PostgreSQL Serial vs Identity

serial Data Type:

• Default Behavior: The serial data type is equivalent to using the bigint data type and defining a default value using the nextval() function of the associated sequence.
• Implicit Usage: You don't need to explicitly reference the sequence in your INSERT statements. The serial data type handles the sequence generation and updates the column value automatically.
• Automatic Sequence Generation: When you define a column as serial in PostgreSQL, it automatically creates a sequence object with the same name as the column. This sequence is used to generate unique integer values for the column.

Example:

CREATE TABLE my_table (
    id serial PRIMARY KEY,
    name text
);

identity Data Type:

• Flexibility: The identity data type provides more flexibility compared to serial as you can customize the sequence properties (e.g., start value, increment, cache size) independently from the column definition.
• Column Default: The identity data type defines the column's default value as the result of the nextval() function for the specified sequence.
• Explicit Sequence Definition: Unlike serial, you need to explicitly create a sequence object before using the identity data type. The sequence name is typically specified as part of the column definition.

CREATE SEQUENCE my_sequence START WITH 1 INCREMENT BY 1;

CREATE TABLE my_table (
    id bigint GENERATED ALWAYS AS IDENTITY (SEQUENCE my_sequence),
    name text
);

Key Differences:

• Usage: serial is often used for simple auto-incrementing primary keys, while identity provides more control and can be used in various scenarios.
• Flexibility: identity offers more flexibility in customizing sequence properties.
• Sequence Creation: serial implicitly creates a sequence, while identity requires explicit creation.

Choosing Between serial and identity:

• Customization: If you require more control over the sequence behavior or want to use the same sequence for multiple columns, identity is preferable.
• Simplicity: If you need a straightforward auto-incrementing primary key, serial is a good choice.

Additional Notes:

• PostgreSQL also supports other sequence-based mechanisms like triggers and functions for more complex scenarios.
• Both serial and identity are primarily used for generating unique integer values for primary keys or other identifying columns.

Example 1: Using serial

CREATE TABLE my_table (
    id serial PRIMARY KEY,
    name text
);

• Explanation: This code creates a table named my_table with two columns:

  • id: A serial data type column that automatically generates unique integer values for each row. It acts as the primary key, ensuring data integrity.
  • name: A text data type column for storing text data.

CREATE SEQUENCE my_sequence START WITH 1 INCREMENT BY 1;

CREATE TABLE my_table (
    id bigint GENERATED ALWAYS AS IDENTITY (SEQUENCE my_sequence),
    name text
);

• • id: A bigint data type column that uses the identity keyword to reference the my_sequence sequence for generating unique values.

Alternative Methods for Sequence Generation in PostgreSQL

While serial and identity are the primary methods for generating sequences in PostgreSQL, there are alternative approaches that you can consider depending on your specific requirements:

Explicit Sequence Creation and Usage:

• Manual Reference: Refer to the sequence in your INSERT statements using the nextval() function to retrieve the next value.
• Sequence Definition: Create a sequence object explicitly using the CREATE SEQUENCE command.

CREATE SEQUENCE my_sequence START WITH 1 INCREMENT BY 1;

INSERT INTO my_table (id, name) VALUES (nextval('my_sequence'), 'John Doe');

Triggers:

• Sequence Manipulation: Within the trigger, use the nextval() function to retrieve the next value from a sequence and assign it to a column.
• Row-Level Triggers: Create a trigger that fires before or after each INSERT or UPDATE operation on a table.

CREATE SEQUENCE my_sequence START WITH 1 INCREMENT BY 1;

CREATE TRIGGER my_trigger BEFORE INSERT ON my_table
FOR EACH ROW
EXECUTE PROCEDURE nextval('my_sequence');

Functions:

• Function Calls: Call the function in your INSERT statements to retrieve the next value.
• Custom Functions: Define a function that encapsulates the sequence generation logic.

CREATE FUNCTION next_id() RETURNS bigint AS $$
DECLARE
  next_value bigint;
BEGIN
  SELECT nextval('my_sequence') INTO next_value;
  RETURN next_value;
END;
$$ LANGUAGE plpgsql;

INSERT INTO my_table (id, name) VALUES (next_id(), 'Jane Doe');

Database Sequences:

• Limited Customization: These sequences may have limited customization options compared to user-defined sequences.
• System-Provided Sequences: Utilize system-provided sequences like pg_sequence for automatic sequence generation.

INSERT INTO my_table (id, name) VALUES (nextval('pg_sequence.my_table_id_seq'), 'John Doe');

Choosing the Right Method:

• Performance: For high-performance scenarios, consider using system-provided sequences or optimizing your custom sequence implementation.
• Customization: If you need more control over sequence behavior or want to integrate sequence generation with other database operations, triggers or functions might be suitable.
• Simplicity: serial and identity are generally the simplest and most convenient options.