ClickHouse DB::Exception: Cannot parse JSON

The "DB::Exception: Cannot parse JSON" error in ClickHouse occurs when the system encounters malformed or invalid JSON data during parsing operations. This error typically arises when importing JSON data into ClickHouse or when querying JSON-formatted data.

Common Causes

1. Malformed JSON structure
2. Incorrect JSON syntax
3. Mismatched data types between JSON and ClickHouse table schema
4. Encoding issues in the JSON data
5. Incomplete or truncated JSON data

Troubleshooting and Resolution Steps

1. Validate JSON data:

   • Use online JSON validators or tools like jq to check the JSON structure.
   • Ensure all JSON objects are properly closed and formatted.

2. Check data types:

   • Verify that the data types in the JSON match the corresponding ClickHouse table schema.
   • Pay attention to number formats, date/time formats, and string representations.

3. Examine encoding:

   • Ensure the JSON data is properly encoded (usually UTF-8).
   • Check for any special characters or escape sequences that might cause parsing issues.

4. Review ClickHouse settings:

   • Verify that the input_format_allow_errors_num and input_format_allow_errors_ratio settings are appropriately configured for your use case.

5. Use error-tolerant parsing:

   • Consider using the JSONEachRow format with the input_format_skip_unknown_fields=1 setting to skip problematic fields.

6. Preprocess data:

   • If possible, preprocess the JSON data to fix known issues before importing into ClickHouse.

7. Check for truncation:

   • Ensure that the JSON data is not being truncated during transfer or storage.

Best Practices

1. Always validate JSON data before importing into ClickHouse.
2. Use appropriate data types in ClickHouse tables to match JSON structure.
3. Consider using ClickHouse's built-in functions for JSON handling, such as JSONExtract and JSONExtractString.
4. Implement error handling and logging in your data pipeline to catch and report JSON parsing issues.
5. Regularly monitor and audit your JSON data sources for consistency and quality.

Frequently Asked Questions

Q: Can ClickHouse handle nested JSON structures?
A: Yes, ClickHouse can handle nested JSON structures. You can use nested data types in your table schema and leverage functions like JSONExtract to work with nested JSON elements.

Q: How can I import large JSON files into ClickHouse efficiently?
A: For large JSON files, consider using the JSONEachRow format and the clickhouse-client command-line tool with the --format_csv_allow_single_quotes=0 option. You can also use the INSERT INTO ... SELECT statement with appropriate JSON parsing functions.

Q: What should I do if only some JSON records are causing parsing errors?
A: You can use the input_format_allow_errors_num and input_format_allow_errors_ratio settings to skip a certain number or percentage of erroneous records. This allows you to import the valid records while logging the problematic ones for further investigation.

Q: How can I debug JSON parsing issues in ClickHouse queries?
A: Use ClickHouse's JSON functions like tryParseJSON or isValidJSON in your queries to identify problematic JSON strings. You can also enable query logging and examine the server logs for detailed error messages.

Q: Is it possible to automatically fix common JSON parsing issues in ClickHouse?
A: While ClickHouse doesn't provide automatic JSON fixing, you can create user-defined functions (UDFs) or use combinations of built-in functions to preprocess and clean JSON data during import or query execution. However, it's generally better to fix issues at the source if possible.