test=# create table u (i int, v int);
CREATE TABLE
Time: 0,668 ms
test=*# copy u (i) from stdin;
Enter data to be copied followed by a newline.
End with a backslash and a period on a line by itself.
>> 1
>> 2
>> 3
>> \.
Time: 3045,274 ms
test=*# update u set v = 5 where i in (select i from u);
UPDATE 3
Time: 0,440 ms
test=*# select * from u;
 i | v
---+---
 1 | 5
 2 | 5
 3 | 5
(3 rows)