When a SQL Server instance restarts, auto-increment columns may exhibit value gaps upon subsequent insertions. This behavior stems from an internal caching optimization introduced in recent versions. Specifically, integer-based identity columns typically reserve chunks of 1,000 values, while BigInt columns reserve 10,000. While this improves write performance, it results in non-contiguous primary keys visible after a service interruption. This can disrupt client-side expectations when continuous numbering is required.

To demonstrate the issue, consider a scenario involving a logging table. Create a table using a default identity configuration:

CREATE TABLE AuditTrail (
    LogID BIGINT IDENTITY(1, 1),
    EventData NVARCHAR(50)
);

Execute initial inserts:

INSERT INTO AuditTrail (EventData) VALUES ('Initial Entry 1');
INSERT INTO AuditTrail (EventData) VALUES ('Initial Entry 2');

Query the current sequence:

SELECT LogID FROM AuditTrail ORDER BY LogID DESC;

Assuming the last ID was 2, stop the database service. Upon restarting SQL Server, proceeed with new entries:

INSERT INTO AuditTrail (EventData) VALUES ('Post-Restart Entry');

Checking the table now reveals the next assigned LogID is likely 10003 rather than 3. This gap represents the pre-allocated memory buffer that remains unused after the restart event.

Microsoft designates this as an intentional feature rather than a defect, prioritizing stability and performance over strict sequence continuity. If continuous IDs are mandatory for reporting or external exposure, specific configurations must override this caching mechanism.

Two primary methods exist to eliminate these gaps: utilizing database sequences or modifying server startup parameters.

Option 1: Database Sequences

Replace the IDENTITY property with a user-defined SEQUENCE. Define the sequence without caching enabled.

CREATE SEQUENCE seq_Audit_ID
    AS BIGINT
    START WITH 1
    INCREMENT BY 1
    MINVALUE 1
    NO MAXVALUE
    NO CACHE;

INSERT INTO AuditTrail (LogID, EventData) 
VALUES (NEXT VALUE FOR seq_Audit_ID, 'Event Data');

By setting NO CACHE, every request fetches a single number from the generator, ensuring no allocation blocks remain in memory during a crash or restart.

Option 2: Trace Flag Configuraton

Disable the identity cache allocation globally via trace flags. Open SQL Server Configuration Manager, locate the specific instance properties, and append -T272 to the startup parameters list. This requires restartnig the Windows service hosting the database engine.

The parameter acts by disabling the caching logic entirely. When applied, all identity columns revert to requesting values one at a time from the lock manager, preventing large jumps.

Implementation Guidance

Evaluate the scope before applying changes. Option 2 impacts all tables system-wide and offers immediate resolution for existing schemas with minimal migration effort. Option 1 provides granular control per table but requires altering existing structures. For legacy applications relying on sequential integers, disabling the cache via startup flags is often the most efficient path to restore expected continuity.