unable to update a single column in upsert: postgres

Question

I came across a scenario where I would be trying to update a single column using UPSERT statement. But I get the error:

ERROR: source for a multiple-column UPDATE item must be a sub-SELECT or ROW() expression

you can use below snippet to reproduce the issue:

drop table if exists source_t;

create table source_t as
(select 1 id, 'dummy1' varcol union
select 3,'dummy3');

drop table if exists dest_t;

create table dest_t as
(select 1 id, 'mmy' varcol
union select 2,'dummy2');



   ALTER TABLE dest_t ADD CONSTRAINT UNQ_CON_dest_t unique  (id);

/***Below does not work***/
insert into    dest_t 
select * from source_t
on conflict on constraint unq_con_dest_t 
do update set (varcol)=( excluded.varcol);

/***Below is working***/
insert into    dest_t 
select * from source_t
on conflict on constraint unq_con_dest_t 
do update set (varcol)=(select excluded.varcol);

/**** Also if the table had another column and if I mentioned it in update , then "select " clause would not be necessary as well***/

Concluding, if the update clause has more than one column it is not necessary to use the "select " clause. Has anyone experienced the same issue by any chance? Is it something known thing/bug in postgres.?

Answer 1

The issue was with usage of brackets after "update set" , below query works without "select" requiring in update:

insert into    dest_t 
select * from source_t
on conflict on constraint unq_con_dest_t 
do update set varcol=excluded.varcol

Also pasting proper justification provided by @a_horse_with_no_name from his comment:

Don't use a row notation for a single column. set (varcol)=( excluded.varcol); should be set varcol=excluded.varcol; The expression varcol is a single column. The expression (varcol) is an anonymous record type with a single field