptz */
-	JSONBTYPE_JSON,				/* JSON */
-	JSONBTYPE_JSONB,			/* JSONB */
-	JSONBTYPE_ARRAY,			/* array */
-	JSONBTYPE_COMPOSITE,		/* composite */
-	JSONBTYPE_JSONCAST,			/* something with an explicit cast to JSON */
-	JSONBTYPE_OTHER				/* all else */
-} JsonbTypeCategory;
-
 typedef struct JsonbAggState
 {
 	JsonbInState *res;
@@ -62,7 +46,7 @@ typedef struct JsonbAggState
 	Oid			val_output_func;
 } JsonbAggState;
 
-static inline Datum jsonb_from_cstring(char *json, int len);
+static inline Datum jsonb_from_cstring(char *json, int len, bool unique_keys);
 static size_t checkStringLen(size_t len);
 static void jsonb_in_object_start(void *pstate);
 static void jsonb_in_object_end(void *pstate);
@@ -71,17 +55,11 @@ static void jsonb_in_array_end(void *pstate);
 static void jsonb_in_object_field_start(void *pstate, char *fname, bool isnull);
 static void jsonb_put_escaped_value(StringInfo out, JsonbValue *scalarVal);
 static void jsonb_in_scalar(void *pstate, char *token, JsonTokenType tokentype);
-static void jsonb_categorize_type(Oid typoid,
-								  JsonbTypeCategory *tcategory,
-								  Oid *outfuncoid);
 static void composite_to_jsonb(Datum composite, JsonbInState *result);
 static void array_dim_to_jsonb(JsonbInState *result, int dim, int ndims, int *dims,
 							   Datum *vals, bool *nulls, int *valcount,
 							   JsonbTypeCategory tcategory, Oid outfuncoid);
 static void array_to_jsonb_internal(Datum array, JsonbInState *result);
-static void jsonb_categorize_type(Oid typoid,
-								  JsonbTypeCategory *tcategory,
-								  Oid *outfuncoid);
 static void datum_to_jsonb(Datum val, bool is_null, JsonbInState *result,
 						   JsonbTypeCategory tcategory, Oid outfuncoid,
 						   bool key_scalar);
@@ -99,7 +77,7 @@ jsonb_in(PG_FUNCTION_ARGS)
 {
 	char	   *json = PG_GETARG_CSTRING(0);
 
-	return jsonb_from_cstring(json, strlen(json));
+	return jsonb_from_cstring(json, strlen(json), false);
 }
 
 /*
@@ -123,7 +101,7 @@ jsonb_recv(PG_FUNCTION_ARGS)
 	else
 		elog(ERROR, "unsupported jsonb version number %d", version);
 
-	return jsonb_from_cstring(str, nbytes);
+	return jsonb_from_cstring(str, nbytes, false);
 }
 
 /*
@@ -164,6 +142,14 @@ jsonb_send(PG_FUNCTION_ARGS)
 	PG_RETURN_BYTEA_P(pq_endtypsend(&buf));
 }
 
+Datum
+jsonb_from_text(text *js, bool unique_keys)
+{
+	return jsonb_from_cstring(VARDATA_ANY(js),
+							  VARSIZE_ANY_EXHDR(js),
+							  unique_keys);
+}
+
 /*
  * Get the type name of a jsonb container.
  */
@@ -254,7 +240,7 @@ jsonb_typeof(PG_FUNCTION_ARGS)
  * Uses the json parser (with hooks) to construct a jsonb.
  */
 static inline Datum
-jsonb_from_cstring(char *json, int len)
+jsonb_from_cstring(char *json, int len, bool unique_keys)
 {
 	JsonLexContext *lex;
 	JsonbInState state;
@@ -264,6 +250,8 @@ jsonb_from_cstring(char *json, int len)
 	memset(&sem, 0, sizeof(sem));
 	lex = makeJsonLexContextCstringLen(json, len, GetDatabaseEncoding(), true);
 
