---
title: "array_length"
slug: "dev-database-reference-sql-functions-lists-array-length"
updated: 2026-10-30T18:19:11Z
published: 2026-10-30T18:19:11Z
canonical: "docs.vastdata.com/dev-database-reference-sql-functions-lists-array-length"
---

> ## 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.

# array_length

*Applies to VAST Cluster 5.5.*

*[Functions](/develop/docs/dev-database-reference-sql-functions-index) › [List, struct and map functions](/develop/docs/dev-database-reference-sql-functions-lists-and-structs) › Size and elements.* Returns the number of elements in a list, NULL elements included. For a list of lists, it counts the outer list. [length](/develop/docs/dev-database-reference-sql-functions-string-length) and `len` return the same count.

## Syntax

```
array_length(list)
array_length(list, dimension)
```

| Argument | Type | Description |
| --- | --- | --- |
| `list` | a list (`ANY[]`) or a fixed-size array | The list to count. |
| `dimension` | `BIGINT` | Which dimension to count. Only `1` works. |

**Returns:** `BIGINT`.

## 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_lists_array_length`. 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.

This block connects, defines the `run()` and `plan()` helpers, and creates a table of four survey responses: a list of answers, one of them unanswered (NULL) in the second row, and a grid of answers as a list of lists. The third row has empty lists and 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_lists_array_length"'

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}.surveys"
run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}")
run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, answers BIGINT[], grid BIGINT[][])")
run(f"INSERT INTO {TABLE} VALUES "
    "(1, [4, 5, 3], [[1, 2], [3]]), "
    "(2, [2, NULL, 5], [[], NULL, [4]]), "
    "(3, [], []), "
    "(4, NULL, NULL)")
```

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

## What it does

`array_length(l)` counts the top-level elements of `l`:

- **NULL elements count.** `array_length([1, NULL, 3])` is `3`. To count only the non-NULL elements, use [list_count](/develop/docs/dev-database-reference-sql-functions-lists-list-count).
- **Only the outer list.** `array_length([[1, 2], [3]])` is `2`, the number of inner lists, whatever their lengths.
- **The dimension must be 1.** `array_length(l, 1)` is the same as `array_length(l)`. Any other dimension fails; see [Limitations and errors](/develop/docs/dev-database-reference-sql-functions-lists-array-length#limitations-and-errors).

## NULLs and edge cases

| Input | Result |
| --- | --- |
| `NULL` list | `NULL` |
| `NULL` dimension | `NULL` |
| `[]` | `0` |
| `[1, NULL, 3]` | `3` |
| `[[1, 2], [3]]` | `2` |
| `array_value(1, 2, 3)` | `3` |
| `array_length([[1, 2], [3]], 2)`, or a dimension of `0` or `-1` | Error: `array_length for lists with dimensions other than 1 not implemented` |
| A number, string, `BLOB`, date or boolean | Error: `No function matches the given name and argument types`, for example `&apos;array_length(STRING_LITERAL)&apos;` |

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-lists-array-length#set-up-the-examples):

```
run("SELECT array_length(NULL), array_length([1, 2], NULL), array_length([]), array_length([1, NULL, 3]), "
    "array_length([[1, 2], [3]]), array_length(array_value(1, 2, 3))")
run("SELECT array_length([[1, 2], [3]], 2)")
run("SELECT array_length('abc')")
```

```
[(None, None, 0, 3, 2, 3)]
OSError: array_length for lists with dimensions other than 1 not implemented
InternalError: Binder Error: No function matches the given name and argument types 'array_length(STRING_LITERAL)'. You might need to add explicit type casts.
```

## Examples

The number of answers and of grid rows in each response. The NULL answer in response 2 is counted:

```
run(f"SELECT id, array_length(answers), array_length(answers, 1), array_length(grid) FROM {TABLE} ORDER BY id LIMIT 10")
```

```
[(1, 3, 3, 2), (2, 3, 3, 3), (3, 0, 0, 0), (4, None, None, None)]
```

Keep the responses with more than one answer:

```
run(f"SELECT id FROM {TABLE} WHERE array_length(answers) > 1 ORDER BY id LIMIT 10")
```

```
[(1,), (2,)]
```

## 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`). A list column is read whole, and every condition on a list runs in the Query Engine, so `array_length()` is always computed there:

- **In the `SELECT` list,** the scan reads the list and the Query Engine counts it for each row it returns.
- **In `WHERE`,** the scan sends the list for every row, and a `FILTER` step in the Query Engine counts it and keeps the rows that match. No rows are skipped in storage.

The plan of the `WHERE` example shows the `FILTER` step above the scan:

