PostgreSQL - BEGIN

The BEGIN command in PostgreSQL is essential for transaction management, allowing a sequence of SQL operations to be executed as a single unit of work. This ensures data consistency and reliability by grouping operations together and enabling us to commit or rollback the entire set of changes.

This article explains the BEGIN command in PostgreSQL, explaining its syntax, usage, examples, and key points for better understanding.

What is BEGIN Command in PostgreSQL?

In PostgreSQL, the BEGIN command initiates a transaction block. Transactions allow us to execute multiple SQL operations as a single atomic unit, meaning either all operations within the block succeed or, if an error occurs, none are applied. This approach ensures data integrity.

By default, PostgreSQL transactions are auto-commit, but to end the transaction block we need to give either COMMIT or ROLLBACK commands. Statements inside the transaction block execute faster than usual because the CPU uses special disk computation for defining transactions.

Syntax

BEGIN;
// statements

-- or

BEGIN TRANSACTION;
// statements

Examples of PostgreSQL BEGIN Statement

Let us take a look at an example of BEGIN Statement in PostgreSQL to better understand the concept. Firstly, we have to create a sample table and go through various scenarios using the below commands to understand how transactions work.

Creating a Sample Table

CREATE TABLE students (
    student_id serial PRIMARY KEY,
    full_name VARCHAR NOT NULL,
    marks INT
);

Inserting Initial Data

INSERT INTO students (
    student_id,
    full_name,
    marks
)
VALUES
    (1, 'Rahul Kumar', NULL),
    (2, 'Abishek Nayak', 5),
    (3, 'Chandra Gupta', 6),
    (4, 'Sanju Sharma', 8);

Now that the table is ready we can look into some examples.

Example 1: Inserting a New Record Within a Transaction

The following example demonstrates how to insert a new record to the 'students table' within a transaction block using BEGIN and COMMIT

Query:

BEGIN;

 INSERT INTO students ( student_id, full_name, marks )
    VALUES
( 5, 'Mehboob Dilse', 10);
        
COMMIT;

Output

Explanation:

This transaction begins with the BEGIN command, adds a new record into the student table. The transaction is then completed with COMMIT, making the insertion permanent. If any errors had occurred before the COMMIT, the transaction could be .

Example 2: Updating a Record within a Transaction Block

The following example shows how to update a record inside a transaction block. This transaction updates the marks of the student with student_id = 1.

Query:

BEGIN;
    UPDATE students
    SET marks = 2
    WHERE 
    student_id = 1 ;
        
COMMIT;

Output

Explanation:

This transaction updates the marks of the student with student_id 1 and then commits the change, ensuring the update is saved permanently.

Important Points About BEGIN Statement in PostgreSQL

• The BEGIN command is used to initiate a transaction in PostgreSQL, marking the start of a transaction block where multiple SQL statements can be executed as a single unit of work.
  
• To end a transaction block, use the COMMIT command to save all changes made within the transaction. Use the ROLLBACK command to undo all changes if something goes wrong.
  
• Using BEGIN allows us to explicitly control the transaction boundaries.

Conclusion

The BEGIN command in PostgreSQL is an important tool for managing transactions, ensuring data integrity, and optimizing performance by grouping statements into a single unit of execution. COMMIT and ROLLBACK commands help us control transaction outcomes, making PostgreSQL a powerful and flexible database management system.