Improve wording of two error messages related to generated columns.

Clarify that you can "insert" into a generated column as long as what
you're inserting is a DEFAULT placeholder.

Also, use ERRCODE_GENERATED_ALWAYS in place of ERRCODE_SYNTAX_ERROR;
there doesn't seem to be any reason to use the less specific errcode.

Discussion: https://postgr.es/m/9q0sgcr416t.fsf@gmx.us

src/backend/rewrite/rewriteHandler.c

@@ -861,7 +861,7 @@ rewriteTargetListIU(List *targetList,
 					if (!apply_default)
 						ereport(ERROR,
 								(errcode(ERRCODE_GENERATED_ALWAYS),
-								 errmsg("cannot insert into column \"%s\"",
+								 errmsg("cannot insert a non-DEFAULT value into column \"%s\"",
 										NameStr(att_tup->attname)),
 								 errdetail("Column \"%s\" is an identity column defined as GENERATED ALWAYS.",
 										   NameStr(att_tup->attname)),
@@ -899,8 +899,8 @@ rewriteTargetListIU(List *targetList,
 
 				if (!apply_default)
 					ereport(ERROR,
-							(errcode(ERRCODE_SYNTAX_ERROR),
-							 errmsg("cannot insert into column \"%s\"",
+							(errcode(ERRCODE_GENERATED_ALWAYS),
+							 errmsg("cannot insert a non-DEFAULT value into column \"%s\"",
 									NameStr(att_tup->attname)),
 							 errdetail("Column \"%s\" is a generated column.",
 									   NameStr(att_tup->attname))));
@@ -923,17 +923,20 @@ rewriteTargetListIU(List *targetList,
 		 */
 		if (commandType == CMD_UPDATE)
 		{
-			if (att_tup->attidentity == ATTRIBUTE_IDENTITY_ALWAYS && new_tle && !apply_default)
+			if (att_tup->attidentity == ATTRIBUTE_IDENTITY_ALWAYS &&
+				new_tle && !apply_default)
 				ereport(ERROR,
 						(errcode(ERRCODE_GENERATED_ALWAYS),
-						 errmsg("column \"%s\" can only be updated to DEFAULT", NameStr(att_tup->attname)),
+						 errmsg("column \"%s\" can only be updated to DEFAULT",
+								NameStr(att_tup->attname)),
 						 errdetail("Column \"%s\" is an identity column defined as GENERATED ALWAYS.",
 								   NameStr(att_tup->attname))));
 
 			if (att_tup->attgenerated && new_tle && !apply_default)
 				ereport(ERROR,
-						(errcode(ERRCODE_SYNTAX_ERROR),
-						 errmsg("column \"%s\" can only be updated to DEFAULT", NameStr(att_tup->attname)),
+						(errcode(ERRCODE_GENERATED_ALWAYS),
+						 errmsg("column \"%s\" can only be updated to DEFAULT",
+								NameStr(att_tup->attname)),
 						 errdetail("Column \"%s\" is a generated column.",
 								   NameStr(att_tup->attname))));
 		}

src/test/regress/expected/generated.out

@@ -93,16 +93,16 @@ LINE 1: ...E gtest_err_8 (a int PRIMARY KEY, b int GENERATED BY DEFAULT...
 INSERT INTO gtest1 VALUES (1);
 INSERT INTO gtest1 VALUES (2, DEFAULT);  -- ok
 INSERT INTO gtest1 VALUES (3, 33);  -- error
-ERROR:  cannot insert into column "b"
+ERROR:  cannot insert a non-DEFAULT value into column "b"
 DETAIL:  Column "b" is a generated column.
 INSERT INTO gtest1 VALUES (3, 33), (4, 44);  -- error
-ERROR:  cannot insert into column "b"
+ERROR:  cannot insert a non-DEFAULT value into column "b"
 DETAIL:  Column "b" is a generated column.
 INSERT INTO gtest1 VALUES (3, DEFAULT), (4, 44);  -- error
-ERROR:  cannot insert into column "b"
+ERROR:  cannot insert a non-DEFAULT value into column "b"
 DETAIL:  Column "b" is a generated column.
 INSERT INTO gtest1 VALUES (3, 33), (4, DEFAULT);  -- error
-ERROR:  cannot insert into column "b"
+ERROR:  cannot insert a non-DEFAULT value into column "b"
 DETAIL:  Column "b" is a generated column.
 INSERT INTO gtest1 VALUES (3, DEFAULT), (4, DEFAULT);  -- ok
 SELECT * FROM gtest1 ORDER BY a;
@@ -193,25 +193,25 @@ SELECT * FROM gtest1v;
 (1 row)
 
