---
title: "random"
slug: "dev-database-reference-sql-functions-numeric-random"
updated: 2026-10-30T18:18:51Z
published: 2026-10-30T18:18:51Z
canonical: "docs.vastdata.com/dev-database-reference-sql-functions-numeric-random"
---

> ## Documentation Index
> Fetch the complete documentation index at: https://docs.vastdata.com/llms.txt
> Use this file to discover all available pages before exploring further.

# random

*Applies to VAST Cluster 5.5.*

*[Functions](/develop/docs/dev-database-reference-sql-functions-index) › [Numeric and math functions](/develop/docs/dev-database-reference-sql-functions-numeric) › Random numbers.* Returns a random `DOUBLE` from 0 up to, not including, 1. Each row gets its own value, and every run of a query gives new values.

## Syntax

```
random()
```

`random` takes no arguments.

**Returns:** `DOUBLE`, from 0 up to, not including, 1.

## Set up the examples

The examples connect with the ADBC driver, as in [VAST Query Engine (ADBC)](/develop/docs/dev-database-integrations-adbc), and work in a schema called `docs_sql_fn_numeric_random`. Set `QE_ENDPOINT`, `AWS_ACCESS_KEY_ID`, `AWS_SECRET_ACCESS_KEY` and `VASTDB_BUCKET` ([SQL reference](/develop/docs/dev-database-reference-sql-index#example-connect-and-use-a-transaction)), and run the blocks in order in one Python session. Below each block is the output you should see. Because `random()` gives new values on every run, the examples print checks and counts rather than the values themselves.

This block connects, defines the `run()` and `plan()` helpers, and creates a table of four sensor readings. Each row has a `sample_key` that was filled with a random number when the row was written; the last row has no values:

```
import os
import re

import adbc_driver_manager
import adbc_driver_manager.dbapi as dbapi
import adbc_driver_vastdb

BUCKET = os.environ["VASTDB_BUCKET"]
SCHEMA = f'"{BUCKET}/docs_sql_fn_numeric_random"'

conn = dbapi.connect(
    driver=adbc_driver_vastdb.get_driver_path(),
    db_kwargs={
        "vast.db.endpoint": os.environ["QE_ENDPOINT"],
        "vast.db.access_key": os.environ["AWS_ACCESS_KEY_ID"],
        "vast.db.secret_key": os.environ["AWS_SECRET_ACCESS_KEY"],
    },
    autocommit=True,
)

def server_message(e):
    """The server's message in a driver error, without the request ID or the echoed statement."""
    text = re.sub(r"\[req=0x[0-9a-f]+\]: ", "", str(e))
    found = (re.search(r'message: "(.*?)", request_id', text, re.S)
             or re.search(r"description: (.*)\)", text, re.S))
    message = found.group(1) if found else text.strip().splitlines()[-1]
    message = re.sub(r"^Plan query: '.*?' failed with: ", "", message, flags=re.S)
    message = re.split(r"\\n|\n", message)[0].replace('\\"', '"')
    return message.replace(BUCKET, "<bucket>")

def run(sql):
    """Run one statement. Print the rows it returns, or the error."""
    try:
        with conn.cursor() as cur:
            cur.execute(sql)
            rows = cur.fetchall() if cur.description else []
    except (adbc_driver_manager.Error, OSError) as e:
        print(f"{type(e).__name__}: {server_message(e)}")
    else:
        print(rows)

def plan(sql):
    """Print the plan without its ~N Rows lines: the table's row count, which can still be 0 just after an insert."""
    with conn.cursor() as cur:
        cur.execute("EXPLAIN " + sql)
        steps = cur.fetchall()[0][1]
    print("\n".join(line for line in steps.splitlines() if "Rows" not in line))

TABLE = f"{SCHEMA}.readings"
run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}")
run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, sensor VARCHAR, reading DOUBLE, sample_key DOUBLE)")
run(f"INSERT INTO {TABLE} VALUES "
    "(1, 'north', 20.5, 0.12), "
    "(2, 'south', 21.0, 0.87), "
    "(3, 'east', 19.5, 0.45), "
    "(4, NULL, NULL, NULL)")
```

```
[]
[]
[(4,)]
```

## What it does

`random()` returns a `DOUBLE` that is at least 0 and less than 1. In a query on a table, it's evaluated for each row, so each row gets its own value; it doesn't depend on the row's columns, so a row whose columns are all NULL gets a value too.

