PostgreSQL database supports 2 types for auto-generated primary keys in tables:

• serial (supported in almost all PostgreSQL versions)
• identity (available from PostgreSQL 10+)

These types can be used as follows:

create table table1 (id serial primary key);
create table table2 (id integer primary key generated always as identity);

They provide the same functionality, but identity is more strict when it comes to creating new records. Consider the two following examples:

insert into table1 (id) values (1);
insert into table2 (id) values (1);

The former succeeds, while the latter raises the following error:

ERROR: cannot insert a non-DEFAULT value into column "id"
DETAIL: Column "id" is an identity column defined as GENERATED ALWAYS.

Even though the first statement succeeded, it will raise an error if we try to create a new record in table1 without providing an id. This is caused by the internal counter being out of sync with the table data:

insert into table1 DEFAULT VALUES

ERROR: duplicate key value violates unique constraint "table1_pkey"
DETAIL: Key (id)=(1) already exists.

As we see, serial types can be tricked to cause troubles with new records. identity has one additional advantage: fewer permissions are required to create a new record - with serial type, a role needs INSERT privilege on the table and USAGE privilege on the sequence. With identity, just INSERT access is enough.