Clickhouse Common Errors

ClickHouse Common Errors

Overview

Quick reference for the most common ClickHouse errors with real error codes, diagnostic queries, and proven solutions. The three highest-frequency errors are inline below; the full catalog of 10 errors plus system-table diagnostics lives in references/error-reference.md.

Prerequisites

• Access to a ClickHouse endpoint — either the native clickhouse-client or the HTTP interface (curl against :8123).
• Permission to read the system.* introspection tables (system.parts, system.processes, system.query_log, system.columns, system.replicas).
• The failing statement's text and, ideally, the raw exception string — the parenthetical name (e.g. MEMORY_LIMIT_EXCEEDED) and numeric code drive lookup.

Instructions

Follow this loop to turn a raw ClickHouse exception into a verified fix:

1. Capture the exception name and code. Read the error string the client returned. If you only have a log file, use Grep to pull the matching line — Grep for DB::Exception or a specific token like MEMORY_LIMIT_EXCEEDED across the log to isolate the failure.
2. Map it to a category. Use the Error Handling code table to classify the error as Schema, Query, Performance, Permissions, Concurrency, Resources, or Insert-pattern.
3. Apply the inline fix for the three top errors (Too Many Parts, Memory Limit, Syntax) below, or open references/error-reference.md for the other seven plus copy-paste diagnostic queries.
4. Confirm with a system table. Re-run the relevant system.* query (part count, system.processes, system.query_log) to prove the condition cleared rather than assuming the fix took.

Top 3 errors (inline)

Too Many Parts (Code 252) — hundreds of tiny inserts outpace merges:

-- Check current part count per table
SELECT database, table, count() AS part_count
FROM system.parts WHERE active GROUP BY database, table ORDER BY part_count DESC;

-- Temporary relief; permanent fix is batching (10K+ rows per INSERT)
ALTER TABLE events MODIFY SETTING parts_to_throw_insert = 1000;  -- default 300

Memory Limit Exceeded (Code 241) — query wants more RAM than max_memory_usage:

SET max_memory_usage = 20000000000;             -- 20GB for this query, OR
SET max_bytes_before_external_group_by = 10000000000;  -- spill big GROUP BY to disk

Syntax Error (Code 62) — most often MySQL habits leaking in:

SELECT "user_id" FROM events;          -- double-quote (not `backtick`) identifiers
SELECT * FROM events LIMIT 10 OFFSET 20;  -- OFFSET keyword, not LIMIT 10, 20

See references/error-reference.md for Unknown Table, Timeout, DateTime parsing, Readonly, No Such Column, Type Mismatch, and Distributed-table errors.

Output

Working through this skill produces:

• A classified diagnosis — the error name, numeric code, and category from the table below.
• A concrete remediation — the exact SET, ALTER, or corrected SQL to run, plus whether it is a temporary relief valve or a permanent fix.
• A verification query against a system.* table confirming the condition cleared (e.g. part count back under threshold, no query stuck in system.processes).

Error Handling

If the error name is not in this table, search the raw exception text against the Error Codes Reference and inspect system.query_log (WHERE type = 'ExceptionWhileProcessing') for the full server-side context.

Examples

Diagnosing a stalled insert pipeline. Inserts start failing with Too many parts (600). Classify as code 252 (Insert pattern), run the system.parts count query to see which table is affected, raise parts_to_throw_insert for immediate relief, then switch the writer to batched inserts. Full walkthrough and the other nine errors are in references/error-reference.md.

Killing a runaway query. A dashboard query hangs. Query system.processes to find its query_id, then KILL QUERY WHERE query_id = '...'. The complete set of diagnostic queries (running queries, recent errors, disk usage, merge health) lives in the Diagnostic Queries section of references/error-reference.md.

Resources

• Error Codes Reference
• System Tables
• Query Log
• references/error-reference.md — full 10-error catalog plus diagnostic queries
• For comprehensive debugging, see the clickhouse-debug-bundle skill.