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395 FUNCTION_THROW_IF_VALUE_IS_NON_ZERO

Error Code 395: FUNCTION_THROW_IF_VALUE_IS_NON_ZERO​

Tip

This error occurs when the throwIf function evaluates to a non-zero (true) value. The throwIf function is designed to intentionally throw an exception when its condition is met, and error code 395 is the standard error code for this behavior.

When you'll see it​

You'll encounter this error in the following situations:

  1. Explicit use of throwIf function:

    • When you deliberately use throwIf() in your query to validate data or enforce business rules
    • Example: SELECT throwIf(number = 2) FROM numbers(5)

  2. HTTP streaming queries:

    • When an exception occurs mid-stream while data is being sent over HTTP
    • The error appears in the response body even after HTTP 200 status has been sent

  3. Testing and validation:

    • When using throwIf to test error handling in applications
    • During data quality checks that use assertions

  4. Custom error codes (optional):

    • With the setting allow_custom_error_code_in_throwif = 1, you can specify custom error codes
    • Example: throwIf(1, 'test', toInt32(49)) - but this is generally not recommended

Potential causes​

  1. Intentional validation failure - The most common cause, where throwIf is working as designed to catch invalid data

  2. Business rule violation - Data doesn't meet expected criteria (e.g., checking for null values, out-of-range numbers, duplicate records)

  3. Test queries - Using throwIf for debugging or testing error handling

  4. HTTP response timing - In HTTP queries, error code 395 can appear mid-response when processing rows incrementally

Quick fixes​

1. For legitimate validation failures:

-- Review the condition causing the exception
SELECT throwIf(number = 3, 'Value 3 is not allowed') FROM numbers(10);

Fix: Adjust your data or query logic to avoid the triggering condition.

2. For HTTP streaming issues:

-- Enable response buffering to get complete results before sending HTTP headers
SELECT * FROM table WHERE condition 
SETTINGS wait_end_of_query=1, http_response_buffer_size=10485760;

3. For unexpected errors in production:

-- Replace throwIf with conditional logic
-- Instead of:
SELECT throwIf(value > 100, 'Value too large')

-- Use:
SELECT if(value > 100, NULL, value) FROM table;

4. For testing/debugging:

-- Use identity() function to bypass optimization and see raw performance
SELECT identity(column) FROM table WHERE NOT throwIf(column IS NULL);

Important notes​

  • The throwIf function is intentional - it's meant to throw exceptions when the condition is true
  • Error code 395 itself is not a bug; it indicates the function is working as designed
  • When using custom error codes (with allow_custom_error_code_in_throwif = 1), thrown exceptions may have unexpected error codes, making debugging harder