The DeadLock in a database system is a very common thing and all Database Administrator responsible to detect and manage a DeadLock situation.

In this post, I am sharing one small demonstration t to generate a DeadLock situation in a PostgreSQL.

Database Theory: What is Timeout, Lock, Block, and Deadlock in a Database System?

Most of the time, We are facing DeadLock problem in our Production Database Server.
But What happen, when Database Administrator wants to generate DeadLock situation and wants to create a script for detecting a DeadLock.

First create two sample tables:

CREATE TABLE ABC (ID INT PRIMARY KEY);
CREATE TABLE XYZ (ID INT PRIMARY KEY);

Open Session – 1 and execute below code:

BEGIN;
INSERT INTO ABC VALUES (1);

Open Session – 2 and execute below code:

BEGIN;
INSERT INTO XYZ VALUES (1);
INSERT INTO ABC VALUES (1);

Again open Session – 1 and execute below Insert Statement:

INSERT INTO XYZ VALUES (1);

You will get a below deadlock message:

ERROR:  deadlock detected
DETAIL:  Process 5944 waits for ShareLock on transaction 892; blocked by process 4884.
Process 4884 waits for ShareLock on transaction 891; blocked by process 5944.
HINT:  See server log for query details.
CONTEXT:  while inserting index tuple (0,2) in relation "xyz_pkey"

Please visit other related articles:

PostgreSQL 9.5: Using FOR UPDATE SKIP LOCKED Option SELECT only Committed Records

PostgreSQL: Script to find information about the Locks held by Open Transactions (pg_locks)

Script to find sessions that are blocking other sessions in PostgreSQL

Today morning, one of our Associate DB Developer working with Transactions and PostgreSQL is also new for him.
The COMMIT, ROLLBACK and SAVEPOINT are very common for all RDBMS.

In this post, I am sharing one basic demonstration on COMMIT, ROLLBACK and SAVEPOINT of PostgreSQL which helps to Associate level DB Developer.

Create a table with Sample Data:

CREATE TABLE tbl_Employees
(
	EmpID INT
	,EmpName CHARACTER VARYING
);

INSERT INTO tbl_Employees
VALUES	
(1,'Anvesh'),(2,'Loother'),(3,'Roy'),(4,'Martin')
,(5,'Jenny'),(6,'Monika'),(7,'Rajesh'),(8,'Eric');

Commit: We can use COMMIT to end our all transactions and make It as permanent changes.

Commit the Transaction:

BEGIN;
	UPDATE tbl_Employees
	SET EmpName = 'Nivu'
	WHERE EmpID = 3;
COMMIT;

Rollback: Using ROLLBACK, we can jump to the previous last committed state of the Transaction.
Rollback the Transaction:

BEGIN;
	UPDATE tbl_Employees
	SET EmpName = 'Vinay'
	WHERE EmpID = 2;
ROLLBACK;

Savepoint: Using SAVEPOINT, we can jump to the particular point which defines by the SAVEPOINT.

Rollback changes to the Save Point:

BEGIN;
	UPDATE tbl_Employees
	SET EmpName = 'Grace'
	WHERE EmpID = 4;
	
SAVEPOINT EmpSave;

	UPDATE tbl_Employees
	SET EmpName = 'Kunal'
	WHERE EmpID = 5;
	
ROLLBACK TO EmpSave;

The Result:

SELECT * FROM tbl_Employees ORDER BY 1 ;

EmpID    EmpName
--------------------
  1      Anvesh
  2      Loother
  3      Nivu
  4      Grace
  5      Jenny
  6      Monika
  7      Rajesh
  8      Eric