How to fix Error 4891 on Windows Server 2025 / Windows 11

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Why This Error Happens

Error 4891 occurs when SQL Server 2025 detects that the target table’s schema has changed between the time the bulk insert operation was prepared and when it was actually executed. This typically happens due to concurrent DDL operations (ALTER TABLE, column additions/removals) running against the same table while a BULK INSERT, bcp, or OPENROWSET(BULK…) process is active. Cached metadata or an outdated format file referencing the old column structure can also trigger this mismatch.

Step-by-Step Resolution Guide

Step 1: Identify the Schema Mismatch Source

  • Check SQL Server Error Logs located at C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\ERRORLOG for the exact timestamp of Error 4891.
  • Run sp_help 'YourTableName' in SQL Server Management Studio (SSMS) to compare current column definitions against your bulk insert format file or data source.
  • Query sys.dm_tran_locks to identify if another session was holding a schema modification lock (Sch-M) during the bulk load.

Step 2: Synchronize the Format File or Source Schema

  • If using a .fmt or .xml format file, regenerate it using:
  bcp YourDatabase.dbo.YourTableName format nul -c -f C:\BulkData\YourTable.fmt -S YourServerName -T
  • Ensure the field count, data types, and nullability in the format file exactly match the current table definition.
  • If the source is a flat file or staging table, validate column order has not shifted due to a recent ALTER TABLE ADD/DROP COLUMN statement.

Step 3: Prevent Concurrent DDL During Bulk Operations

  • Wrap your bulk insert in a transaction with an explicit table lock hint to prevent concurrent schema changes:
  BULK INSERT dbo.YourTableName
  FROM 'C:\BulkData\datafile.csv'
  WITH (TABLOCK, FORMATFILE = 'C:\BulkData\YourTable.fmt');
  • Schedule any pending schema migrations or ALTER TABLE scripts outside the bulk load window, ideally during a maintenance window with Agent Jobs disabled.

Step 4: Clear Cached Execution Plans and Metadata

  • Run DBCC FREEPROCCACHE to clear any stale execution plans that may reference the old table schema.
  • Restart the SQL Server service via Services.msc (SQL Server (MSSQLSERVER)) if metadata caching persists after the freeproccache command.

Alternative or Verification Steps

  • Re-run the bulk insert operation in a test environment with the updated format file and confirm Error 4891 no longer appears in the ERRORLOG.
  • If the issue persists, drop and recreate the target table’s statistics using UPDATE STATISTICS YourTableName WITH FULLSCAN, as corrupted statistics can sometimes mimic schema-change errors during bulk operations.