```
plan(f"SELECT id FROM {TABLE} WHERE array_length(answers) > 1")
```

```
┌───────────────────────────┐
│         PROJECTION        │
│    ────────────────────   │
│             #1            │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│           FILTER          │
│    ────────────────────   │
│(array_length(answers) > 1)│
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│         VAST_SCAN         │
│    ────────────────────   │
│    Function: VAST_SCAN    │
│                           │
│        Projections:       │
│          answers          │
│             id            │
│                           │
└───────────────────────────┘
```

## Make it faster

- **An empty-list test isn't faster.** `answers = []` and `answers IS NULL` also run in a `FILTER` step: no condition on a list column is pushed down.
- **Store the count if you filter on it often.** Compute it when you write the rows and keep it in its own column: `answer_count &gt; 1` is pushed down to VAST DataBase, `array_length(answers) &gt; 1` isn't.
- **Combine it with a pushed-down condition.** In `id &gt; 100 AND array_length(answers) &gt; 1`, `id &gt; 100` is still evaluated in VAST DataBase, so `array_length()` runs only on the rows that pass it.

```
plan(f"SELECT id FROM {TABLE} WHERE answers = []")
```

```
┌───────────────────────────┐
│         PROJECTION        │
│    ────────────────────   │
│             #1            │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│           FILTER          │
│    ────────────────────   │
│       (answers = [])      │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│         VAST_SCAN         │
│    ────────────────────   │
│    Function: VAST_SCAN    │
│                           │
│        Projections:       │
│          answers          │
│             id            │
│                           │
└───────────────────────────┘
```

## Limitations and errors

- **Only the first dimension.** `array_length(l, 2)` fails with `array_length for lists with dimensions other than 1 not implemented`, as do `0` and negative dimensions. To get the length of the longest inner list, use [list_transform](/develop/docs/dev-database-reference-sql-functions-lists-list-transform) and [list_max](/develop/docs/dev-database-reference-sql-functions-lists-list-max): `list_max(list_transform(grid, x -&gt; len(x)))`. It returns NULL for an empty list.
- **Lists only.** A string, a number, a `BLOB`, a date or a boolean fails with `No function matches the given name and argument types`. For the length of a string, use [length](/develop/docs/dev-database-reference-sql-functions-string-length).
- **Other SQL dialects' names aren't available.** `cardinality` works only on maps (`Binder Error: Cardinality can only operate on MAPs`), and `array_size` fails with `Scalar Function with name array_size does not exist!`. Use `array_length` or `len`.

```
run(f"SELECT id, list_max(list_transform(grid, x -> len(x))) FROM {TABLE} ORDER BY id LIMIT 10")
run("SELECT cardinality([1, 2, 3])")
run("SELECT array_size([1, 2])")
```

```
[(1, 2), (2, 1), (3, None), (4, None)]
InternalError: Binder Error: Cardinality can only operate on MAPs
InternalError: Catalog Error: Scalar Function with name array_size does not exist!
```

## Similar functions

| Function | Counts | Result for `[3, 1, NULL, 3, 10]` |
| --- | --- | --- |
| `array_length(l)`, `array_length(l, 1)` | elements, NULLs included | `5` |
| `len(l)`, [length](/develop/docs/dev-database-reference-sql-functions-string-length) `(l)` | elements, NULLs included | `5` |
| [list_count](/develop/docs/dev-database-reference-sql-functions-lists-list-count) | non-NULL elements | `4` |
| [list_unique](/develop/docs/dev-database-reference-sql-functions-lists-list-unique) | distinct non-NULL elements | `3` |

```
run("SELECT array_length(l), array_length(l, 1), len(l), length(l), list_count(l), list_unique(l) "
    "FROM (SELECT [3, 1, NULL, 3, 10] AS l)")
```

```
[(5, 5, 5, 5, 4, 3)]
```

Clean up:

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

```
[]
[]
```

## See also

- [List, struct and map functions](/develop/docs/dev-database-reference-sql-functions-lists-and-structs): all the list functions.
- [length](/develop/docs/dev-database-reference-sql-functions-string-length), [list_count](/develop/docs/dev-database-reference-sql-functions-lists-list-count) and [list_unique](/develop/docs/dev-database-reference-sql-functions-lists-list-unique).
- [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.