Hi,
I saw a user wanted to restrict the rows copied by COPY TO using a WHERE
clause, but ran into an error.
As you know, COPY FROM supports WHERE, but COPY TO does not:
=# copy t1 to stdout where i > 5;
ERROR: WHERE clause not allowed with COPY TO
LINE 1: copy t1 to stdout where i > 5;
^
In such cases, we can do with specifying a query as the target of COPY:
=# copy (select i from t1 where i > 5) to stdout;
However, as shown in the first example, no hint is provided in the error
message.
For views or others, COPY TO already provides a helpful hint message:
=# copy v1 to stdout;
ERROR: cannot copy from view "v1"
HINT: Try the COPY (SELECT ...) TO variant.
Considering what the user was trying to do, it might be helpful to
provide a similar hint in this case as well.
I’ve attached a patch that adds such a hint.
What do you think?
--
Regards,
--
Atsushi Torikoshi
Seconded from NTT DATA Japan Corporation to SRA OSS K.K.
From d1b4e23a1b24782e9de16d748508c46354e42423 Mon Sep 17 00:00:00 2001
From: Atsushi Torikoshi <torikos...@oss.nttdata.com>
Date: Sun, 3 Aug 2025 17:55:09 +0900
Subject: [PATCH v1] COPY TO: provide hint when WHERE clause is used
Provide a hint suggesting COPY query TO when a WHERE clause
is specified, similar to the hint already given for views or others.
---
src/backend/parser/gram.y | 1 +
1 file changed, 1 insertion(+)
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index db43034b9db..9fd48acb1f8 100644
--- a/src/backend/parser/gram.y
+++ b/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;
--
2.48.1
