How to fix Error 9826 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: Error 9826 – DECOMPRESS Function Failure in SQL Server 2025

Why This Error Happens

Error 9826 occurs when the DECOMPRESS built-in function receives a varbinary input that was never compressed using the standard GZIP algorithm expected by SQL Server, or when the compressed data has been truncated or altered during storage or transit. This commonly happens when data compressed by a third-party tool (using a different compression standard) is fed into DECOMPRESS, or when a COMPRESS-generated blob is accidentally modified by an ETL process, string conversion, or encoding change (e.g., converting binary data to a non-binary collation). Additionally, physical corruption of the underlying page due to disk I/O errors can trigger this error even on data that was originally compressed correctly.

Step-by-Step Resolution Guide

Step 1: Verify the Data Source and Compression Method

  • Confirm that the binary data being passed to DECOMPRESS was actually generated using the native COMPRESS() function in T-SQL.
  • Run a quick check on the source column: SELECT TOP 10 YourColumn, DATALENGTH(YourColumn) FROM YourTable WHERE YourColumn IS NOT NULL;
  • If the data originates from an external application (e.g., a .NET service using System.IO.Compression), ensure it uses the GZipStream class specifically, as SQL Server’s COMPRESS function relies on the GZIP standard.

Step 2: Check for Data Truncation or Collation Conflicts

  • Inspect the target column’s data type; it must be VARBINARY(MAX). Using a fixed-length VARBINARY(n) can silently truncate the compressed stream, leading to corruption.
  • Review any ETL pipelines (SSIS packages or Azure Data Factory flows) to ensure the binary stream is not being cast to VARCHAR or NVARCHAR at any point, as this will corrupt the byte-order and binary integrity of the GZIP header.
  • Validate the connection string drivers (ODBC/JDBC) are configured to handle binary large objects (BLOBs) without character encoding conversion.

Step 3: Run DBCC CHECKDB to Rule Out Physical Corruption

  • If the data was previously verified as valid but is now throwing Error 9826, check for underlying disk or page corruption.
  • Execute the following in SQL Server Management Studio (SSMS):
    DBCC CHECKDB ('YourDatabaseName') WITH NO_INFOMSGS, ALL_ERRORMSGS;
  • If corruption is found in the specific pages holding the compressed blob, you may need to restore the table from the last known good backup rather than attempting a repair, as REPAIR_ALLOW_DATA_LOSS may not recover compressed binary integrity.

Step 4: Implement Error Handling for Legacy Data

  • If migrating from an older system where mixed compressed/uncompressed data exists in the same column, implement a TRY_CAST or TRY-CATCH block to handle decompression gracefully rather than failing the entire batch.
  • Update your query logic to the following pattern to bypass invalid rows: 
  BEGIN TRY
      SELECT DECOMPRESS(YourColumn) FROM YourTable;
  END TRY
  BEGIN CATCH
      PRINT 'Invalid compression data found in row ID: ' + CAST(id AS VARCHAR);
  END CATCH

Alternative or Verification Steps

  • Verification: After applying the fix, re-run the SELECT DECOMPRESS(YourColumn) query on the specific row IDs that previously failed. If the output returns readable text/binary data instead of a NULL or error, the fix was successful.
  • Alternative: If the data must remain compatible with external non-SQL systems, consider switching from the native COMPRESS/DECOMPRESS functions to a CLR Stored Procedure that explicitly defines the compression algorithm (e.g., Deflate vs. GZip) to ensure byte-for-byte compatibility across platforms.