Troubleshooting Guide: SQL Server Error 1999 – Invalid Column Type for Included Index

Why This Error Happens

This error occurs because SQL Server 2025 restricts certain data types—such as text, ntext, image, geography, geometry, xml (in some configurations), or varchar(max)/nvarchar(max)/varbinary(max) under specific legacy compatibility levels—from being used as included (INCLUDE) columns in a nonclustered index. The root cause is typically a schema design mismatch where a column’s data type was changed (e.g., to a large object or spatial type) after the index was originally created, or a migrated script from an older SQL Server version attempts to include an incompatible type. This is not a permissions or connectivity issue; it is strictly a schema/index-definition conflict enforced by the Database Engine’s metadata validation layer.

Step-by-Step Resolution Guide

Step 1: Identify the Offending Column and Index

• Open SQL Server Management Studio (SSMS) 21 or Azure Data Studio connected to your Windows Server 2025 instance.
• Run the following query to extract the exact table and column referenced in the error message:

  SELECT t.name AS TableName, c.name AS ColumnName, ty.name AS DataType
  FROM sys.columns c
  JOIN sys.tables t ON c.object_id = t.object_id
  JOIN sys.types ty ON c.user_type_id = ty.user_type_id
  WHERE t.name = 'YourTableName';

• Confirm the data type matches a restricted category (LOB, spatial, or deprecated types).

Step 2: Modify or Recreate the Index Definition

• Drop the existing index using DROP INDEX IX_YourIndexName ON dbo.YourTableName;
• Recreate the index without the invalid column in the INCLUDE clause:

  CREATE NONCLUSTERED INDEX IX_YourIndexName
  ON dbo.YourTableName (ColumnA, ColumnB)
  INCLUDE (ValidColumn1, ValidColumn2);

• If the LOB/spatial column must be indexed, consider using a computed column cast to a supported type, or implement Full-Text Search/Spatial indexing separately.

Step 3: Validate Compatibility Level Settings

• Check the database compatibility level, as some type restrictions behave differently across versions:

  SELECT compatibility_level FROM sys.databases WHERE name = 'YourDatabaseName';

• If necessary, align it with SQL Server 2025 standards (170) using:

  ALTER DATABASE YourDatabaseName SET COMPATIBILITY_LEVEL = 170;

• Re-run your original index creation script to confirm the schema now validates correctly.

Step 4: Clear Cached Execution Plans (If Using Deployment Scripts)

• If the error persists only during automated deployments (e.g., DACPAC or SSDT), clear the plan cache to rule out stale metadata:

  DBCC FREEPROCCACHE;

• Redeploy the schema change script via SQL Server Data Tools or sqlpackage.exe.

Alternative or Verification Steps

• Verify the fix by querying sys.index_columns joined with sys.columns to confirm the is_included_column flag is only set to 1 for supported data types.
• If the index logic absolutely requires the LOB/spatial data to be queryable alongside the index, use a Filtered Index or a Covering View with SCHEMABINDING as an architectural alternative instead of direct inclusion.