diff --git a/src/backend/utils/adt/json.c b/src/backend/utils/adt/json.c
index a576843234..1d6b752a28 100644
--- a/src/backend/utils/adt/json.c
+++ b/src/backend/utils/adt/json.c
@@ -94,6 +94,7 @@ static void datum_to_json(Datum val, bool is_null, StringInfo result,
 			  bool key_scalar);
 static void add_json(Datum val, bool is_null, StringInfo result,
 		 Oid val_type, bool key_scalar);
+static text *catenate_stringinfo_string(StringInfo buffer, const char *addon);
 
 /* the null action object used for pure validation */
 static JsonSemAction nullSemAction =
@@ -175,7 +176,7 @@ lex_expect(JsonParseContext ctx, JsonLexContext *lex, JsonTokenType token)
 
 /* utility function to check if a string is a valid JSON number */
 extern bool
-IsValidJsonNumber(const char * str, int len)
+IsValidJsonNumber(const char *str, int len)
 {
 	bool		numeric_error;
 	JsonLexContext dummy_lex;
@@ -200,7 +201,7 @@ IsValidJsonNumber(const char * str, int len)
 
 	json_lex_number(&dummy_lex, dummy_lex.input, &numeric_error);
 
-	return ! numeric_error;
+	return !numeric_error;
 }
 
 /*
@@ -1370,7 +1371,7 @@ datum_to_json(Datum val, bool is_null, StringInfo result,
 	text	   *jsontext;
 
 	/* callers are expected to ensure that null keys are not passed in */
-	Assert( ! (key_scalar && is_null));
+	Assert(!(key_scalar && is_null));
 
 	if (is_null)
 	{
@@ -1404,6 +1405,7 @@ datum_to_json(Datum val, bool is_null, StringInfo result,
 			break;
 		case JSONTYPE_NUMERIC:
 			outputstr = OidOutputFunctionCall(outfuncoid, val);
+
 			/*
 			 * Don't call escape_json for a non-key if it's a valid JSON
 			 * number.
@@ -1798,6 +1800,8 @@ to_json(PG_FUNCTION_ARGS)
 
 /*
  * json_agg transition function
+ *
+ * aggregate input column as a json array value.
  */
 Datum
 json_agg_transfn(PG_FUNCTION_ARGS)
@@ -1884,18 +1888,18 @@ json_agg_finalfn(PG_FUNCTION_ARGS)
 
 	state = PG_ARGISNULL(0) ? NULL : (StringInfo) PG_GETARG_POINTER(0);
 
+	/* NULL result for no rows in, as is standard with aggregates */
 	if (state == NULL)
 		PG_RETURN_NULL();
 
-	appendStringInfoChar(state, ']');
-
-	PG_RETURN_TEXT_P(cstring_to_text_with_len(state->data, state->len));
+	/* Else return state with appropriate array terminator added */
+	PG_RETURN_TEXT_P(catenate_stringinfo_string(state, "]"));
 }
 
 /*
  * json_object_agg transition function.
  *
- * aggregate two input columns as a single json value.
+ * aggregate two input columns as a single json object value.
  */
 Datum
 json_object_agg_transfn(PG_FUNCTION_ARGS)
@@ -1909,7 +1913,7 @@ json_object_agg_transfn(PG_FUNCTION_ARGS)
 	if (!AggCheckCallContext(fcinfo, &aggcontext))
 	{
 		/* cannot be called directly because of internal-type argument */
-		elog(ERROR, "json_agg_transfn called in non-aggregate context");
+		elog(ERROR, "json_object_agg_transfn called in non-aggregate context");
 	}
 
 	if (PG_ARGISNULL(0))
@@ -1976,7 +1980,6 @@ json_object_agg_transfn(PG_FUNCTION_ARGS)
 
 /*
  * json_object_agg final function.
- *
  */
 Datum
 json_object_agg_finalfn(PG_FUNCTION_ARGS)
@@ -1988,12 +1991,32 @@ json_object_agg_finalfn(PG_FUNCTION_ARGS)
 
 	state = PG_ARGISNULL(0) ? NULL : (StringInfo) PG_GETARG_POINTER(0);
 
+	/* NULL result for no rows in, as is standard with aggregates */
 	if (state == NULL)
 		PG_RETURN_NULL();
 
-	appendStringInfoString(state, " }");
+	/* Else return state with appropriate object terminator added */
+	PG_RETURN_TEXT_P(catenate_stringinfo_string(state, " }"));
+}
 
-	PG_RETURN_TEXT_P(cstring_to_text_with_len(state->data, state->len));
+/*
+ * Helper function for aggregates: return given StringInfo's contents plus
+ * specified trailing string, as a text datum.  We need this because aggregate
+ * final functions are not allowed to modify the aggregate state.
+ */
+static text *
+catenate_stringinfo_string(StringInfo buffer, const char *addon)
+{
+	/* custom version of cstring_to_text_with_len */
+	int			buflen = buffer->len;
+	int			addlen = strlen(addon);
+	text	   *result = (text *) palloc(buflen + addlen + VARHDRSZ);
+
+	SET_VARSIZE(result, buflen + addlen + VARHDRSZ);
+	memcpy(VARDATA(result), buffer->data, buflen);
+	memcpy(VARDATA(result) + buflen, addon, addlen);
+
+	return result;
 }
 
 /*