 INSERT INTO gtest1v VALUES (4, 8);  -- error
-ERROR:  cannot insert into column "b"
+ERROR:  cannot insert a non-DEFAULT value into column "b"
 DETAIL:  Column "b" is a generated column.
 INSERT INTO gtest1v VALUES (5, DEFAULT);  -- ok
 INSERT INTO gtest1v VALUES (6, 66), (7, 77);  -- error
-ERROR:  cannot insert into column "b"
+ERROR:  cannot insert a non-DEFAULT value into column "b"
 DETAIL:  Column "b" is a generated column.
 INSERT INTO gtest1v VALUES (6, DEFAULT), (7, 77);  -- error
-ERROR:  cannot insert into column "b"
+ERROR:  cannot insert a non-DEFAULT value into column "b"
 DETAIL:  Column "b" is a generated column.
 INSERT INTO gtest1v VALUES (6, 66), (7, DEFAULT);  -- error
-ERROR:  cannot insert into column "b"
+ERROR:  cannot insert a non-DEFAULT value into column "b"
 DETAIL:  Column "b" is a generated column.
 INSERT INTO gtest1v VALUES (6, DEFAULT), (7, DEFAULT);  -- ok
 ALTER VIEW gtest1v ALTER COLUMN b SET DEFAULT 100;
 INSERT INTO gtest1v VALUES (8, DEFAULT);  -- error
-ERROR:  cannot insert into column "b"
+ERROR:  cannot insert a non-DEFAULT value into column "b"
 DETAIL:  Column "b" is a generated column.
 INSERT INTO gtest1v VALUES (8, DEFAULT), (9, DEFAULT);  -- error
-ERROR:  cannot insert into column "b"
+ERROR:  cannot insert a non-DEFAULT value into column "b"
 DETAIL:  Column "b" is a generated column.
 SELECT * FROM gtest1v;
  a | b  

src/test/regress/expected/identity.out

@@ -107,20 +107,20 @@ SELECT * FROM itest4;
 -- VALUES RTEs
 CREATE TABLE itest5 (a int generated always as identity, b text);
 INSERT INTO itest5 VALUES (1, 'a');  -- error
-ERROR:  cannot insert into column "a"
+ERROR:  cannot insert a non-DEFAULT value into column "a"
 DETAIL:  Column "a" is an identity column defined as GENERATED ALWAYS.
 HINT:  Use OVERRIDING SYSTEM VALUE to override.
 INSERT INTO itest5 VALUES (DEFAULT, 'a');  -- ok
 INSERT INTO itest5 VALUES (2, 'b'), (3, 'c');  -- error
-ERROR:  cannot insert into column "a"
+ERROR:  cannot insert a non-DEFAULT value into column "a"
 DETAIL:  Column "a" is an identity column defined as GENERATED ALWAYS.
 HINT:  Use OVERRIDING SYSTEM VALUE to override.
 INSERT INTO itest5 VALUES (DEFAULT, 'b'), (3, 'c');  -- error
-ERROR:  cannot insert into column "a"
+ERROR:  cannot insert a non-DEFAULT value into column "a"
 DETAIL:  Column "a" is an identity column defined as GENERATED ALWAYS.
 HINT:  Use OVERRIDING SYSTEM VALUE to override.
 INSERT INTO itest5 VALUES (2, 'b'), (DEFAULT, 'c');  -- error
-ERROR:  cannot insert into column "a"
+ERROR:  cannot insert a non-DEFAULT value into column "a"
 DETAIL:  Column "a" is an identity column defined as GENERATED ALWAYS.
 HINT:  Use OVERRIDING SYSTEM VALUE to override.
 INSERT INTO itest5 VALUES (DEFAULT, 'b'), (DEFAULT, 'c');  -- ok
@@ -197,7 +197,7 @@ SELECT * FROM itest1;
 -- GENERATED ALWAYS
 -- This is an error:
 INSERT INTO itest2 VALUES (10, 'xyz');
-ERROR:  cannot insert into column "a"
+ERROR:  cannot insert a non-DEFAULT value into column "a"
 DETAIL:  Column "a" is an identity column defined as GENERATED ALWAYS.
 HINT:  Use OVERRIDING SYSTEM VALUE to override.
 -- This inserts the row as presented:
@@ -313,7 +313,7 @@ SELECT * FROM itestv10;
 (4 rows)
 
 INSERT INTO itestv11 VALUES (10, 'xyz');
-ERROR:  cannot insert into column "a"
+ERROR:  cannot insert a non-DEFAULT value into column "a"
 DETAIL:  Column "a" is an identity column defined as GENERATED ALWAYS.
 HINT:  Use OVERRIDING SYSTEM VALUE to override.
 INSERT INTO itestv11 OVERRIDING SYSTEM VALUE VALUES (11, 'xyz');

src/test/regress/expected/updatable_views.out

@@ -1475,10 +1475,10 @@ INSERT INTO rw_view1 (id) VALUES (2);
 INSERT INTO base_tbl (id, idplus1) VALUES (3, DEFAULT);
 INSERT INTO rw_view1 (id, idplus1) VALUES (4, DEFAULT);
 INSERT INTO base_tbl (id, idplus1) VALUES (5, 6);  -- error
-ERROR:  cannot insert into column "idplus1"
+ERROR:  cannot insert a non-DEFAULT value into column "idplus1"
 DETAIL:  Column "idplus1" is a generated column.
 INSERT INTO rw_view1 (id, idplus1) VALUES (6, 7);  -- error
-ERROR:  cannot insert into column "idplus1"
+ERROR:  cannot insert a non-DEFAULT value into column "idplus1"
 DETAIL:  Column "idplus1" is a generated column.
 SELECT * FROM base_tbl;
  id | idplus1