Troubleshooting Corrupted Indexes in ClickHouse


To troubleshoot index corruption in ClickHouse, you can follow these steps:

  1. Check the error logs: Look for any error messages related to index corruption in the ClickHouse logs. This can give you a clue as to which table or index is corrupted.
  2. Check the system tables: You can check the system tables such as system.tables and system.columns to see if there are any errors related to the corrupted table or index.
  3. Check the data: You can use the SELECT statement to check the data in the corrupted table or index. If the data is corrupted, you may see unexpected or incorrect values.
  4. Check the metadata: You can use the SHOW CREATE TABLE statement to check the metadata of the corrupted table or index. This can help you to understand the structure of the table or index and identify any issues.
  5. Repair the index: If the index is corrupted, you can use the OPTIMIZE statement to repair the index. This will rebuild the index and fix any corruption issues.
  6. Check the disk space: Ensure that the disk has enough space for the data and indexes.
  7. Check the hardware: ensure that the hardware is running fine.
  8. Check the configuration: ensure that the configuration is appropriate for the load and performance requirements.
  9. Check the version and updates: ensure that the software version is up to date, and all the updates are installed.
  10. Seek help: if the problem persists, seek help from ChistaDATA 24*7 Enterprise Support

Python script to Monitor Corrupted ClickHouse Indexes

import os
import subprocess

def check_index_corruption(database, table):
output = subprocess.check_output(["clickhouse-client", "-d", database, "-q", f"SELECT count() FROM {table} WHERE isCorrupted()"], stderr=subprocess.STDOUT)
output = output.decode("utf-8")
if "0" in output:
return f"Indexes for table {table} in database {database} are not corrupted"
return f"Indexes for table {table} in database {database} are corrupted"
except subprocess.CalledProcessError as e:
return e.output.decode("utf-8")

database = "mydatabase"
table = "mytable"
print(check_index_corruption(database, table))


This script uses the ClickHouse command-line client to check for corrupted indexes in a specified table in a specified database. It runs the SELECT count() query with the isCorrupted() function, which returns the number of corrupted parts of the index. If the output is “0”, then the indexes are not corrupted, otherwise, it returns the message that the indexes are corrupted. It is always recommended to keep monitoring your indexes and take the appropriate actions in case of corruption.

To read more about troubleshooting Indexes in ClickHouse, do consider reading the below articles:

About Shiv Iyer 227 Articles
Open Source Database Systems Engineer with a deep understanding of Optimizer Internals, Performance Engineering, Scalability and Data SRE. Shiv currently is the Founder, Investor, Board Member and CEO of multiple Database Systems Infrastructure Operations companies in the Transaction Processing Computing and ColumnStores ecosystem. He is also a frequent speaker in open source software conferences globally.