Mutation Tracking and Analysis

Track and diagnose ALTER UPDATE, ALTER DELETE, and other mutation operations.

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Diagnostics

Run all queries from the file checks.sql and analyze the results.

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Problem Investigation

Why Is Mutation Stuck?

Check for competing operations using these ad-hoc queries:

-- Active merges on same table
select
    database,
    table,
    is_mutation,
    elapsed,
    progress,
    num_parts
from system.merges
where database = '{database}' and table = '{table}'

-- Replication queue for same table
select
    type,
    create_time,
    is_currently_executing,
    num_tries,
    last_exception
from system.replication_queue
where database = '{database}' and table = '{table}'
order by create_time
limit 20

-- Part mutations status
select
    name,
    active,
    mutation_version,
    modification_time
from system.parts
where database = '{database}' and table = '{table}'
order by mutation_version desc
limit 30

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Canceling Mutations

To kill a stuck mutation:

-- Find mutation_id first
select mutation_id, command from system.mutations
where database = '{database}' and table = '{table}' and not is_done;

-- Then kill it
-- KILL MUTATION WHERE database = '{database}' AND table = '{table}' AND mutation_id = '{mutation_id}';

Warning: Killed mutations leave table in partially-mutated state.

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Best Practices

Mutation Anti-Patterns

Anti-Pattern	Problem	Solution
Frequent small UPDATEs	Creates many mutations	Batch updates together
DELETE without WHERE	Full table rewrite	Use TTL instead
UPDATE on high-cardinality column	Slow, lots of IO	Restructure data model
Many concurrent mutations	Queue builds up	Serialize mutations

Monitoring Mutations

Set alerts for:

• Mutations pending > 10
• Mutation age > 1 hour
• latest_fail_reason not empty

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Cross-Module Triggers

Finding	Load Module	Reason
Mutation blocked by merge	altinity-expert-clickhouse-merges	Merge backlog
Mutation OOM	altinity-expert-clickhouse-memory	Memory limits
Mutation slow due to disk	altinity-expert-clickhouse-storage	IO bottleneck
Replicated mutation stuck	altinity-expert-clickhouse-replication	Replication issues

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Settings Reference

Setting	Notes
mutations_sync	0=async, 1=wait current replica, 2=wait all
max_mutations_in_flight	Max concurrent mutations
number_of_mutations_to_delay	Delay INSERTs threshold
number_of_mutations_to_throw	Throw error threshold