ClickHouse Query Testing

ClickHouse Query Testing

Use this skill to run ad-hoc ClickHouse queries for analytics, metrics analysis, and debugging.

Running Queries

Use the included query script:

node .claude/​skills/​clickhouse-query/​query.mjs "SELECT count() FROM views"

Options

Examples

# Count rows in a table
node .claude/​skills/​clickhouse-query/​query.mjs "SELECT count() FROM views"

# Query with filters
node .claude/​skills/​clickhouse-query/​query.mjs "SELECT * FROM modelEvents WHERE modelId = 123 LIMIT 10"

# Check query execution plan
node .claude/​skills/​clickhouse-query/​query.mjs --explain "SELECT * FROM views WHERE userId = 1"

# Override default 30s timeout for longer queries
node .claude/​skills/​clickhouse-query/​query.mjs --timeout 60 "SELECT ... (complex aggregation)"

# Query from file
node .claude/​skills/​clickhouse-query/​query.mjs -f my-query.sql

# JSON output for processing
node .claude/​skills/​clickhouse-query/​query.mjs --json "SELECT type, count() FROM modelEvents GROUP BY type"

Safety Features

1. Read-only by default: Blocks INSERT/​ALTER/​DROP unless --writable flag is used
2. 30 second timeout: Prevents runaway queries (override with --timeout)
3. Explicit permission required: Before using --writable, you MUST ask the user for permission

When to Use --writable

Only use the --writable flag when:

• The user explicitly requests write access
• You need to insert test data
• You're running maintenance operations

IMPORTANT: Always ask the user for permission before running with --writable.

Common Tables

Querying Replica Clusters

IMPORTANT: Production uses a ClickHouse replica cluster. When querying system tables (logs, metrics, etc.), you must use clusterAllReplicas() to get data from all nodes.

System Tables on Replica Clusters

-- WRONG: Only queries the node you're connected to
SELECT * FROM system.query_log WHERE event_time > now() - INTERVAL 1 HOUR

-- CORRECT: Queries all replicas in the cluster
SELECT * FROM clusterAllReplicas(default, system.query_log)
WHERE event_time > now() - INTERVAL 1 HOUR

Common System Table Queries

-- Find recent queries across all nodes
SELECT
    hostname(),
    event_time,
    query_duration_ms,
    formatReadableSize(memory_usage) AS memory,
    query
FROM clusterAllReplicas(default, system.query_log)
WHERE type = 'QueryFinish'
    AND event_time > now() - INTERVAL 5 MINUTE
ORDER BY event_time DESC
LIMIT 20

-- Find expensive queries by memory usage (last 24 hours)
SELECT
    count() as query_count,
    user,
    sum(memory_usage) AS total_memory,
    normalized_query_hash
FROM clusterAllReplicas(default, system.query_log)
WHERE event_time > now() - INTERVAL 1 DAY
    AND query_kind = 'Select'
    AND type = 'QueryFinish'
GROUP BY normalized_query_hash, user
ORDER BY total_memory DESC
LIMIT 10

-- Search query logs by pattern
SELECT event_time, query_id, query, type
FROM clusterAllReplicas(default, merge('system', '^query_log*'))
WHERE query ILIKE '%some_table%'
    AND event_time > now() - INTERVAL 5 MINUTE

-- Debug a specific query across all nodes
SELECT hostname(), message
FROM clusterAllReplicas(default, system.text_log)
WHERE query_id = 'your-query-id-here'
ORDER BY event_time_microseconds ASC

When to Use clusterAllReplicas()

ClickHouse SQL Tips

-- Use count() not COUNT(*)
SELECT count() FROM views

-- Date filtering with toDate()
SELECT * FROM views WHERE toDate(time) = today()

-- Last 7 days
SELECT * FROM modelEvents WHERE time > now() - INTERVAL 7 DAY

-- Aggregations
SELECT type, count() as cnt FROM modelEvents GROUP BY type ORDER BY cnt DESC