transactions, concurrency, isolation levels and deadlock

Author: ptyadana

Complete SQL Mastery - Mosh/Transactions and Concurrency.sql

@@ -0,0 +1,111 @@
+/************************************************/
+/* Transactions and Concurrency				    */
+/************************************************/
+/*
+ACID properties
+
+Atomicity
+Consistency
+Isolation
+Durability
+*/
+
+/************************************************/
+/* Creating Transactions			     	    */
+/************************************************/
+SHOW VARIABLES like 'autocommit';
+SET autocommit = 0;
+
+USE mosh_sql_store;
+
+START TRANSACTION;
+
+INSERT INTO orders (customer_id, order_date, status)
+VALUES(1,'2020-04-05',1);
+
+INSERT INTO ordessr_items
+VALUES(LAST_INSERT_ID(),1,1,1);
+
+COMMIT;
+
+SET autocommit = 1;
+
+/************************************************/
+/* Concurrency and Locking			     	    */
+/************************************************/
+
+/************************************************/
+/* Cocurrency Problems                          */
+/************************************************/
+
+/*
+	- Lost Updates
+    - Dirty Reads
+    - Non Repating Reads
+    - Phantom Reads
+    
+    
+ 1) Lost Updates
+ - This problem occurs when multiple transactions execute concurrently and updates from one or more transactions get lost.
+ 
+ Solution Isolation Level: LOCK
+ 
+ 
+ 2) Dirty Reads
+ - Reading the data written by an uncommitted transaction is called as dirty read.
+ 
+  Solution Isolation Level: READ COMMITTED
+ 
+ 
+ 3) Non Repating Reads / Unrepeatable Reads
+ - This problem occurs when a transaction gets to read unrepeated 
+ i.e. different values of the same variable in its different read operations even when it has not updated its value.
+ 
+ Solution Isolation Level: REPEATABLE READ
+ 
+ 
+ 4) Phantom Reads
+ - This problem occurs when a transaction reads some variable from the buffer 
+ and when it reads the same variable later, it finds that the variable does not exist.
+ 
+  Solution Isolation Level:	SERIALIZABLE
+
+*/
+
+/************************************************/
+/* Transaction Isolation Levels                 */
+/************************************************/
+/*
+
+- the higher the isolation level, the lower the concurrency issues as it can solve these.
+- but the price is cost is very expensive as it can slow the queries.
+- SERIALIZABLE make sure each queries are running in sequences and wait other to be completed.
+
+									Lost Updates		Dirty Reads			Non-Repeating Reads			Phantom Reads
+
+(isolation level)
+4) SERIALIZABLE						   SOLVED              SOLVED                 SOLVED                  SOLVED
+
+3) REPEATABLE READ					   SOLVED              SOLVED                 SOLVED
+
+2) READ COMMITED                                           SOLVED
+
+1) READ UNCOMMITED 
+*/
+
+SHOW VARIABLES LIKE 'transaction_isolation';
+
+/*for session only*/
+SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE;
+
+/*for global*/
+SET GLOBAL TRANSACTION ISOLATION LEVEL SERIALIZABLE;
+
+
+
+/************************************************/
+/* DeadLocks                                    */
+/************************************************/
+/*
+A deadlock is a situation in which two or more transactions are waiting for one another to give up locks.
+*/