SQL Server 2012 : LOCKS (part 3) - Lock Modes - Conversion Lock Modes

Conversion Lock Modes (SIX), (SIU), (UIX)

SQL Server also provides the facility to convert shared, update, or exclusive locks to shared with intent exclusive (SIX), shared with intent update (SIU), or update with intent exclusive (UIX). This happens when a statement inside a transaction already holds a lock at a coarse granularity (a table) but now needs to modify a component of the resource held at a much finer granularity (a row). The lock held against the coarse granularity needs to reflect this.

Consider the following example of a SIX lock (code file Ch6ConversionLockModes.sql):

USE AdventureWorks2012;
 
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
 
BEGIN TRANSACTION; 
 
SELECT  BusinessEntityID
        ,FirstName
        ,MiddleName
        ,LastName
        ,Suffix
FROM   Person.Person;
 
SELECT   resource_type
        ,resource_subtype 
        ,resource_description
        ,resource_associated_entity_id
        ,request_mode
        ,request_status
FROM    sys.dm_tran_locks
WHERE   request_session_id= @@SPID;
 
UPDATE  Person.Person
SET     Suffix      = 'Junior'
WHERE   FirstName   = 'Syed'
AND     LastName    = 'Abbas';
 
SELECT   resource_type
        ,resource_subtype 
        ,resource_description
        ,resource_associated_entity_id
        ,request_mode
        ,request_status
FROM    sys.dm_tran_locks
WHERE   request_session_id= @@SPID;
 
ROLLBACK TRANSACTION;

A transaction has selected all rows from the Person.Person table. This generates a table-level shared lock, as shown in Figure 3.

FIGURE 3

The transaction continues through to update a single row. This triggers the need to convert the table-level shared lock to a SIX lock as the row must be exclusively locked. Figure 4 clearly shows that the row is locked with an exclusive KEY lock, but also that the table/object has converted its lock from shared (S) to shared with intent exclusive (SIX).

FIGURE 4