Optimized Halloween Protection in SQL Server

Optimized Halloween Protection in SQL Server 2025

The Halloween Problem is a classic and subtle issue in relational database systems that arises during certain data modification operations—specifically UPDATE, INSERT, DELETE, or MERGE statements. The core of the problem is that, under specific circumstances, a row can be modified more than once during a single statement execution, even though the intention is for each row to be updated only once.

How Does the Halloween Problem Occur?

The issue typically surfaces when:

* An update operation modifies a column that is also used in the index being scanned to identify the rows to update.

* As the update progresses, changing the indexed column can cause the physical location of the row in the index to change.

* This movement can cause the row to reappear ahead of the scan cursor, meaning the same row is encountered and updated again—potentially leading to multiple updates or even infinite loops.

Halloween Protection: The Traditional Solution

To prevent this, database engines implement Halloween protection. The classic method is to separate the read and write phases of the update operation:

* First, the engine reads and collects all the rows that qualify for the update.

* Only after collecting this set does, it perform the updates, ensuring no row is modified more than once.

Technical Implementation:

SQL Server and other engines often use a blocking operator (like an eager spool or sort) in the query execution plan. This operator materializes the qualifying rows into a temporary structure (often in tempdb) before any updates are performed.

While effective, this approach can be expensive in terms of performance and resource usage, especially for large tables, because it requires copying and storing potentially large amounts of data.

Optimized Halloween Protection in SQL Server 2025

SQL Server 2025 introduces a major enhancement with Optimized Halloween Protection, designed to address the performance overhead of traditional methods.

Key Improvements

* No More Spool Operator by Default: Instead of always inserting a blocking operator like a spool (which copies rows to tempdb), SQL Server 2025 can now, in many cases, avoid this extra step. This is achieved by leveraging the Accelerated Data Recovery (ADR) feature, which efficiently tracks which rows have already been updated without needing to physically copy them.

* Performance Gains: Removing the need for spooling means significantly less tempdb usage, lower CPU time, and faster execution for large updates. For example, demo results show that with optimized protection enabled, updates can run more than twice as fast, with dramatically reduced resource consumption.

* Automatic and Configurable: This feature is enabled by default in SQL Server 2025, but can also be controlled at the database or query level using configuration settings or query hints.

How Does It Work?

* Traditional Approach: Reads all qualifying rows, stores them in a spool (temporary storage), then updates them—ensuring no row is updated twice, but with a performance penalty.

* Optimized Approach (SQL Server 2025): Uses internal tracking (via ADR) to remember which rows have been updated, eliminating the need for a spool in many scenarios. This makes the update process more efficient, especially for large tables or frequent DML operations.

When Is Halloween Protection Needed?

Halloween protection is only required when:

* The update operation modifies a column that is part of the index being used to find the rows to update.

* There is a risk that updating a row could cause it to be re-encountered by the scan cursor.

If neither condition applies, SQL Server can safely skip the blocking operator, further optimizing performance.

Practical Implications for Developers

* Better Performance: Large update operations, especially those that previously triggered Halloween protection, will now run faster and use fewer resources.

* No Code Changes Needed: The feature is on by default in SQL Server 2025; just upgrade to benefit.

* Granular Control: If needed, you can enable or disable optimized Halloween protection for specific queries or databases.

How to Enable Optimized Halloween Protection -

To use this feature, your database must meet three requirements:

* Accelerated Database Recovery (ADR) must be ON

* Compatibility level must be 170

* OPTIMIZED_HALLOWEEN_PROTECTION must be ON (it is by default)

Enable it using:

ALTER DATABASE [YourDatabase] SET ACCELERATED_DATABASE_RECOVERY = ON;

ALTER DATABASE [YourDatabase] SET COMPATIBILITY_LEVEL = 170;

ALTER DATABASE SCOPED CONFIGURATION SET OPTIMIZED_HALLOWEEN_PROTECTION = ON;

Note: Enabling ADR requires an exclusive lock on the database. You may need to use:

ALTER DATABASE [YourDatabase] SET ACCELERATED_DATABASE_RECOVERY = ON WITH ROLLBACK IMMEDIATE;