What happens when you declare a field as a primary key in PostgreSQL?

When you declare a field as a primary key in PostgreSQL, the following happens:

• A uniqueness constraint (UNIQUE) is imposed on this field, meaning the values in this column must be unique.
• The field automatically becomes NOT NULL — NULL values are not allowed in a primary key.
• To ensure fast search and uniqueness verification, a unique index is created on this field.

Thus, the primary key guarantees the unique identification of each row in the table and ensures data integrity.

Example:

CREATE TABLE users (
    id SERIAL PRIMARY KEY,
    name TEXT
);

Here, id is the primary key, unique, and cannot be NULL.