Added Lab-DeadLock

Author: PradeepBhadiyadra

Lab-DeadLock

@@ -0,0 +1,58 @@
+
+## Open a Terminal we call this session-1
+connect to database server
+connect to world database 
+
+\c world
+
+BEGIN ;  # THIS WILL KEEP CONNECTION ALIVE
+BEGIN  # will get this output
+
+UPDATE country SET continent='Europe' WHERE code='USA';   
+UPDATE 1  # will give this output - started query on row - USA , hence row - USA  is in use
+
+
+
+## Open another Terminal we call this session-2
+
+connect to database server
+connect to world database 
+
+\c world
+
+BEGIN ;  
+BEGIN # will get this output
+
+UPDATE country SET continent='Asia' WHERE code='GBR';
+UPDATE 1  # will give this output - started query on row - GBR , hence row - GBR  is in use
+
+UPDATE country SET indepyear=1111 WHERE code='USA';  # now this will go in waiting as row - USA is still in use by session 1
+
+
+## Go to session 1
+
+UPDATE country SET indepyear=1112 WHERE code='GBR';  # row - GBR is in use by session 2 hence it will generate deadlock error
+
+ERROR:  deadlock detected
+DETAIL:  Process 22208 waits for ShareLock on transaction 806; blocked by process 22256.
+Process 22256 waits for ShareLock on transaction 805; blocked by process 22208.
+HINT:  See server log for query details.
+CONTEXT:  while updating tuple (4,27) in relation "country"
+
+postgresql automatically break the lock 
+
+
+## Go to session 2
+
+and now query is completed
+
+
+now end both session by executing below command in both session and notice the output
+
+end;
+
+session 2 will say - COMMIT
+but 
+session 1 will say - ROLLBACK - that means - " UPDATE country SET indepyear=1112 WHERE code='GBR'; " was not successfully executed
+
+For more details we can see the log in our PostgreSQL server