Improve plpgsql's error reporting for no-such-column cases.

Given a column reference foo.bar, where there is a composite plpgsql
variable foo but it doesn't contain a column bar, the pre-9.0 coding would
immediately throw a "record foo has no field bar" error.  In 9.0 the parser
hook instead falls through to let the core parser see if it can resolve the
reference.  If not, you get a complaint about "missing FROM-clause entry
for table foo", which while in some sense correct isn't terribly helpful.
Complicate things a bit so that we can throw the old error message if
neither the core parser nor the hook are able to resolve the column
reference, while not changing the behavior in any other case.
Per bug #5757 from Andrey Galkin.

src/pl/plpgsql/src/pl_comp.c

@@ -99,7 +99,8 @@ static void add_dummy_return(PLpgSQL_function *function);
 static Node *plpgsql_pre_column_ref(ParseState *pstate, ColumnRef *cref);
 static Node *plpgsql_post_column_ref(ParseState *pstate, ColumnRef *cref, Node *var);
 static Node *plpgsql_param_ref(ParseState *pstate, ParamRef *pref);
-static Node *resolve_column_ref(PLpgSQL_expr *expr, ColumnRef *cref);
+static Node *resolve_column_ref(ParseState *pstate, PLpgSQL_expr *expr,
+				   ColumnRef *cref, bool error_if_no_field);
 static Node *make_datum_param(PLpgSQL_expr *expr, int dno, int location);
 static PLpgSQL_row *build_row_from_class(Oid classOid);
 static PLpgSQL_row *build_row_from_vars(PLpgSQL_variable **vars, int numvars);
@@ -945,7 +946,7 @@ plpgsql_pre_column_ref(ParseState *pstate, ColumnRef *cref)
 	PLpgSQL_expr *expr = (PLpgSQL_expr *) pstate->p_ref_hook_state;
 
 	if (expr->func->resolve_option == PLPGSQL_RESOLVE_VARIABLE)
-		return resolve_column_ref(expr, cref);
+		return resolve_column_ref(pstate, expr, cref, false);
 	else
 		return NULL;
 }
@@ -965,7 +966,17 @@ plpgsql_post_column_ref(ParseState *pstate, ColumnRef *cref, Node *var)
 	if (expr->func->resolve_option == PLPGSQL_RESOLVE_COLUMN && var != NULL)
 		return NULL;			/* there's a table column, prefer that */
 
-	myvar = resolve_column_ref(expr, cref);
+	/*
+	 * If we find a record/row variable but can't match a field name, throw
+	 * error if there was no core resolution for the ColumnRef either.  In
+	 * that situation, the reference is inevitably going to fail, and
+	 * complaining about the record/row variable is likely to be more
+	 * on-point than the core parser's error message.  (It's too bad we
+	 * don't have access to transformColumnRef's internal crerr state here,
+	 * as in case of a conflict with a table name this could still be less
+	 * than the most helpful error message possible.)
+	 */
+	myvar = resolve_column_ref(pstate, expr, cref, (var == NULL));
 
 	if (myvar != NULL && var != NULL)
 	{
@@ -1010,9 +1021,13 @@ plpgsql_param_ref(ParseState *pstate, ParamRef *pref)
  * resolve_column_ref		attempt to resolve a ColumnRef as a plpgsql var
  *
  * Returns the translated node structure, or NULL if name not found
+ *
+ * error_if_no_field tells whether to throw error or quietly return NULL if
+ * we are able to match a record/row name but don't find a field name match.
  */
 static Node *
-resolve_column_ref(PLpgSQL_expr *expr, ColumnRef *cref)
+resolve_column_ref(ParseState *pstate, PLpgSQL_expr *expr,
+				   ColumnRef *cref, bool error_if_no_field)
 {
 	PLpgSQL_execstate *estate;
 	PLpgSQL_nsitem *nse;
@@ -1147,9 +1162,16 @@ resolve_column_ref(PLpgSQL_expr *expr, ColumnRef *cref)
 				/*
 				 * We should not get here, because a RECFIELD datum should
 				 * have been built at parse time for every possible qualified
-				 * reference to fields of this record.	But if we do, fall out
-				 * and return NULL.
+				 * reference to fields of this record.	But if we do, handle
+				 * it like field-not-found: throw error or return NULL.
 				 */
+				if (error_if_no_field)
+					ereport(ERROR,
+							(errcode(ERRCODE_UNDEFINED_COLUMN),
+							 errmsg("record \"%s\" has no field \"%s\"",
+									(nnames_field == 1) ? name1 : name2,
+									colname),
+							 parser_errposition(pstate, cref->location)));
 			}
 			break;
 		case PLPGSQL_NSTYPE_ROW:
@@ -1170,7 +1192,14 @@ resolve_column_ref(PLpgSQL_expr *expr, ColumnRef *cref)
 												cref->location);
 					}
 				}
-				/* Not found, so return NULL */
+				/* Not found, so throw error or return NULL */
+				if (error_if_no_field)
+					ereport(ERROR,
+							(errcode(ERRCODE_UNDEFINED_COLUMN),
+							 errmsg("record \"%s\" has no field \"%s\"",
+									(nnames_field == 1) ? name1 : name2,
+									colname),
+							 parser_errposition(pstate, cref->location)));
 			}
 			break;
 		default: