Add hint about not qualifying UPDATE...SET target with relation name.

Target columns in UPDATE ... SET must not be qualified with the target
table; we disallow this because it'd create ambiguity about which name
is the column name in case of field-qualified names.  However, newbies
have been seen to expect that they could qualify a target name just
like other names.  The error message when they do is confusing:
"column "foo" of relation "foo" does not exist".  To improve matters,
issue a HINT if the invalid name is qualified and matches the
relation's alias.

James Coleman (editorialized a bit by me)

Discussion: https://postgr.es/m/CAAaqYe8S2Qa060UV-YF5GoSd5PkEhLV94x-fEi3=TOtpaXCV+w@mail.gmail.com

src/backend/parser/analyze.c

@@ -2518,6 +2518,9 @@ transformUpdateTargetList(ParseState *pstate, List *origTlist)
 					 errmsg("column \"%s\" of relation \"%s\" does not exist",
 							origTarget->name,
 							RelationGetRelationName(pstate->p_target_relation)),
+					 (origTarget->indirection != NIL &&
+					  strcmp(origTarget->name, pstate->p_target_nsitem->p_names->aliasname) == 0) ?
+					 errhint("SET target columns cannot be qualified with the relation name.") : 0,
 					 parser_errposition(pstate, origTarget->location)));
 
 		updateTargetListEntry(pstate, tle, origTarget->name,

src/test/regress/expected/insert_conflict.out

@@ -272,6 +272,12 @@ ERROR:  invalid reference to FROM-clause entry for table "insertconflicttest"
 LINE 1: ...onfruit') on conflict (key) do update set fruit = insertconf...
                                                              ^
 HINT:  Perhaps you meant to reference the table alias "ict".
+-- Check helpful hint when qualifying set column with target table
+insert into insertconflicttest values (3, 'Kiwi') on conflict (key, fruit) do update set insertconflicttest.fruit = 'Mango';
+ERROR:  column "insertconflicttest" of relation "insertconflicttest" does not exist
+LINE 1: ...3, 'Kiwi') on conflict (key, fruit) do update set insertconf...
+                                                             ^
+HINT:  SET target columns cannot be qualified with the relation name.
 drop index key_index;
 --
 -- Composite key tests

src/test/regress/expected/update.out

@@ -44,6 +44,12 @@ SELECT * FROM update_test;
  10 | 20 | 
 (2 rows)
 
+-- error, you're not supposed to qualify the target column
+UPDATE update_test t SET t.b = t.b + 10 WHERE t.a = 10;
+ERROR:  column "t" of relation "update_test" does not exist
+LINE 1: UPDATE update_test t SET t.b = t.b + 10 WHERE t.a = 10;
+                                 ^
+HINT:  SET target columns cannot be qualified with the relation name.
 --
 -- Test VALUES in FROM
 --

src/test/regress/sql/insert_conflict.sql

@@ -118,6 +118,9 @@ insert into insertconflicttest AS ict values (6, 'Passionfruit') on conflict (ke
 insert into insertconflicttest AS ict values (6, 'Passionfruit') on conflict (key) do update set fruit = ict.fruit; -- ok, alias
 insert into insertconflicttest AS ict values (6, 'Passionfruit') on conflict (key) do update set fruit = insertconflicttest.fruit; -- error, references aliased away name
 
+-- Check helpful hint when qualifying set column with target table
+insert into insertconflicttest values (3, 'Kiwi') on conflict (key, fruit) do update set insertconflicttest.fruit = 'Mango';
+
 drop index key_index;
 
 --

src/test/regress/sql/update.sql

@@ -31,6 +31,9 @@ UPDATE update_test t SET b = t.b + 10 WHERE t.a = 10;
 
 SELECT * FROM update_test;
 
+-- error, you're not supposed to qualify the target column
+UPDATE update_test t SET t.b = t.b + 10 WHERE t.a = 10;
+
 --
 -- Test VALUES in FROM
 --