Scale it for other ranges: `random() * 10` is from 0 up to 10, and `CAST(floor(random() * 6) AS BIGINT) + 1` is a whole number from 1 to 6.

## NULLs and edge cases

| Input | Result |
| --- | --- |
| `random()` | A `DOUBLE` with `random() &gt;= 0 AND random() &lt; 1` true |
| `typeof(random())` | `&apos;DOUBLE&apos;` |
| 10,000 values from `range(10000)` | The smallest is at least 0, the largest less than 1 |
| `WHERE random() IS NOT NULL` | Every row, including the row whose columns are all NULL |
| `random(42)` | Error: `No function matches the given name and argument types &apos;random(INTEGER_LITERAL)&apos;` |
| `rand()` | Error: `Scalar Function with name rand does not exist!` |

The same cases, as the cluster returns them. They use the table and helpers from [Set up the examples](/develop/docs/dev-database-reference-sql-functions-numeric-random#set-up-the-examples):

```
run("SELECT random() >= 0 AND random() < 1, typeof(random())")
run("SELECT min(r) >= 0, max(r) < 1, count(*) FROM (SELECT random() AS r FROM range(10000))")
run(f"SELECT id FROM {TABLE} WHERE random() IS NOT NULL ORDER BY id LIMIT 10")
run("SELECT random(42)")
run("SELECT rand()")
```

```
[(True, 'DOUBLE')]
[(True, True, 10000)]
[(1,), (2,), (3,), (4,)]
InternalError: Binder Error: No function matches the given name and argument types 'random(INTEGER_LITERAL)'. You might need to add explicit type casts.
InternalError: Catalog Error: Scalar Function with name rand does not exist!
```

## Examples

A random value for each row. The block prints how many rows came back, how many different values they hold, and whether every value is from 0 up to 1:

```
with conn.cursor() as cur:
    cur.execute(f"SELECT random() FROM {TABLE} ORDER BY id LIMIT 10")
    values = [row[0] for row in cur.fetchall()]
print(len(values), len(set(values)), all(0 <= v < 1 for v in values))
```

```
4 4 True
```

A whole number from 1 to 6, like a die. Over 1,000 rolls, the smallest and largest values come out as 1 and 6:

```
run("SELECT min(d), max(d) FROM (SELECT CAST(floor(random() * 6) AS BIGINT) + 1 AS d FROM range(1000))")
```

```
[(1, 6)]
```

Keep about half of the rows: `random() &lt; 0.5` keeps each row with a chance of one in two. Of 10,000 rows, between 4,500 and 5,500 are kept:

```
run("SELECT count(*) BETWEEN 4500 AND 5500 FROM (SELECT random() AS r FROM range(10000)) WHERE r < 0.5")
```

```
[(True,)]
```

Pick two rows at random. The block prints how many ids came back and whether they're all ids of the table:

```
with conn.cursor() as cur:
    cur.execute(f"SELECT id FROM {TABLE} ORDER BY random() LIMIT 2")
    ids = [row[0] for row in cur.fetchall()]
print(len(ids), set(ids) <= {1, 2, 3, 4})
```

```
2 True
```

## How it runs in VAST

The Query Engine runs on the cluster's CNodes, and a query on a table starts with a scan in VAST DataBase (`VAST_SCAN`). `random()` is always computed by the Query Engine:

- **In the `SELECT` list,** the Query Engine computes `random()` for each row the scan returns. `random()` reads no column, so the scan of `SELECT random() FROM readings` projects no columns.
- **In `WHERE`,** the scan sends every row, and a `FILTER` step in the Query Engine evaluates the condition on each one. A random sample is taken after the rows have left storage, so `random() &lt; 0.01` still reads the whole table.

