LOCK | SQL Server Interview Questions


  1. What are locks?
    Microsoft® SQL Server™ 2000 uses locking to ensure transactional integrity and database consistency. Locking prevents users from reading data being changed by other users, and prevents multiple users from changing the same data at the same time. If locking is not used, data within the database may become logically incorrect, and queries executed against that data may produce unexpected results. 
  2. What are the different types of locks?
    SQL Server uses these resource lock modes.
Lock mode
Description
Shared (S)
Used for operations that do not change or update data (read-only operations), such as a SELECT statement.
Update (U)
Used on resources that can be updated. Prevents a common form of deadlock that occurs when multiple sessions are reading, locking, and potentially updating resources later.
Exclusive (X)
Used for data-modification operations, such as INSERT, UPDATE, or DELETE. Ensures that multiple updates cannot be made to the same resource at the same time.
Intent
Used to establish a lock hierarchy. The types of intent locks are: intent shared (IS), intent exclusive (IX), and shared with intent exclusive (SIX).
Schema
Used when an operation dependent on the schema of a table is executing. The types of schema locks are: schema modification (Sch-M) and schema stability (Sch-S).
Bulk Update (BU)
Used when bulk-copying data into a table and the TABLOCK hint is specified.
  1. What is a dead lock? Give a practical sample? How you can minimize the deadlock situation? What is a deadlock and what is a live lock? How will you go about resolving deadlocks?
    Deadlock is a situation when two processes, each having a lock on one piece of data, attempt to acquire a lock on the other's piece. Each process  would wait indefinitely for the other to release the lock, unless one of the user processes is terminated. SQL Server detects deadlocks and terminates one user's process.
    A livelock is one, where a request for an exclusive lock is repeatedly denied because a series of overlapping shared locks keeps interfering. SQL Server detects the situation after four denials and refuses further shared locks. (A livelock also occurs when read transactions monopolize a table or page, forcing a write transaction to wait indefinitely.) 
  2. What is isolation level?
    An isolation level determines the degree of isolation of data between concurrent transactions. The default SQL Server isolation level is Read Committed. A lower isolation level increases concurrency, but at the expense of data correctness. Conversely, a higher isolation level ensures that data is correct, but can affect concurrency negatively. The isolation level required by an application determines the locking behavior SQL Server uses.
    SQL-92 defines the following isolation levels, all of which are supported by SQL Server:
    • Read uncommitted (the lowest level where transactions are isolated only enough to ensure that physically corrupt data is not read).
    • Read committed (SQL Server default level).
    • Repeatable read.
    • Serializable (the highest level, where transactions are completely isolated from one another).
Isolation level
Dirty read
Nonrepeatable read
Phantom
Read uncommitted
Yes
Yes
Yes
Read committed
No
Yes
Yes
Repeatable read
No
No
Yes
Serializable
No
No
No
  1. nolock? What is the difference between the REPEATABLE READ and SERIALIZE isolation levels?
    Locking Hints -     A range of table-level locking hints can be specified using the SELECT, INSERT, UPDATE, and DELETE statements to direct Microsoft® SQL Server 2000 to the type of locks to be used. Table-level locking hints can be used when a finer control of the types of locks acquired on an object is required. These locking hints override the current transaction isolation level for the session.
Locking hint
Description
HOLDLOCK
Hold a shared lock until completion of the transaction instead of releasing the lock as soon as the required table, row, or data page is no longer required. HOLDLOCK is equivalent to SERIALIZABLE.
NOLOCK
Do not issue shared locks and do not honor exclusive locks. When this option is in effect, it is possible to read an uncommitted transaction or a set of pages that are rolled back in the middle of a read. Dirty reads are possible. Only applies to the SELECT statement.
PAGLOCK
Use page locks where a single table lock would usually be taken.
READCOMMITTED
Perform a scan with the same locking semantics as a transaction running at the READ COMMITTED isolation level. By default, SQL Server 2000 operates at this isolation level.
READPAST
Skip locked rows. This option causes a transaction to skip rows locked by other transactions that would ordinarily appear in the result set, rather than block the transaction waiting for the other transactions to release their locks on these rows. The READPAST lock hint applies only to transactions operating at READ COMMITTED isolation and will read only past row-level locks. Applies only to the SELECT statement.
READUNCOMMITTED
Equivalent to NOLOCK.
REPEATABLEREAD
Perform a scan with the same locking semantics as a transaction running at the REPEATABLE READ isolation level.
ROWLOCK
Use row-level locks instead of the coarser-grained page- and table-level locks.
SERIALIZABLE
Perform a scan with the same locking semantics as a transaction running at the SERIALIZABLE isolation level. Equivalent to HOLDLOCK.
TABLOCK
Use a table lock instead of the finer-grained row- or page-level locks. SQL Server holds this lock until the end of the statement. However, if you also specify HOLDLOCK, the lock is held until the end of the transaction.
TABLOCKX
Use an exclusive lock on a table. This lock prevents others from reading or updating the table and is held until the end of the statement or transaction.
UPDLOCK
Use update locks instead of shared locks while reading a table, and hold locks until the end of the statement or transaction. UPDLOCK has the advantage of allowing you to read data (without blocking other readers) and update it later with the assurance that the data has not changed since you last read it.
XLOCK
Use an exclusive lock that will be held until the end of the transaction on all data processed by the statement. This lock can be specified with either PAGLOCK or TABLOCK, in which case the exclusive lock applies to the appropriate level of granularity.
  1. For example, if the transaction isolation level is set to SERIALIZABLE, and the table-level locking hint NOLOCK is used with the SELECT statement, key-range locks typically used to maintain serializable transactions are not taken.
    USE pubs
    GO
    SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
    GO
    BEGIN TRANSACTION
    SELECT au_lname FROM authors WITH (NOLOCK)
    GO
8. What is escalation of locks?
Lock escalation is the process of converting a lot of low level locks (like row locks, page locks) into higher level locks (like table locks). Every lock is a memory structure too many locks would mean, more memory being occupied by locks. To prevent this from happening, SQL Server escalates the many fine-grain locks to fewer coarse-grain locks. Lock escalation threshold was definable in SQL Server 6.5, but from SQL Server 7.0 onwards it's dynamically managed by SQL Server.