[CF 6016] v2 - COPY TO: provide hint when WHERE clause is used

This branch was automatically generated by a robot using patches from an
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https://commitfest.postgresql.org/patch/6016

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Patch(es): https://www.postgresql.org/message-id/CAM6-o=DVOd-jpO6WparhuPjOG5QkNiyspdPzeOrDT6x_JAqDMw@mail.gmail.com
Author(s): Atsushi Torikoshi

Author: Commitfest Bot

src/backend/parser/gram.y

@@ -3442,6 +3442,7 @@ CopyStmt:	COPY opt_binary qualified_name opt_column_list
 						ereport(ERROR,
 								(errcode(ERRCODE_SYNTAX_ERROR),
 								 errmsg("WHERE clause not allowed with COPY TO"),
+								 errhint("Try the COPY (SELECT ... WHERE ...) TO variant."),
 								 parser_errposition(@11)));
 
 					n->options = NIL;

src/test/regress/expected/copy2.out

@@ -163,6 +163,7 @@ COPY x TO stdout WHERE a = 1;
 ERROR:  WHERE clause not allowed with COPY TO
 LINE 1: COPY x TO stdout WHERE a = 1;
                          ^
+HINT:  Try the COPY (SELECT ... WHERE ...) TO variant.
 COPY x from stdin WHERE a = 50004;
 COPY x from stdin WHERE a > 60003;
 COPY x from stdin WHERE f > 60003;