+	state.unique_keys = unique_keys;
+
 	sem.semstate = (void *) &state;
 
 	sem.object_start = jsonb_in_object_start;
@@ -298,6 +286,7 @@ jsonb_in_object_start(void *pstate)
 	JsonbInState *_state = (JsonbInState *) pstate;
 
 	_state->res = pushJsonbValue(&_state->parseState, WJB_BEGIN_OBJECT, NULL);
+	_state->parseState->unique_keys = _state->unique_keys;
 }
 
 static void
@@ -620,7 +609,7 @@ add_indent(StringInfo out, bool indent, int level)
  * output function OID.  If the returned category is JSONBTYPE_JSONCAST,
  * we return the OID of the relevant cast function instead.
  */
-static void
+void
 jsonb_categorize_type(Oid typoid,
 					  JsonbTypeCategory *tcategory,
 					  Oid *outfuncoid)
@@ -1127,6 +1116,18 @@ add_jsonb(Datum val, bool is_null, JsonbInState *result,
 	datum_to_jsonb(val, is_null, result, tcategory, outfuncoid, key_scalar);
 }
 
+Datum
+to_jsonb_worker(Datum val, JsonbTypeCategory tcategory, Oid outfuncoid)
+{
+	JsonbInState result;
+
+	memset(&result, 0, sizeof(JsonbInState));
+
+	datum_to_jsonb(val, false, &result, tcategory, outfuncoid, false);
+
+	return JsonbPGetDatum(JsonbValueToJsonb(result.res));
+}
+
 bool
 to_jsonb_is_immutable(Oid typoid)
 {
@@ -1168,7 +1169,6 @@ to_jsonb(PG_FUNCTION_ARGS)
 {
 	Datum		val = PG_GETARG_DATUM(0);
 	Oid			val_type = get_fn_expr_argtype(fcinfo->flinfo, 0);
-	JsonbInState result;
 	JsonbTypeCategory tcategory;
 	Oid			outfuncoid;
 
@@ -1180,11 +1180,7 @@ to_jsonb(PG_FUNCTION_ARGS)
 	jsonb_categorize_type(val_type,
 						  &tcategory, &outfuncoid);
 
-	memset(&result, 0, sizeof(JsonbInState));
-
-	datum_to_jsonb(val, false, &result, tcategory, outfuncoid, false);
-
-	PG_RETURN_POINTER(JsonbValueToJsonb(result.res));
+	PG_RETURN_DATUM(to_jsonb_worker(val, tcategory, outfuncoid));
 }
 
 Datum

src/backend/utils/adt/ruleutils.c

@@ -10092,7 +10092,9 @@ get_json_constructor_options(JsonConstructorExpr *ctor, StringInfo buf)
 	if (ctor->unique)
 		appendStringInfoString(buf, " WITH UNIQUE KEYS");
 
-	get_json_returning(ctor->returning, buf, true);
+	if (ctor->type != JSCTOR_JSON_PARSE &&
+		ctor->type != JSCTOR_JSON_SCALAR)
+		get_json_returning(ctor->returning, buf, true);
 }
 
 static void
@@ -10106,6 +10108,15 @@ get_json_constructor(JsonConstructorExpr *ctor, deparse_context *context,
 
 	switch (ctor->type)
 	{
+		case JSCTOR_JSON_PARSE:
+			funcname = "JSON";
+			break;
+		case JSCTOR_JSON_SCALAR:
+			funcname = "JSON_SCALAR";
+			break;
+		case JSCTOR_JSON_SERIALIZE:
+			funcname = "JSON_SERIALIZE";
+			break;
 		case JSCTOR_JSON_OBJECT:
 			funcname = "JSON_OBJECT";
 			break;

src/include/executor/execExpr.h

@@ -680,6 +680,11 @@ typedef struct ExprEvalStep
 			Datum	   *arg_values;
 			bool	   *arg_nulls;
 			Oid		   *arg_types;
+			struct
+			{
+				int			category;
+				Oid			outfuncid;
+			}		   *arg_type_cache;		/* cache for datum_to_json[b]() */
 			int			nargs;
 		}			json_constructor;
 

src/include/nodes/nodes.h

@@ -205,6 +205,9 @@ typedef enum NodeTag
 	T_JsonFormat,
 	T_JsonReturning,
 	T_JsonValueExpr,
+	T_JsonParseExpr,
+	T_JsonScalarExpr,
+	T_JsonSerializeExpr,
 	T_JsonConstructorExpr,
 	T_JsonExpr,
 	T_JsonCoercion,

src/include/nodes/parsenodes.h

@@ -1676,6 +1676,41 @@ typedef struct JsonKeyValue
 	JsonValueExpr *value;		/* JSON value expression */
 } JsonKeyValue;
 
+/*
+ * JsonParseExpr -
+ *		untransformed representation of JSON()
+ */
+typedef struct JsonParseExpr
+{
+	NodeTag		type;
+	JsonValueExpr *expr;		/* string expression */
+	bool		unique_keys;	/* WITH UNIQUE KEYS? */
+	int			location;		/* token location, or -1 if unknown */
+} JsonParseExpr;
+
+/*
+ * JsonScalarExpr -
+ *		untransformed representation of JSON_SCALAR()
+ */
+typedef struct JsonScalarExpr
+{
+	NodeTag		type;
+	Expr	   *expr;			/* scalar expression */
+	int			location;		/* token location, or -1 if unknown */
+} JsonScalarExpr;
+
+/*
+ * JsonSerializeExpr -
+ *		untransformed representation of JSON_SERIALIZE() function
+ */
+typedef struct JsonSerializeExpr
+{
+	NodeTag		type;
+	JsonValueExpr *expr;		/* json value expression */
+	JsonOutput *output;			/* RETURNING clause, if specified  */
+	int			location;		/* token location, or -1 if unknown */
+} JsonSerializeExpr;
+
 /*
  * JsonObjectConstructor -
  *		untransformed representation of JSON_OBJECT() constructor

src/include/nodes/primnodes.h

@@ -1339,7 +1339,10 @@ typedef enum JsonConstructorType
 	JSCTOR_JSON_OBJECT = 1,
 	JSCTOR_JSON_ARRAY = 2,
 	JSCTOR_JSON_OBJECTAGG = 3,
-	JSCTOR_JSON_ARRAYAGG = 4
+	JSCTOR_JSON_ARRAYAGG = 4,
+	JSCTOR_JSON_SCALAR = 5,
+	JSCTOR_JSON_SERIALIZE = 6,
+	JSCTOR_JSON_PARSE = 7
 } JsonConstructorType;
 
 /*

src/include/parser/kwlist.h

@@ -232,13 +232,15 @@ PG_KEYWORD("is", IS, TYPE_FUNC_NAME_KEYWORD, BARE_LABEL)
 PG_KEYWORD("isnull", ISNULL, TYPE_FUNC_NAME_KEYWORD, AS_LABEL)
 PG_KEYWORD("isolation", ISOLATION, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("join", JOIN, TYPE_FUNC_NAME_KEYWORD, BARE_LABEL)
-PG_KEYWORD("json", JSON, UNRESERVED_KEYWORD, BARE_LABEL)
+PG_KEYWORD("json", JSON, COL_NAME_KEYWORD, BARE_LABEL)
 PG_KEYWORD("json_array", JSON_ARRAY, COL_NAME_KEYWORD, BARE_LABEL)
 PG_KEYWORD("json_arrayagg", JSON_ARRAYAGG, COL_NAME_KEYWORD, BARE_LABEL)
 PG_KEYWORD("json_exists", JSON_EXISTS, COL_NAME_KEYWORD, BARE_LABEL)
 PG_KEYWORD("json_object", JSON_OBJECT, COL_NAME_KEYWORD, BARE_LABEL)
 PG_KEYWORD("json_objectagg", JSON_OBJECTAGG, COL_NAME_KEYWORD, BARE_LABEL)
 PG_KEYWORD("json_query", JSON_QUERY, COL_NAME_KEYWORD, BARE_LABEL)
+PG_KEYWORD("json_scalar", JSON_SCALAR, COL_NAME_KEYWORD, BARE_LABEL)
+PG_KEYWORD("json_serialize", JSON_SERIALIZE, COL_NAME_KEYWORD, BARE_LABEL)
 PG_KEYWORD("json_value", JSON_VALUE, COL_NAME_KEYWORD, BARE_LABEL)
 PG_KEYWORD("keep", KEEP, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("key", KEY, UNRESERVED_KEYWORD, BARE_LABEL)

