--- title: "list_filter" slug: "dev-database-reference-sql-functions-lists-list-filter" updated: 2026-10-30T18:19:06Z published: 2026-10-30T18:19:06Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-lists-list-filter" --- > ## 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. # list_filter *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) › Lambdas.* Returns the elements of a list for which a lambda, a small condition written in the query, is true, in their original order. `array_filter` and `filter` are the same function. ## Syntax ``` list_filter(l, x -> condition) list_filter(l, (x, i) -> condition) array_filter(l, x -> condition) filter(l, x -> condition) ``` | Argument | Type | Description | | --- | --- | --- | | `l` | a list of any type | The list to filter. | | `x -> condition` | lambda | `x` stands for each element in turn. With `(x, i) -> condition`, `i` is the element's position, starting at 1. The condition can use other columns of the row. | **Returns:** a list of the same type as `l`. **Aliases:** `array_filter`, `filter`. ## 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_list_filter`. 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 five rows of lists (numbers, words, measurements and lists of lists) and a threshold `n`. Row 3 has empty lists and row 4 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_list_filter"' 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, "") 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}.samples" run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}") run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, nums BIGINT[], words VARCHAR[], vals DOUBLE[], nested BIGINT[][], n BIGINT)") run(f"INSERT INTO {TABLE} VALUES " "(1, [1, 2, 3], ['a', 'b', 'c'], [0.5, 2.5, -1.0], [[1, 2], [3]], 2), " "(2, [3, NULL, 3], ['b', NULL, 'B'], [2.0, NULL], [[], NULL, [4]], -1), " "(3, [], [], [], [], 0), " "(4, NULL, NULL, NULL, NULL, NULL), " "(5, [-5, 0, -5, 10], ['\u00e9', 'Zo\u00eb', '\U0001F1EB\U0001F1F7', ''], [1e308, 1e308, -1e-308], [[5, NULL], [], [6, 7, 8]], 10)") ``` ``` [] [] [(5,)] ``` ## What it does `list_filter(l, x -> condition)` tests `condition` for each element of `l`, with `x` set to that element, and keeps the elements for which it's true. `list_filter([1, 2, 3], x -> x > 1)` returns `[2, 3]`. - **Only true keeps an element.** An element for which the condition is false or NULL is dropped: `list_filter([1, NULL, 3], x -> x > 1)` returns `[3]`, because `NULL > 1` is NULL. - **To keep or find NULL elements, test for them.** `x -> x IS NOT NULL` removes them; `x -> x IS NULL` keeps only them. - **The position is available.** In `(x, i) -> condition`, `i` is the position of `x`, starting at 1: `list_filter([10, 20, 30], (x, i) -> i % 2 = 1)` returns `[10, 30]`. - **The condition can use the row's columns.** `list_filter(nums, x -> x >= n)` compares each element with the row's `n`. - **`NaN` is larger than any number,** so `x -> x > 1` keeps a `NaN` element. - **Write a condition that returns `BOOLEAN`.** A lambda that returns another type isn't rejected: `x -> x + 1` and `x -> 'yes'` keep every element, and on `[0, 1, 2]`, `x -> x` drops the `0`. ## NULLs and edge cases | Input | Result | | --- | --- | | `list_filter(NULL::BIGINT[], x -> x > 1)` | `NULL` | | `list_filter([], x -> x > 1)` | `[]` | | `list_filter([1, NULL, 3], x -> x > 1)` | `[3]` | | `list_filter([1, 2], x -> NULL)` | `[]` | | `list_filter([1, NULL, 3], x -> x IS NULL)` | `[NULL]` | | `list_filter([1, NULL, 3], x -> x IS NOT NULL)` | `[1, 3]` | | `list_filter(['NaN'::DOUBLE, 1.0, 2.0], x -> x > 1)` | `[nan, 2.0]` | | `list_filter([1, 2, 3], x -> x + 1)` | `[1, 2, 3]` | | `list_filter([0, 1, 2], x -> x)` | `[1, 2]` | | `list_filter([1], NULL)` | Error: `Binder Error: Invalid lambda expression!` | | `list_filter([1, 2], (x, i, j) -> true)` | Error: `Binder Error: This lambda function only supports up to two lambda parameters!` | | A number, string, date or other non-list | Error: `Binder Error: failed to bind function, either: Invalid LIST argument during lambda function binding!` | The same cases, as the cluster returns them. They use the helpers from [Set up the examples](/develop/docs/dev-database-reference-sql-functions-lists-list-filter#set-up-the-examples): ``` run("SELECT list_filter(NULL::BIGINT[], x -> x > 1), list_filter([], x -> x > 1), " "list_filter([1, NULL, 3], x -> x > 1), list_filter([1, 2], x -> NULL)") run("SELECT list_filter([1, NULL, 3], x -> x IS NULL), list_filter([1, NULL, 3], x -> x IS NOT NULL), " "list_filter(['NaN'::DOUBLE, 1.0, 2.0], x -> x > 1)") run("SELECT list_filter([1, 2, 3], x -> x + 1), list_filter([0, 1, 2], x -> x)") run("SELECT list_filter([1], NULL)") run("SELECT list_filter([1, 2], (x, i, j) -> true)") run("SELECT list_filter(123, x -> x > 1)") ``` ``` [(None, [], [3], [])] [([None], [1, 3], [nan, 2.0])] [([1, 2, 3], [1, 2])] InternalError: Binder Error: Invalid lambda expression! InternalError: Binder Error: This lambda