init random data guide

Author: sdairs

docs/guides/generating-test-data.md

@@ -0,0 +1,143 @@
+---
+sidebar_label: 'Generating random test data'
+title: 'Generating random test data in ClickHouse'
+slug: /guides/generating-test-data
+description: 'Learn about Generating Random Test Data in ClickHouse'
+show_related_blogs: true
+---
+
+# Generating Random Test Data in ClickHouse
+
+Generating random data is useful when testing new use cases or benchmarking your implementation. ClickHouse has a [wide range of functions for generating random data](/sql-reference/functions/random-functions) that, in many cases, avoid the need for an external data generator.
+
+This guide provides several examples of how to generate random datasets in ClickHouse with different randomness requirements.
+
+## Simple Uniform Dataset
+
+**Use-case**: Generate a quick dataset of user events with random timestamps and event types.
+
+```sql
+CREATE TABLE user_events (
+  event_id UUID,
+  user_id UInt32,
+  event_type LowCardinality(String),
+  event_time DateTime
+) ENGINE = MergeTree
+ORDER BY event_time;
+
+INSERT INTO user_events
+SELECT
+  generateUUIDv4() AS event_id,
+  rand() % 10000 AS user_id,
+  arrayJoin(['click','view','purchase']) AS event_type,
+  now() - INTERVAL rand() % 3600*24 SECOND AS event_time
+FROM numbers(1000000);
+```
+
+* `rand() % 10000`: uniform distribution of 10k users
+* `arrayJoin(...)`: randomly selects one of three event types
+* Timestamps spread over the previous 24 hours
+
+---
+
+## Exponential Distribution
+
+**Use-case**: Simulate purchase amounts where most values are low, but a few are high.
+
+```sql
+CREATE TABLE purchases (
+  dt DateTime,
+  customer_id UInt32,
+  total_spent Float32
+) ENGINE = MergeTree
+ORDER BY dt;
+
+INSERT INTO purchases
+SELECT
+  now() - INTERVAL randUniform(1,1_000_000) SECOND AS dt,
+  number AS customer_id,
+  15 + round(randExponential(1/10), 2) AS total_spent
+FROM numbers(500000);
+```
+
+* Uniform timestamps over recent period
+* `randExponential(1/10)` — most totals near 0, offset by 15 as a minimum ([ClickHouse][1], [ClickHouse][2], [Atlantic.Net][3], [GitHub][4])
+
+---
+
+## Time-Distributed Events (Poisson)
+
+**Use-case**: Simulate event arrivals that cluster around a specific period (e.g., peak hour).
+
+```sql
+CREATE TABLE events (
+  dt DateTime,
+  event_type String
+) ENGINE = MergeTree
+ORDER BY dt;
+
+INSERT INTO events
+SELECT
+  toDateTime('2022-12-12 12:00:00')
+    - ((12 + randPoisson(12)) * 3600) AS dt,
+  'click' AS event_type
+FROM numbers(200000);
+```
+
+* Events peak around noon, with Poisson-distributed deviation
+
+---
+
+## Time-Varying Normal Distribution
+
+**Use-case**: Emulate system metrics (e.g., CPU usage) that vary over time.
+
+```sql
+CREATE TABLE cpu_metrics (
+  host String,
+  ts DateTime,
+  usage Float32
+) ENGINE = MergeTree
+ORDER BY (host, ts);
+
+INSERT INTO cpu_metrics
+SELECT
+  arrayJoin(['host1','host2','host3']) AS host,
+  now() - INTERVAL number SECOND AS ts,
+  greatest(0.0, least(100.0,
+    randNormal(50 + 30*sin(toUInt32(ts)%86400/86400*2*pi()), 10)
+  )) AS usage
+FROM numbers(10000);
+```
+
+* `usage` follows a diurnal sine wave + randomness
+* Values bounded to \[0,100]
+
+---
+
+## Categorical & Nested Data
+
+**Use-case**: Create user profiles with multi-valued interests.
+
+```sql
+CREATE TABLE user_profiles (
+  user_id UInt32,
+  interests Array(String),
+  scores Array(UInt8)
+) ENGINE = MergeTree
+ORDER BY user_id;
+
+INSERT INTO user_profiles
+SELECT
+  number AS user_id,
+  arrayShuffle(['sports','music','tech'])[1 + rand() % 3 : 1 + rand() % 3] AS interests,
+  [rand() % 100, rand() % 100, rand() % 100] AS scores
+FROM numbers(20000);
+```
+
+* Random array length between 1–3
+* Three per-user scores for each interest
+
+:::tip
+Read the [Generating Random Data in ClickHouse](https://clickhouse.com/blog/generating-random-test-distribution-data-for-clickhouse) blog for even more examples.
+:::