update test1 set user_name = 'c' where user_name = 'a' returning uid;

create table test1 (
	uid bigserial not null primary key,
	user_name varchar(1000) null
);

insert into test1(user_name) values('a');
insert into test1(user_name) values('a');
insert into test1(user_name) values('a');
insert into test1(user_name) values('a');
insert into test1(user_name) values('b');
insert into test1(user_name) values('a');
insert into test1(user_name) values('b');
insert into test1(user_name) values('b');
insert into test1(user_name) values('b');

select * from test1;

update test1 set user_name = 'c' where user_name = 'a' returning uid;