Difference between a procedure and a function in PostgreSQL.

In PostgreSQL, both procedures and functions are database objects that encapsulate a series of SQL and procedural statements. However, they have some differences in terms of their behavior and usage:

1. Return Type:

   • Function: Functions must have a specified return type. They return a value and can be used in SQL queries like any other expression.
   • Procedure: Procedures do not have a return type. They typically perform an action or a series of actions without returning a value directly.

2. Usage in Queries:

   • Function: Functions can be called from within SQL queries and used in expressions. They can be part of a SELECT statement, WHERE clause, or used in other SQL expressions.
   • Procedure: Procedures are generally called using the CALL statement. They are not used in the same way as functions within SQL queries.

3. Transaction Control:

   • Function: Functions can be part of a transaction and can participate in the transaction control statements (e.g., COMMIT or ROLLBACK).
   • Procedure: Procedures can also be part of a transaction, and they can include transaction control statements. However, procedures are often used for encapsulating a series of actions that may include multiple transactions.

4. Atomicity:

   • Functions are atomic, meaning they are executed within a single transaction and cannot be interrupted.
   • Procedures are not atomic and can be interrupted or rolled back.

5. Data Modification:

   • Functions can modify data within the database.
   • Procedures can also modify data within the database.

Example illustrating the differences:

-- Function example
CREATE FUNCTION get_employee_count() RETURNS INTEGER AS $$
DECLARE
    emp_count INTEGER;
BEGIN
    SELECT COUNT(*) INTO emp_count FROM employees;
    RETURN emp_count;
END;
$$ LANGUAGE plpgsql;

-- Procedure example
CREATE PROCEDURE update_employee_salary(amount INT) AS $$
BEGIN
    UPDATE employees SET salary = salary + amount;
END;
$$ LANGUAGE plpgsql;

To call the function, you would use it in a SQL query:

SELECT get_employee_count();

To call the procedure, you would use the CALL statement:

CALL update_employee_salary(500);

Summary: functions are primarily used to compute and return values, while procedures are used to perform actions or tasks.