The plan of a half sample shows the `FILTER` step above the scan:

```
plan(f"SELECT id FROM {TABLE} WHERE random() < 0.5")
```

```
┌───────────────────────────┐
│           FILTER          │
│    ────────────────────   │
│      (random() < 0.5)     │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│         VAST_SCAN         │
│    ────────────────────   │
│    Function: VAST_SCAN    │
│      Projections: id      │
│                           │
└───────────────────────────┘
```

## Make it faster

- **Store a random number with each row if you sample often.** Fill a column such as `sample_key` with a random value from 0 to 1 when you write the rows. A comparison of that column with a constant is pushed down to VAST DataBase, so the rows outside the sample never leave storage, and the sample is the same on every run. The plan for `sample_key &lt; 0.5` has no `FILTER` step:

```
plan(f"SELECT id FROM {TABLE} WHERE sample_key < 0.5")
run(f"SELECT id FROM {TABLE} WHERE sample_key < 0.5 ORDER BY id LIMIT 10")
```

```
┌───────────────────────────┐
│         VAST_SCAN         │
│    ────────────────────   │
│    Function: VAST_SCAN    │
│      Projections: id      │
│                           │
└───────────────────────────┘
[(1,), (3,)]
```

## Limitations and errors

- **No seed argument.** `random(42)` fails with `No function matches the given name and argument types &apos;random(INTEGER_LITERAL)&apos;`. For a sample that's the same on every run, store the random value in a column, as in [Make it faster](/develop/docs/dev-database-reference-sql-functions-numeric-random#make-it-faster). [setseed](/develop/docs/dev-database-reference-sql-functions-numeric-setseed) accepts a seed, but it doesn't make `random()` repeat.
- **`rand` isn't available.** `rand()` fails with `Scalar Function with name rand does not exist!`. Use `random()`.
- **Values change on every run.** `random()` isn't repeatable: print checks on its values, as these examples do, or store the values in a column.

```
run("SELECT random(42)")
run("SELECT rand()")
```

```
InternalError: Binder Error: No function matches the given name and argument types 'random(INTEGER_LITERAL)'. You might need to add explicit type casts.
InternalError: Catalog Error: Scalar Function with name rand does not exist!
```

## Similar functions

| Function | Returns |
| --- | --- |
| `random()` | A random `DOUBLE` from 0 up to, not including, 1 |
| [setseed](/develop/docs/dev-database-reference-sql-functions-numeric-setseed) `(x)` | NULL; it checks a seed from -1 to 1, and doesn't make `random()` repeat |
| [gen_random_uuid](/develop/docs/dev-database-reference-sql-functions-binary-hash-uuid-gen-random-uuid) `()` | A random `UUID` |

```
run("SELECT random() >= 0 AND random() < 1, setseed(0.5) IS NULL, typeof(gen_random_uuid())")
```

```
[(True, True, 'UUID')]
```

Clean up:

```
run(f"DROP TABLE IF EXISTS {TABLE}")
run(f"DROP SCHEMA IF EXISTS {SCHEMA}")
conn.close()
```

```
[]
[]
```

## See also

- [Numeric and math functions](/develop/docs/dev-database-reference-sql-functions-numeric): all the numeric functions.
- [setseed](/develop/docs/dev-database-reference-sql-functions-numeric-setseed).
- [Where filters run](/develop/docs/dev-database-reference-sql-select#where-filters-run) and [pushdowns](/develop/docs/dev-database-reference-pushdowns).
- [EXPLAIN](/develop/docs/dev-database-reference-sql-explain): how to read a plan.