Optimized locking is a per-database SQL Server 2025 feature that can reduce how many low-level locks write transactions hold and for how long. It uses transaction ID (TID) locking and, when read committed snapshot isolation (RCSI) is enabled, lock after qualification (LAQ). It is off by default in SQL Server 2025, requires accelerated database recovery (ADR), and can reduce—but does not eliminate—lock-related blocking.
What is optimized locking in SQL Server 2025?
Optimized locking changes how the database engine protects rows modified by INSERT, UPDATE, DELETE, and MERGE. Rather than retaining many row or page locks until a transaction ends, the engine can release low-level locks sooner and use a transaction ID lock to represent the transaction’s protection. Microsoft says the design aims to reduce lock blocking and lock-memory consumption for concurrent transactions; the benefit depends on workload and configuration.
Transaction ID (TID) locking
Each modified row is associated with the transaction ID of the transaction that last changed it. A transaction-level TID lock can protect the modified rows, allowing low-level row or page locks to be released as work proceeds instead of retaining many such locks until commit or rollback.
Lock after qualification (LAQ)
When RCSI is enabled, LAQ evaluates a write predicate against the latest committed row version before acquiring a lock for that qualification check. If a row qualifies but has an active writer, the transaction can still have to wait. LAQ changes how the engine determines whether a row qualifies; it does not mean that writes happen without protection.
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What changes compared with conventional locking?
| Behavior | Conventional locking | With optimized locking |
|---|---|---|
| Row and page locks | Many low-level locks may be retained until transaction end. | Low-level locks can be released sooner; a TID lock protects the transaction’s modifications. |
| Lock-memory demand and escalation | Large numbers of held locks can use more lock memory and make escalation more likely. | Fewer and shorter-lived low-level locks can reduce lock-memory use and the likelihood of escalation. |
| Write predicate qualification | A writer generally acquires locks as it evaluates and modifies rows. | With RCSI, LAQ can test the latest committed row version before acquiring a lock for qualification. |
Microsoft’s illustration uses a transaction that updates 1,000 rows: conventionally, 1,000 exclusive row locks might remain until transaction end, whereas optimized locking can release low-level locks as rows are updated while a TID lock remains. This is an explanatory example, not a benchmark or a promised performance result.
The feature primarily affects row and page locks for data modifications. It does not change every lock type: schema and other database or object locks remain, and the documentation does not claim all forms of blocking disappear.
Is optimized locking enabled by default?
No. Microsoft lists optimized locking as supported in SQL Server 2025 (17.x), off by default, and configurable per database. SQL Server 2022 (16.x) and earlier are listed as unsupported. Azure SQL Database, Azure SQL Managed Instance, and SQL database in Microsoft Fabric also support the feature, but their documented defaults are service-specific; do not infer their defaults from SQL Server 2025’s on-premises default.
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Does optimized locking require ADR or RCSI?
ADR is required: enable accelerated database recovery in the database before enabling optimized locking. RCSI is not stated as a prerequisite for optimized locking as a whole, but it is required for LAQ and is recommended by Microsoft for the greatest benefit.
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Microsoft identifies RCSI together with the default READ COMMITTED isolation level as the configuration most likely to benefit. Readers use statement-level row versions, while LAQ evaluates writer predicates against the latest committed value. A writer waits when a qualifying row has an active writer. In this configuration, the engine handles and retries detected update conflicts.
Stricter isolation levels and SNAPSHOT
Under REPEATABLE READ or SERIALIZABLE, row and page locks can remain until transaction end, increasing blocking and lock-memory use and reducing the advantage of optimized locking. Under SNAPSHOT, update conflicts behave as they did before optimized locking; the application must detect and handle or retry them. Changing isolation levels solely to obtain a locking benefit requires workload validation.
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Locking hints
Hints including UPDLOCK, READCOMMITTEDLOCK, XLOCK, and HOLDLOCK remain honored, but can reduce the optimization’s benefit. READCOMMITTEDLOCK is available when an application intentionally needs blocking behavior under RCSI.
How do I enable optimized locking in SQL Server 2025?
Use a SQL Server 2025 database that is online, and ensure no other active connections to it remain while changing the setting. Microsoft requires that the connection issuing ALTER DATABASE be the only active database connection for this change. First check the database options:
SELECT name,
is_accelerated_database_recovery_on,
is_read_committed_snapshot_on,
is_optimized_locking_on
FROM sys.databases
WHERE name = DB_NAME();Confirm that ADR is enabled. RCSI is recommended for the greatest benefit and is necessary for LAQ; evaluate its use against your application’s behavior before changing it. Then enable optimized locking for the database:
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ALTER DATABASE [YourDatabase]
SET OPTIMIZED_LOCKING = ON;Replace YourDatabase with the database name. The setting is per database, not a server-wide switch. To turn it off, use:
ALTER DATABASE [YourDatabase]
SET OPTIMIZED_LOCKING = OFF;You can also check the current database’s setting with:
SELECT DATABASEPROPERTYEX(DB_NAME(), 'IsOptimizedLockingOn')
AS IsOptimizedLockingOn;For the documented option and connection requirements, see Microsoft’s ALTER DATABASE SET options reference. The feature’s availability, configuration, and status checks are covered in Microsoft’s optimized locking documentation.
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Does optimized locking eliminate blocking?
No. It can reduce certain lock-related blocking by shortening the duration and reducing the number of low-level locks. A qualifying row with an active writer can still make another writer wait, and schema locks, stricter isolation settings, locking hints, and other lock types can still cause blocking. Optimized locking should be understood as a change to write-locking behavior, not a guarantee of nonblocking transactions.
How should you judge its impact?
Microsoft’s feature and configuration documentation describes qualitative benefits, not a general throughput percentage or named benchmark. Results depend on the database’s write patterns, transaction sizes, isolation settings, hints, and contention. Measure the workload before and after enabling the feature under representative conditions, and verify that application conflict handling and latency remain acceptable. Microsoft’s transaction locking and row versioning guide explains the isolation and conflict behavior to account for.
- Optimized locking is not used for modifications in tempdb or temporary tables.
- It is not used on read-only secondary replicas, where DML cannot run.
For Microsoft’s SQL Server 2025 feature summary, see What’s new in SQL Server 2025.
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