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.?

• user1822: 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.
• ypercubeᵀᴹ: @a_horse_with_no_name I believe the problem is not with the parenthese on the left side of the = sign, (varcol) = but with the right side: = (excluded.varcol). This does not parse as a row expression. If one uses set (varcol) = ROW(excluded.varcol), it should work fine.

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

• ypercubeᵀᴹ: This can be used as well: set (varcol) = ROW(excluded.varcol)