src/include/utils/json.h

@@ -16,16 +16,35 @@
 
 #include "lib/stringinfo.h"
 
+typedef enum					/* type categories for datum_to_json */
+{
+	JSONTYPE_NULL,				/* null, so we didn't bother to identify */
+	JSONTYPE_BOOL,				/* boolean (built-in types only) */
+	JSONTYPE_NUMERIC,			/* numeric (ditto) */
+	JSONTYPE_DATE,				/* we use special formatting for datetimes */
+	JSONTYPE_TIMESTAMP,
+	JSONTYPE_TIMESTAMPTZ,
+	JSONTYPE_JSON,				/* JSON itself (and JSONB) */
+	JSONTYPE_ARRAY,				/* array */
+	JSONTYPE_COMPOSITE,			/* composite */
+	JSONTYPE_CAST,				/* something with an explicit cast to JSON */
+	JSONTYPE_OTHER				/* all else */
+} JsonTypeCategory;
+
 /* functions in json.c */
 extern void escape_json(StringInfo buf, const char *str);
 extern char *JsonEncodeDateTime(char *buf, Datum value, Oid typid,
 								const int *tzp);
 extern bool to_json_is_immutable(Oid typoid);
+extern void json_categorize_type(Oid typoid, JsonTypeCategory *tcategory,
+								 Oid *outfuncoid);
+extern Datum to_json_worker(Datum val, JsonTypeCategory tcategory,
+							Oid outfuncoid);
 extern Datum json_build_object_worker(int nargs, Datum *args, bool *nulls,
 									  Oid *types, bool absent_on_null,
 									  bool unique_keys);
 extern Datum json_build_array_worker(int nargs, Datum *args, bool *nulls,
 									 Oid *types, bool absent_on_null);
-extern bool json_validate(text *json, bool check_unique_keys);
+extern bool json_validate(text *json, bool check_unique_keys, bool throw_error);
 
 #endif							/* JSON_H */

src/include/utils/jsonb.h

@@ -376,6 +376,22 @@ typedef struct JsonbIterator
 	struct JsonbIterator *parent;
 } JsonbIterator;
 
+/* unlike with json categories, we need to treat json and jsonb differently */
+typedef enum					/* type categories for datum_to_jsonb */
+{
+	JSONBTYPE_NULL,				/* null, so we didn't bother to identify */
+	JSONBTYPE_BOOL,				/* boolean (built-in types only) */
+	JSONBTYPE_NUMERIC,			/* numeric (ditto) */
+	JSONBTYPE_DATE,				/* we use special formatting for datetimes */
+	JSONBTYPE_TIMESTAMP,		/* we use special formatting for timestamp */
+	JSONBTYPE_TIMESTAMPTZ,		/* ... and timestamptz */
+	JSONBTYPE_JSON,				/* JSON */
+	JSONBTYPE_JSONB,			/* JSONB */
+	JSONBTYPE_ARRAY,			/* array */
+	JSONBTYPE_COMPOSITE,		/* composite */
+	JSONBTYPE_JSONCAST,			/* something with an explicit cast to JSON */
+	JSONBTYPE_OTHER				/* all else */
+} JsonbTypeCategory;
 
