How to fix Error 10635 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 10635 – Online Index Operation Failure on LOB Columns

Why This Error Happens

This error occurs because SQL Server’s online index operation (ONLINE = ON) feature has architectural limitations when dealing with Large Object (LOB) data types such as VARCHAR(MAX), NVARCHAR(MAX), VARBINARY(MAX), XML, or legacy types like TEXT, NTEXT, and IMAGE. The database engine cannot maintain row versioning and concurrent read/write access for LOB pointers during an online rebuild, as the row-versioning mechanism used for concurrency does not fully cover the pointer structures for these large data chunks. This is a hard-coded engine restriction rather than a configuration or permissions error, meaning it occurs regardless of server health or user privileges.

Step-by-Step Resolution Guide

Step 1: Identify the Offending Column and Index

  • Open SSMS (SQL Server Management Studio) 21 connected to your SQL Server 2025 instance.
  • Run the following query to find which index and column is triggering the conflict:
    SELECT i.name AS IndexName, c.name AS ColumnName, t.name AS DataType FROM sys.indexes i JOIN sys.index_columns ic ON i.object_id = ic.object_id AND i.index_id = ic.index_id JOIN sys.columns c ON ic.object_id = c.object_id AND ic.column_id = c.column_id JOIN sys.types t ON c.user_type_id = t.user_type_id WHERE t.name IN ('text', 'ntext', 'image', 'xml', 'varchar', 'nvarchar', 'varbinary') AND i.object_id = OBJECT_ID('YourTableName');
  • Confirm if the data type listed matches a MAX or LOB-based type.

Step 2: Switch the Operation to OFFLINE Mode

  • Since LOB columns are not fully supported in online mode, you must modify your T-SQL script to disable the online flag.
  • Navigate to your index creation or rebuild script, typically found in your deployment .sql files or SSMS query window.
  • Change the syntax from WITH (ONLINE = ON) to WITH (ONLINE = OFF).
  • Example command: ALTER INDEX [YourIndexName] ON [dbo].[YourTableName] REBUILD WITH (ONLINE = OFF, SORT_IN_TEMPDB = ON);

Step 3: Schedule the Maintenance During Low-Traffic Windows

  • Since the operation must now run offline, it will acquire a Schema Modification (Sch-M) lock, blocking user access to the table.
  • Use SQL Server Agent on Windows Server 2025 to schedule this job during a maintenance window (e.g., 2:00 AM – 4:00 AM).
  • Navigate to SQL Server Agent > Jobs > New Job in SSMS to automate this specific ALTER INDEX script.

Step 4: Evaluate Data Type Compatibility (Optional Long-Term Fix)

  • If frequent online rebuilds are a business requirement, evaluate if the LOB data can be refactored.
  • Check if VARCHAR(MAX) data is actually small enough to fit into a standard VARCHAR(8000) or NVARCHAR(4000), which supports online operations.
  • Modify the schema using ALTER TABLE YourTableName ALTER COLUMN YourColumnName VARCHAR(8000); if business logic allows for the data truncation/change.

Alternative or Verification Steps

  • Verification: After running the index rebuild with ONLINE = OFF, check the sys.indexes catalog view to ensure the is_disabled flag is set to 0 and verify the operation completed without further 10635 errors in the SQL Server Error Log located at C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\.
  • Alternative: If downtime is strictly prohibited, consider using the RESUMABLE = ON option in conjunction with ONLINE = OFF for very large tables; this allows you to pause and resume the offline index build without restarting the entire process, minimizing the impact of the required lock.