Merge pull request #360 from NikhilBhargav/main

Types of Locks in SQL Server

Author: milos-simic

sql-constraints/sql-locks/sqlserver-locks.sql

@@ -0,0 +1,138 @@
+-- Change schema to University
+USE University;
+
+--Shared Lock (S) 
+
+--Session 1
+-- Set the isolation level to REPEATABLE READ to hold shared locks until commit/rollback
+SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
+
+BEGIN TRAN;
+SELECT * 
+FROM Course 
+WHERE id='CS111';
+
+--Check the locks held by this session:
+SELECT request_session_id, resource_type, resource_description, request_mode, resource_associated_entity_id 
+FROM sys.dm_tran_locks
+WHERE request_session_id = @@SPID;
+
+--Commit later
+COMMIT TRAN;
+
+--Session 2
+SELECT * 
+FROM Course 
+WHERE id='CS111';
+
+BEGIN TRAN;
+
+UPDATE Course
+SET credits = 8
+WHERE id='CS111';
+
+ROLLBACK TRAN;
+
+--Sesion 3
+SELECT
+    session_id,
+    blocking_session_id,
+    command,
+    status,
+    wait_type,
+    wait_time,
+    last_wait_type,
+    open_transaction_count,
+    text
+FROM sys.dm_exec_requests
+CROSS APPLY sys.dm_exec_sql_text(sql_handle)
+WHERE blocking_session_id <> 0;
+
+--EXEC sp_who2;
+
+--Exclusive Lock (X) 
+
+--Session 1
+
+-- Set the isolation level to REPEATABLE READ to hold shared locks until commit/rollback
+SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
+
+BEGIN TRAN;
+UPDATE Course
+SET credits = 8
+WHERE id='CS111';
+
+--Check the locks held by this session:
+SELECT request_session_id, resource_type, resource_description, request_mode, resource_associated_entity_id
+FROM sys.dm_tran_locks
+WHERE request_session_id = @@SPID;
+
+--Rollback later
+ROLLBACK TRAN;
+
+--Session 2
+BEGIN TRAN;
+SELECT * 
+FROM Course 
+WHERE id='CS111';
+
+COMMIT TRAN;
+
+
+--Update Lock (U) 
+
+--Session 1
+SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
+
+BEGIN TRAN;
+SELECT * 
+FROM Course
+WITH (UPDLOCK) 
+WHERE id='CS111';
+
+--Check the locks held by this session:
+SELECT request_session_id, resource_type, resource_description, request_mode 
+FROM sys.dm_tran_locks
+WHERE request_session_id = @@SPID;
+
+--Session 2
+SELECT * 
+FROM Course
+WHERE id='CS111';
+
+UPDATE Course
+SET credits = 8
+WHERE id='CS111';
+
+--Shared with Intent Exclusive (SIX)
+
+--Session 1
+SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
+
+BEGIN TRAN;
+
+-- Explicitly request a SIX lock:
+SELECT * 
+FROM Course
+WITH (TABLOCKX, HOLDLOCK)
+WHERE id='CS111';
+
+UPDATE Course
+SET credits = 8
+WHERE id='CS111';
+
+--Rollback later
+ROLLBACK TRAN;
+
+SELECT request_session_id, resource_type, resource_description, request_mode 
+FROM sys.dm_tran_locks
+WHERE request_session_id = @@SPID;
+
+--Session 2
+BEGIN TRAN;
+
+SELECT * 
+FROM Course
+WHERE id='CS111';
+
+COMMIT TRAN;

sql-transactions/sql-locks/sqlserver-locks.sql

@@ -0,0 +1,134 @@
+-- Change schema to University
+USE University;
+
+--Shared Lock (S) 
+
+--Session 1
+-- Set the isolation level to REPEATABLE READ to hold shared locks until commit/rollback
+SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
+
+BEGIN TRAN;
+SELECT id, name, credits 
+FROM Course 
+WHERE id='CS111';
+
+--Check the locks held by this session:
+SELECT request_session_id, resource_type, request_mode 
+FROM sys.dm_tran_locks
+WHERE request_session_id = @@SPID
+AND resource_type = 'KEY';
+
+--Commit later
+COMMIT TRAN;
+
+--Session 2
+SELECT id, name, credits 
+FROM Course 
+WHERE id='CS111';
+
+BEGIN TRAN;
+
+UPDATE Course
+SET credits = 8
+WHERE id='CS111';
+
+ROLLBACK TRAN;
+
+--Sesion 3
+SELECT
+    session_id,
+    blocking_session_id,
+    command,
+    status
+FROM sys.dm_exec_requests
+CROSS APPLY sys.dm_exec_sql_text(sql_handle)
+WHERE blocking_session_id <> 0;
+
+--EXEC sp_who2;
+
+--Exclusive Lock (X) 
+
+--Session 1
+
+-- Set the isolation level to REPEATABLE READ to hold shared locks until commit/rollback
+SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
+
+BEGIN TRAN;
+UPDATE Course
+SET credits = 8
+WHERE id='CS111';
+
+--Check the locks held by this session:
+SELECT request_session_id, resource_type, request_mode
+FROM sys.dm_tran_locks
+WHERE request_session_id = @@SPID;
+
+--Rollback later
+ROLLBACK TRAN;
+
+--Session 2
+BEGIN TRAN;
+SELECT id, name, credits 
+FROM Course 
+WHERE id='CS111';
+
+COMMIT TRAN;
+
+
+--Update Lock (U) 
+
+--Session 1
+SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
+
+BEGIN TRAN;
+SELECT id, name, credits 
+FROM Course
+WITH (UPDLOCK) 
+WHERE id='CS111';
+
+--Check the locks held by this session:
+SELECT request_session_id, resource_type, request_mode 
+FROM sys.dm_tran_locks
+WHERE request_session_id = @@SPID;
+
+--Session 2
+SELECT id, name, credits 
+FROM Course
+WHERE id='CS111';
+
+UPDATE Course
+SET credits = 8
+WHERE id='CS111';
+
+--Shared with Intent Exclusive (SIX)
+
+--Session 1
+SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
+
+BEGIN TRAN;
+
+-- Explicitly request a SIX lock:
+SELECT id, name, credits 
+FROM Course
+WITH (TABLOCKX, HOLDLOCK)
+WHERE id='CS111';
+
+UPDATE Course
+SET credits = 8
+WHERE id='CS111';
+
+--Rollback later
+ROLLBACK TRAN;
+
+SELECT request_session_id, resource_type, request_mode 
+FROM sys.dm_tran_locks
+WHERE request_session_id = @@SPID;
+
+--Session 2
+BEGIN TRAN;
+
+SELECT * 
+FROM Course
+WHERE id='CS111';
+
+COMMIT TRAN;