ClickHouse: Columnar Analytics Database

What is ClickHouse?

ClickHouse is an open-source columnar database management system designed for online analytical processing (OLAP). Developed by Yandex, it can process billions of rows in milliseconds and is optimized for analytical workloads requiring real-time query performance on large datasets.

Unlike traditional row-based databases, ClickHouse stores data in columns, achieving exceptional compression ratios and query performance for analytics. It excels in scenarios like real-time dashboards, business intelligence, log analysis, and time-series data processing.

ClickHouse Analytics Calculator

ClickHouse Architecture Features

CREATE TABLE events (
  date Date,
  user_id UInt32
) ENGINE = MergeTree(date, user_id, 8192)

ClickHouse Specialized Features

CREATE TABLE user_events (
  user_id UInt32,
  event_types Array(String),
  timestamps Array(DateTime),
  metadata Nested(
    key String,
    value String
  )
) ENGINE = MergeTree ORDER BY user_id;

-- Time window aggregation
SELECT
  toStartOfHour(timestamp) as hour,
  count() as events,
  uniq(user_id) as unique_users
FROM events
WHERE timestamp >= now() - INTERVAL 24 HOUR
GROUP BY hour
ORDER BY hour;

SELECT
  uniq(user_id) as exact_unique,
  uniqHLL12(user_id) as approx_unique,
  quantile(0.95)(response_time) as p95_latency,
  quantileTDigest(0.99)(response_time) as p99_latency
FROM requests;

Real-World ClickHouse Implementations

ClickHouse Use Case Patterns