function only supports up to two lambda parameters! InternalError: Binder Error: failed to bind function, either: Invalid LIST argument during lambda function binding! ``` ## Examples Keep the numbers above 1, the numbers at least the row's `n`, and the elements after the first position: ``` run(f"SELECT id, list_filter(nums, x -> x > 1), filter(nums, x -> x >= n), array_filter(nums, (x, i) -> i > 1) " f"FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, [2, 3], [2, 3], [2, 3]), (2, [3, 3], [3, 3], [None, 3]), (3, [], [], []), (4, None, None, None), (5, [10], [10], [0, -5, 10])] ``` Filter text and lists of lists: drop the word `'b'`, and keep the inner lists with more than one element: ``` run(f"SELECT id, list_filter(words, x -> x <> 'b'), list_filter(nested, x -> len(x) > 1) " f"FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, ['a', 'c'], [[1, 2]]), (2, ['B'], []), (3, [], []), (4, None, None), (5, ['é', 'Zoë', '🇫🇷', ''], [[5, None], [6, 7, 8]])] ``` The rows that have at least one number above 1: filter the list, then test its length: ``` run(f"SELECT id FROM {TABLE} WHERE len(list_filter(nums, x -> x > 1)) > 0 ORDER BY id LIMIT 10") ``` ``` [(1,), (2,), (5,)] ``` ## 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`). The scan reads a list column whole, and `list_filter()` and its lambda always run in the Query Engine. Its condition filters the elements of a list, not the rows of the table: - **In the `SELECT` list,** a projection step filters each row's list after the scan. - **In `WHERE`,** the scan sends the list for every row, and a `FILTER` step runs the lambda 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 len(list_filter(nums, x -> x > 1)) > 0") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #1 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ (len(list_filter(nums)) > │ │ 0) │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ nums │ │ id │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Add a condition on a plain column.** A comparison of a column with a constant, such as `id > 1`, is evaluated in VAST DataBase, so the lambda runs only on the rows that pass it. The plan for `id > 1` alone has no `FILTER` step: ``` plan(f"SELECT id FROM {TABLE} WHERE id > 1") ``` ``` ┌───────────────────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ Projections: id │ │ │ └───────────────────────────┘ ``` - **Test for one value with `list_contains`.** To ask whether a list holds a value, [list_contains](/develop/docs/dev-database-reference-sql-functions-lists-list-contains)`(l, 3)` says it in one function; `len(list_filter(l, x -> x = 3)) > 0` gives the same answer. - **Store values you filter on in their own column.** A condition on a list always runs in the Query Engine; a comparison of a plain column with a constant is pushed down. ## Limitations and errors - **No functions to remove values by name.** `array_remove`, `array_except` and `list_except` fail with `Scalar Function with name ... does not exist!`. Use `list_filter`: `list_filter(l, x -> x <> 1)` removes every `1`, and `list_filter(a, x -> NOT list_contains(b, x))` removes the elements of `b` from `a`. - **The `x -> condition` form only.** `lambda x: x > 1` fails with `Parser Error: syntax error at or near "x"`. At most two lambda parameters. ``` run("SELECT array_remove([1, 2, 1], 1)") run("SELECT list_filter([1, 2, 1], x -> x <> 1), list_filter([1, 2, 3], x -> NOT list_contains([2], x))") ``` ``` InternalError: Catalog Error: Scalar Function with name array_remove does not exist! [([2], [1, 3])] ``` ## Similar functions | Function | What it does | Result for `[3, 1, NULL, 3, 10]` | | --- | --- | --- | | `list_filter(l, x -> x > 1)` | keeps the elements for which a lambda is true | `[3, 3, 10]` | | [List comprehensions](/develop/docs/dev-database-reference-sql-functions-lists-list-comprehension) `[x FOR x IN l IF x > 1]` | the same, written as a comprehension | `[3, 3, 10]` | | [list_contains](/develop/docs/dev-database-reference-sql-functions-lists-list-contains)`(l, 3)` | whether the list holds a value | `true` | | [list_where](/develop/docs/dev-database-reference-sql-functions-lists-list-where) | keeps the elements where a list of booleans is true | | | [list_transform](/develop/docs/dev-database-reference-sql-functions-lists-list-transform)`(l, x -> x * 2)` | applies a lambda to each element | `[6, 2, NULL, 6, 20]` | ``` run("SELECT list_filter(l, x -> x > 1), [x FOR x IN l IF x > 1], list_contains(l, 3), list_transform(l, x -> x * 2) " "FROM (SELECT [3, 1, NULL, 3, 10] AS l)") ``` ``` [([3, 3, 10], [3, 3, 10], True, [6, 2, None, 6, 20])] ``` 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#lambdas): all the list functions, and how lambdas work. - [list_transform](/develop/docs/dev-database-reference-sql-functions-lists-list-transform) and [list_reduce](/develop/docs/dev-database-reference-sql-functions-lists-list-reduce): the other lambda functions. - [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.