Only evaluate default values as required when doing COPY FROM

Commit 9f8377f7a2 was a little too eager in fetching default values.
Normally this would not matter, but if the default value is not valid
for the type (e.g. a varchar that's too long) it caused an unnecessary
error.

Complaint and fix from Laurenz Albe

Backpatch to release 16.

Discussion: https://postgr.es/m/75a7b7483aeb331aa017328d606d568fc715b90d.camel@cybertec.at

src/backend/commands/copyfrom.c

@@ -1571,7 +1571,14 @@ BeginCopyFrom(ParseState *pstate,
 		/* Get default info if available */
 		defexprs[attnum - 1] = NULL;
 
-		if (!att->attgenerated)
+		/*
+		 * We only need the default values for columns that do not appear in
+		 * the column list, unless the DEFAULT option was given. We never need
+		 * default values for generated columns.
+		 */
+		if ((cstate->opts.default_print != NULL ||
+			 !list_member_int(cstate->attnumlist, attnum)) &&
+			!att->attgenerated)
 		{
 			Expr	   *defexpr = (Expr *) build_column_default(cstate->rel,
 																attnum);

src/test/regress/expected/copy.out

@@ -240,3 +240,20 @@ SELECT * FROM header_copytest ORDER BY a;
 (5 rows)
 
 drop table header_copytest;
+-- test COPY with overlong column defaults
+create temp table oversized_column_default (
+    col1 varchar(5) DEFAULT 'more than 5 chars',
+    col2 varchar(5));
+-- normal COPY should work
+copy oversized_column_default from stdin;
+-- error if the column is excluded
+copy oversized_column_default (col2) from stdin;
+ERROR:  value too long for type character varying(5)
+\.
+invalid command \.
+-- error if the DEFAULT option is given
+copy oversized_column_default from stdin (default '');
+ERROR:  value too long for type character varying(5)
+\.
+invalid command \.
+drop table oversized_column_default;

src/test/regress/sql/copy.sql

@@ -268,3 +268,18 @@ a	c	b
 
 SELECT * FROM header_copytest ORDER BY a;
 drop table header_copytest;
+
+-- test COPY with overlong column defaults
+create temp table oversized_column_default (
+    col1 varchar(5) DEFAULT 'more than 5 chars',
+    col2 varchar(5));
+-- normal COPY should work
+copy oversized_column_default from stdin;
+\.
+-- error if the column is excluded
+copy oversized_column_default (col2) from stdin;
+\.
+-- error if the DEFAULT option is given
+copy oversized_column_default from stdin (default '');
+\.
+drop table oversized_column_default;