What is the point of "grant select on sequence api.todos_id_seq to my_user"?

Question

In step one on this page it says to add select on sequence. Being relatively new to Postgres, what is a sequence and why does it need separate permissions and why is there what looks like an extra table for the sequence I didn't create?

-- run this in psql using the database created
-- in the previous tutorial

create role todo_user nologin;
grant todo_user to authenticator;

grant usage on schema api to todo_user;
grant all on api.todos to todo_user;
grant usage, select on sequence api.todos_id_seq to todo_user;

Answer 1

I should mark the question as "needs focus", since you ask three questions, but it is simple enough:

• A sequence is an object that efficiently generates unique numbers, even when used by many sessions in parallel.

• Sequences have permissions just like all other objects. By default, only the owner (the user that created the sequence) can use it. If you grant SELECT on a sequence to a user, that user can read the current value from the sequence, but it cannot use the nextval function to get the next value.

• Sequences are “relations” in PostgreSQL (that is, something with columns, whose metadata are stored in pg_class). This is an implementation detail of PostgreSQL that changed somewhat in v10. The “table” now contains a single row that contains those metadata that are changed by nextval.