 /* Support functions */
 extern uint32 getJsonbOffset(const JsonbContainer *jc, int index);
@@ -403,6 +419,7 @@ extern void JsonbHashScalarValueExtended(const JsonbValue *scalarVal,
 										 uint64 *hash, uint64 seed);
 
 /* jsonb.c support functions */
+extern Datum jsonb_from_text(text *js, bool unique_keys);
 extern char *JsonbToCString(StringInfo out, JsonbContainer *in,
 							int estimated_len);
 extern char *JsonbToCStringIndent(StringInfo out, JsonbContainer *in,
@@ -418,6 +435,10 @@ extern Datum jsonb_set_element(Jsonb *jb, Datum *path, int path_len,
 extern Datum jsonb_get_element(Jsonb *jb, Datum *path, int npath,
 							   bool *isnull, bool as_text);
 extern bool to_jsonb_is_immutable(Oid typoid);
+extern void jsonb_categorize_type(Oid typoid, JsonbTypeCategory *tcategory,
+								  Oid *outfuncoid);
+extern Datum to_jsonb_worker(Datum val, JsonbTypeCategory tcategory,
+							 Oid outfuncoid);
 extern Datum jsonb_build_object_worker(int nargs, Datum *args, bool *nulls,
 									   Oid *types, bool absent_on_null,
 									   bool unique_keys);

