> ## Documentation Index
> Fetch the complete documentation index at: https://private-7c7dfe99-parallel-read-in-order-multi-part.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

> The GenerateRandom table engine produces random data for given table schema.

# GenerateRandom table engine

The GenerateRandom table engine produces random data for given table schema.

Usage examples:

* Use in test to populate reproducible large table.
* Generate random input for fuzzing tests.

<h2 id="usage-in-clickhouse-server">
  Usage in ClickHouse Server
</h2>

```sql theme={null}
ENGINE = GenerateRandom([random_seed [,max_string_length [,max_array_length]]])
[SETTINGS null_ratio = ..., max_json_depth = ..., max_json_keys_per_object = ...]
```

The `max_array_length` and `max_string_length` parameters specify maximum length of all
array or map columns and strings correspondingly in generated data.

The `SETTINGS` clause controls how `Nullable`, `Variant`, `Dynamic` and `JSON` values are generated:
`null_ratio` (default `0.0625`) is the probability that a value is `NULL` and the base probability
that a `JSON` key is absent from a row - a minority of sparse keys are absent several times more
often, `max_json_depth` (default `3`) bounds the nesting depth of generated `JSON` objects,
and `max_json_keys_per_object` (default `8`) bounds the number of generated keys on one level of an
object. The [`generateRandom`](/reference/functions/table-functions/generate) table function accepts
the same settings.

Generate table engine supports only `SELECT` queries.

It supports all [DataTypes](/reference/data-types/index) that can be stored in a table,
except `AggregateFunction`, `Interval`, `Nothing` and `QBit`.

<h2 id="example">
  Example
</h2>

**1.** Set up the `generate_engine_table` table:

```sql theme={null}
CREATE TABLE generate_engine_table (name String, value UInt32) ENGINE = GenerateRandom(1, 5, 3)
```

**2.** Query the data:

```sql theme={null}
SELECT * FROM generate_engine_table LIMIT 3
```

```text theme={null}
┌─name─┬──────value─┐
│ c4xJ │ 1412771199 │
│ r    │ 1791099446 │
│ 7#$  │  124312908 │
└──────┴────────────┘
```

<h2 id="details-of-implementation">
  Details of Implementation
</h2>

* Not supported:
  * `ALTER`
  * `SELECT ... SAMPLE`
  * `INSERT`
  * Indices
  * Replication