An identity column is a special column that is generated automatically from an implicit sequence. It can be used to generate key values.
   To create an identity column, use the GENERATED ...
   AS IDENTITY clause in CREATE TABLE, for example:
CREATE TABLE people (
    id bigint GENERATED ALWAYS AS IDENTITY,
    ...,
);
or alternatively
CREATE TABLE people (
    id bigint GENERATED BY DEFAULT AS IDENTITY,
    ...,
);
See CREATE TABLE for more details.
   If an INSERT command is executed on the table with the
   identity column and no value is explicitly specified for the identity
   column, then a value generated by the implicit sequence is inserted.  For
   example, with the above definitions and assuming additional appropriate
   columns, writing
INSERT INTO people (name, address) VALUES ('A', 'foo');
INSERT INTO people (name, address) VALUES ('B', 'bar');
   would generate values for the id column starting at 1
   and result in the following table data:
id | name | address ----+------+--------- 1 | A | foo 2 | B | bar
   Alternatively, the keyword DEFAULT can be specified in
   place of a value to explicitly request the sequence-generated value, like
INSERT INTO people (id, name, address) VALUES (DEFAULT, 'C', 'baz');
   Similarly, the keyword DEFAULT can be used in
   UPDATE commands.
  
Thus, in many ways, an identity column behaves like a column with a default value.
   The clauses ALWAYS and BY DEFAULT in
   the column definition determine how explicitly user-specified values are
   handled in INSERT and UPDATE
   commands.  In an INSERT command, if
   ALWAYS is selected, a user-specified value is only
   accepted if the INSERT statement specifies
   OVERRIDING SYSTEM VALUE.  If BY
   DEFAULT is selected, then the user-specified value takes
   precedence.  Thus, using BY DEFAULT results in a
   behavior more similar to default values, where the default value can be
   overridden by an explicit value, whereas ALWAYS provides
   some more protection against accidentally inserting an explicit value.
  
The data type of an identity column must be one of the data types supported by sequences. (See CREATE SEQUENCE.) The properties of the associated sequence may be specified when creating an identity column (see CREATE TABLE) or changed afterwards (see ALTER TABLE).
   An identity column is automatically marked as NOT NULL.
   An identity column, however, does not guarantee uniqueness.  (A sequence
   normally returns unique values, but a sequence could be reset, or values
   could be inserted manually into the identity column, as discussed above.)
   Uniqueness would need to be enforced using a PRIMARY KEY
   or UNIQUE constraint.
  
   In table inheritance hierarchies, identity columns and their properties in
   a child table are independent of those in its parent tables.  A child table
   does not inherit identity columns or their properties automatically from
   the parent. During INSERT or UPDATE,
   a column is treated as an identity column if that column is an identity
   column in the table named in the statement, and the corresponding identity
   properties are applied.
  
Partitions inherit identity columns from the partitioned table. They cannot have their own identity columns. The properties of a given identity column are consistent across all the partitions in the partition hierarchy.