src/test/regress/expected/sqljson.out

@@ -1,3 +1,270 @@
+-- JSON()
+SELECT JSON();
+ERROR:  syntax error at or near ")"
+LINE 1: SELECT JSON();
+                    ^
+SELECT JSON(NULL);
+ json 
+------
+ 
+(1 row)
+
+SELECT JSON('{ "a" : 1 } ');
+     json     
+--------------
+ { "a" : 1 } 
+(1 row)
+
+SELECT JSON('{ "a" : 1 } ' FORMAT JSON);
+     json     
+--------------
+ { "a" : 1 } 
+(1 row)
+
+SELECT JSON('{ "a" : 1 } ' FORMAT JSON ENCODING UTF8);
+ERROR:  JSON ENCODING clause is only allowed for bytea input type
+LINE 1: SELECT JSON('{ "a" : 1 } ' FORMAT JSON ENCODING UTF8);
+                                   ^
+SELECT JSON('{ "a" : 1 } '::bytea FORMAT JSON ENCODING UTF8);
+     json     
+--------------
+ { "a" : 1 } 
+(1 row)
+
+SELECT pg_typeof(JSON('{ "a" : 1 } '));
+ pg_typeof 
+-----------
+ json
+(1 row)
+
+SELECT JSON('   1   '::json);
+  json   
+---------
+    1   
+(1 row)
+
+SELECT JSON('   1   '::jsonb);
+ json 
+------
+ 1
+(1 row)
+
+SELECT JSON('   1   '::json WITH UNIQUE KEYS);
+ERROR:  cannot use non-string types with WITH UNIQUE KEYS clause
+LINE 1: SELECT JSON('   1   '::json WITH UNIQUE KEYS);
+               ^
+SELECT JSON(123);
+ERROR:  cannot cast type integer to json
+LINE 1: SELECT JSON(123);
+                    ^
+SELECT JSON('{"a": 1, "a": 2}');
+       json       
+------------------
+ {"a": 1, "a": 2}
+(1 row)
+
+SELECT JSON('{"a": 1, "a": 2}' WITH UNIQUE KEYS);
+ERROR:  duplicate JSON object key value
+SELECT JSON('{"a": 1, "a": 2}' WITHOUT UNIQUE KEYS);
+       json       
+------------------
+ {"a": 1, "a": 2}
+(1 row)
+
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON('123');
+         QUERY PLAN          
+-----------------------------
+ Result
+   Output: JSON('123'::json)
+(2 rows)
+
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON('123' FORMAT JSON);
+         QUERY PLAN          
+-----------------------------
+ Result
+   Output: JSON('123'::json)
+(2 rows)
+
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON('123'::bytea FORMAT JSON);
+                  QUERY PLAN                   
+-----------------------------------------------
+ Result
+   Output: JSON('\x313233'::bytea FORMAT JSON)
+(2 rows)
+
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON('123'::bytea FORMAT JSON ENCODING UTF8);
+                         QUERY PLAN                          
+-------------------------------------------------------------
+ Result
+   Output: JSON('\x313233'::bytea FORMAT JSON ENCODING UTF8)
+(2 rows)
+
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON('123' WITH UNIQUE KEYS);
+                  QUERY PLAN                  
+----------------------------------------------
+ Result
+   Output: JSON('123'::text WITH UNIQUE KEYS)
+(2 rows)
+
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON('123' WITHOUT UNIQUE KEYS);
+         QUERY PLAN          
+-----------------------------
+ Result
+   Output: JSON('123'::json)
+(2 rows)
+
+-- JSON_SCALAR()
+SELECT JSON_SCALAR();
+ERROR:  syntax error at or near ")"
+LINE 1: SELECT JSON_SCALAR();
+                           ^
+SELECT JSON_SCALAR(NULL);
+ json_scalar 
+-------------
+ 
+(1 row)
+
+SELECT JSON_SCALAR(NULL::int);
+ json_scalar 
+-------------
+ 
+(1 row)
+
+SELECT JSON_SCALAR(123);
+ json_scalar 
+-------------
+ 123
+(1 row)
+
+SELECT JSON_SCALAR(123.45);
+ json_scalar 
+-------------
+ 123.45
+(1 row)
+
+SELECT JSON_SCALAR(123.45::numeric);
+ json_scalar 
+-------------
+ 123.45
+(1 row)
+
+SELECT JSON_SCALAR(true);
+ json_scalar 
+-------------
+ true
+(1 row)
+
+SELECT JSON_SCALAR(false);
+ json_scalar 
+-------------
+ false
+(1 row)
+
+SELECT JSON_SCALAR(' 123.45');
+ json_scalar 
+-------------
+ " 123.45"
+(1 row)
+
+SELECT JSON_SCALAR('2020-06-07'::date);
+ json_scalar  
+--------------
+ "2020-06-07"
+(1 row)
+
+SELECT JSON_SCALAR('2020-06-07 01:02:03'::timestamp);
+      json_scalar      
+-----------------------
+ "2020-06-07T01:02:03"
+(1 row)
+
+SELECT JSON_SCALAR('{}'::json);
+ json_scalar 
+-------------
+ {}
+(1 row)
+
+SELECT JSON_SCALAR('{}'::jsonb);
+ json_scalar 
+-------------
+ {}
+(1 row)
+
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON_SCALAR(123);
+         QUERY PLAN         
+----------------------------
+ Result
+   Output: JSON_SCALAR(123)
+(2 rows)
+
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON_SCALAR('123');
+             QUERY PLAN             
+------------------------------------
+ Result
+   Output: JSON_SCALAR('123'::text)
+(2 rows)
+
+-- JSON_SERIALIZE()
+SELECT JSON_SERIALIZE();
+ERROR:  syntax error at or near ")"
+LINE 1: SELECT JSON_SERIALIZE();
+                              ^
+SELECT JSON_SERIALIZE(NULL);
+ json_serialize 
+----------------
+ 
+(1 row)
+
+SELECT JSON_SERIALIZE(JSON('{ "a" : 1 } '));
+ json_serialize 
+----------------
+ { "a" : 1 } 
+(1 row)
+
+SELECT JSON_SERIALIZE('{ "a" : 1 } ');
+ json_serialize 
+----------------
+ { "a" : 1 } 
+(1 row)
+
+SELECT JSON_SERIALIZE('1');
+ json_serialize 
+----------------
+ 1
+(1 row)
+
+SELECT JSON_SERIALIZE('1' FORMAT JSON);
+ json_serialize 
+----------------
+ 1
+(1 row)
+
+SELECT JSON_SERIALIZE('{ "a" : 1 } ' RETURNING bytea);
+       json_serialize       
+----------------------------
+ \x7b20226122203a2031207d20
+(1 row)
+
+SELECT pg_typeof(JSON_SERIALIZE(NULL));
+ pg_typeof 
+-----------
+ text
+(1 row)
+
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON_SERIALIZE('{}');
+                     QUERY PLAN                      
+-----------------------------------------------------
+ Result
+   Output: JSON_SERIALIZE('{}'::json RETURNING text)
+(2 rows)
+
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON_SERIALIZE('{}' RETURNING bytea);
+                      QUERY PLAN                      
+------------------------------------------------------
+ Result
+   Output: JSON_SERIALIZE('{}'::json RETURNING bytea)
+(2 rows)
+
 -- JSON_OBJECT()
 SELECT JSON_OBJECT();
  json_object 

