ClickHouse DB::Exception: Unknown compression method

The "DB::Exception: Unknown compression method" error in ClickHouse occurs when the server encounters a compression codec it does not recognize. The UNKNOWN_COMPRESSION_METHOD error typically appears when reading data that was compressed with a codec not available in the current ClickHouse build, or when a data file header specifies an invalid compression method byte. This can happen after a downgrade, when transferring data between incompatible versions, or when data files have been corrupted.

Impact

Queries that need to read parts compressed with the unrecognized method will fail immediately. If the affected parts belong to critical tables, this can block entire query workloads. Background merges involving those parts will also fail, potentially causing part count growth over time if left unaddressed.

Common Causes

1. Data was written by a newer version of ClickHouse that supports a compression codec not available in the current (older) version
2. A ClickHouse downgrade was performed without considering codec compatibility
3. Data files were transferred from a different ClickHouse installation that was compiled with additional codec support
4. Corruption in part data files caused the compression method byte in the block header to become invalid
5. Custom or experimental codecs were enabled on the source server but are not available on the destination

Troubleshooting and Resolution Steps

1. Check which compression codecs your server supports:

   SELECT * FROM system.table_functions WHERE name = 'format';
   -- Or simply try creating a table with the codec in question:
   CREATE TABLE test_codec (x UInt32 CODEC(ZSTD)) ENGINE = Memory;
   

2. Identify the affected table and parts:

   SELECT name, partition, modification_time
   FROM system.parts
   WHERE table = 'your_table' AND database = 'your_database'
   ORDER BY modification_time DESC;
   

3. Check the ClickHouse version to confirm codec support:

   SELECT version();
   

   Compare the version against the ClickHouse changelog to verify whether the codec in question is supported.

4. Upgrade ClickHouse if the data was written by a newer version. The simplest fix is often to upgrade to a version that supports the compression method used in the data.

5. If data is corrupted, attempt to recover by detaching and re-attaching the affected parts:

   ALTER TABLE your_table DETACH PART 'part_name';
   

   Then inspect the part files on disk for signs of corruption. If backups are available, restore the affected parts from a known good backup.

6. Re-compress data if you need to downgrade. Before downgrading, alter columns to use a universally supported codec:

   ALTER TABLE your_table MODIFY COLUMN your_column CODEC(LZ4);
   

   Then force a merge to rewrite all parts:

   OPTIMIZE TABLE your_table FINAL;
   

Best Practices

• Before downgrading ClickHouse, verify that all codecs used in your tables are supported by the target version.
• Stick to well-established codecs like LZ4 and ZSTD for maximum compatibility across versions.
• When migrating data between clusters, ensure both clusters run compatible ClickHouse versions.
• Maintain regular backups so that corrupted parts can be restored without data loss.

Frequently Asked Questions

Q: Can I read data compressed with a newer codec on an older ClickHouse version?
A: No. If the older version does not include support for that codec, it cannot decompress the data. You must either upgrade the server or re-compress the data using a supported codec on the newer server before migrating.

Q: How do I find out which codec a specific column uses?
A: You can query the system.columns table:

SELECT database, table, name, compression_codec FROM system.columns WHERE compression_codec != '';

Q: Does this error always mean corruption?
A: Not necessarily. The most common cause is a version mismatch rather than corruption. However, if you are confident the codec should be supported, corruption in the block header is a possibility worth investigating.