Why UPDATE permission does not work without SELECT permission in PostgreSQL?

Question

What is the purpose of UPDATE permission if it does not allow to update without SELECT permission?

Generally, I understand that update internally causes select to find target rows, but this "internal select" does not leak to a user, so it is unclear if it is a bug or there is a "deeper meaning" of that.

Assume initially my_user has only USAGE on my_schema and no grants on my_table

Case 1:

GRANT UPDATE ON TABLE my_schema.my_table TO my_user;

UPDATE my_table
   SET my_col = 'X';

>> SQL Error [42501]: ERROR: permission denied for table my_table

Case 2:

GRANT SELECT ON TABLE my_schema.my_table TO my_user;
GRANT UPDATE ON TABLE my_schema.my_table TO my_user;

UPDATE my_table
   SET my_col = 'X';

>> SUCCESS

Answer 1

Your assumption about "internal select" does not leak to a user is erroneous.

The returning keyword is very powerful and can be used to proxy a select statement:

UPDATE my_table
SET my_col = my_col
RETURNING *;

--> will show the same as select * from my_table

However, the select privilege is required only because the value of my_col is read. If you were to use a constant instead, it would work with just the update privilege and it wouldn't let you return the row.

REVOKE select ON my_table FROM my_user;

update my_table set my_col =1;
UPDATE 7
update my_table set my_col =1 returning *;
ERROR:  permission denied for table test