src/test/regress/sql/sqljson.sql

@@ -1,3 +1,60 @@
+-- JSON()
+SELECT JSON();
+SELECT JSON(NULL);
+SELECT JSON('{ "a" : 1 } ');
+SELECT JSON('{ "a" : 1 } ' FORMAT JSON);
+SELECT JSON('{ "a" : 1 } ' FORMAT JSON ENCODING UTF8);
+SELECT JSON('{ "a" : 1 } '::bytea FORMAT JSON ENCODING UTF8);
+SELECT pg_typeof(JSON('{ "a" : 1 } '));
+
+SELECT JSON('   1   '::json);
+SELECT JSON('   1   '::jsonb);
+SELECT JSON('   1   '::json WITH UNIQUE KEYS);
+SELECT JSON(123);
+
+SELECT JSON('{"a": 1, "a": 2}');
+SELECT JSON('{"a": 1, "a": 2}' WITH UNIQUE KEYS);
+SELECT JSON('{"a": 1, "a": 2}' WITHOUT UNIQUE KEYS);
+
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON('123');
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON('123' FORMAT JSON);
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON('123'::bytea FORMAT JSON);
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON('123'::bytea FORMAT JSON ENCODING UTF8);
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON('123' WITH UNIQUE KEYS);
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON('123' WITHOUT UNIQUE KEYS);
+
+
+-- JSON_SCALAR()
+SELECT JSON_SCALAR();
+SELECT JSON_SCALAR(NULL);
+SELECT JSON_SCALAR(NULL::int);
+SELECT JSON_SCALAR(123);
+SELECT JSON_SCALAR(123.45);
+SELECT JSON_SCALAR(123.45::numeric);
+SELECT JSON_SCALAR(true);
+SELECT JSON_SCALAR(false);
+SELECT JSON_SCALAR(' 123.45');
+SELECT JSON_SCALAR('2020-06-07'::date);
+SELECT JSON_SCALAR('2020-06-07 01:02:03'::timestamp);
+SELECT JSON_SCALAR('{}'::json);
+SELECT JSON_SCALAR('{}'::jsonb);
+
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON_SCALAR(123);
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON_SCALAR('123');
+
+-- JSON_SERIALIZE()
+SELECT JSON_SERIALIZE();
+SELECT JSON_SERIALIZE(NULL);
+SELECT JSON_SERIALIZE(JSON('{ "a" : 1 } '));
+SELECT JSON_SERIALIZE('{ "a" : 1 } ');
+SELECT JSON_SERIALIZE('1');
+SELECT JSON_SERIALIZE('1' FORMAT JSON);
+SELECT JSON_SERIALIZE('{ "a" : 1 } ' RETURNING bytea);
+SELECT pg_typeof(JSON_SERIALIZE(NULL));
+
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON_SERIALIZE('{}');
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON_SERIALIZE('{}' RETURNING bytea);
+
 -- JSON_OBJECT()
 SELECT JSON_OBJECT();
 SELECT JSON_OBJECT(RETURNING json);