--- title: "list_reduce" slug: "dev-database-reference-sql-functions-lists-list-reduce" updated: 2026-10-30T18:19:06Z published: 2026-10-30T18:19:06Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-lists-list-reduce" --- > ## 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_reduce *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.* Folds a list into one value: a lambda combines the result so far with each element in turn, from left to right, starting from the first element. `array_reduce` and `reduce` are the same function. ## Syntax ``` list_reduce(l, (acc, x) -> expression) list_reduce(l, (acc, x, i) -> expression) array_reduce(l, (acc, x) -> expression) reduce(l, (acc, x) -> expression) ``` | Argument | Type | Description | | --- | --- | --- | | `l` | a list of any type | The list to fold. It must not be empty. | | `(acc, x) -> expression` | lambda | `acc` is the result so far and `x` the next element. With `(acc, x, i) -> expression`, `i` is the position of `x`, starting at 2 for the first call. | **Returns:** a value of the element type of `l`. **Aliases:** `array_reduce`, `reduce`. ## 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_reduce`. 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. 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_reduce"' 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[][])") run(f"INSERT INTO {TABLE} VALUES " "(1, [1, 2, 3], ['a', 'b', 'c'], [0.5, 2.5, -1.0], [[1, 2], [3]]), " "(2, [3, NULL, 3], ['b', NULL, 'B'], [2.0, NULL], [[], NULL, [4]]), " "(3, [], [], [], []), " "(4, NULL, NULL, NULL, NULL), " "(5, [-5, 0, -5, 10], ['\u00e9', 'Zo\u00eb', '\U0001F1EB\U0001F1F7', ''], [1e308, 1e308, -1e-308], [[5, NULL], [], [6, 7, 8]])") ``` ``` [] [] [(5,)] ``` ## What it does `list_reduce(l, (acc, x) -> expression)` starts with the first element of `l` as `acc`. For each following element `x`, it computes `expression` and makes the result the new `acc`. The last `acc` is the result: - `list_reduce([1, 2, 3], (acc, x) -> acc + x)` computes `(1 + 2) + 3` and returns `6`. - `list_reduce(['a', 'b', 'c'], (acc, x) -> acc || x)` returns `'abc'`; with `x || acc` it returns `'cba'`. - A list of one element returns that element: `list_reduce([7], ...)` is `7`. - With `(acc, x, i)`, `i` is the position of `x`: `list_reduce([1, 2, 3], (acc, x, i) -> acc + x * i)` is `1 + 2 * 2 + 3 * 3 = 14`. Two things to know: - **There's no initial value, so an empty list is an error.** `list_reduce` on `[]` stops the whole query with `Cannot perform list_reduce on an empty input list`, even if only one row of the table has an empty list. To start from a value of your own, put it in front of the list with [list_prepend](/develop/docs/dev-database-reference-sql-functions-lists-list-prepend): `list_reduce(list_prepend(0, l), ...)` returns `0` for an empty list. Or guard the call with `CASE WHEN len(l) > 0 THEN list_reduce(...) END`, or `WHERE len(l) > 0`. - **The result has the element type.** The lambda's result is converted to it: on an `INTEGER` list, `(acc, x) -> acc * 1.5` returns `3` for `[1, 2, 3]`, and `acc::VARCHAR || x::VARCHAR` returns the `INTEGER` `123`. ## NULLs and edge cases | Input | Result | | --- | --- | | `list_reduce(NULL::BIGINT[], (acc, x) -> acc + x)` | `NULL` | | `list_reduce([1, NULL, 2], (acc, x) -> acc + x)` | `NULL`: the NULL element is passed to the lambda, and `acc + NULL` is NULL | | `list_reduce([7], (acc, x) -> acc + x)` | `7` | | `list_reduce([3, 1, 2], (acc, x) -> greatest(acc, x))` | `3` | | `list_reduce(list_prepend(0, []::BIGINT[]), (acc, x) -> acc + x)` | `0` | | `list_reduce([]::BIGINT[], (acc, x) -> acc + x)` | Error: `Cannot perform list_reduce on an empty input list` | | `list_reduce([1, 2], (acc, x) -> acc + x, 100)` | Error: `Binder Error: failed to bind function, either: Invalid number of function arguments!` | | `list_reduce([1, 2], x -> x)` | Error: `Binder Error: list_reduce expects a function with 2 or 3 arguments` | | `list_reduce([9223372036854775807, 1], (acc, x) -> acc + x)` | Error: `Overflow in addition of INT64 (9223372036854775807 + 1)!` | | 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-reduce#set-up-the-examples): ``` run("SELECT list_reduce(NULL::BIGINT[], (acc, x) -> acc + x), list_reduce([1, NULL, 2], (acc, x) -> acc + x), " "list_reduce([7], (acc, x) -> acc + x), list_reduce([3, 1, 2], (acc, x) -> greatest(acc, x))") run("SELECT list_reduce(list_prepend(0, []::BIGINT[]), (acc, x) -> acc + x)") run("SELECT list_reduce([]::BIGINT[], (acc, x) -> acc + x)") run("SELECT list_reduce([1, 2], (acc, x) -> acc + x, 100)") run("SELECT list_reduce([1, 2], x -> x)") run("SELECT list_reduce([9223372036854775807, 1], (acc, x) -> acc + x)") run("SELECT list_reduce(123, (acc, x) -> acc + x)") ``` ``` [(None, None, 7, 3)] [(0,)] OSError: Cannot perform list_reduce on an empty input list InternalError: Binder Error: failed to bind function, either: Invalid number of function arguments! InternalError: Binder Error: list_reduce expects a function with 2 or 3 arguments OSError: Overflow in addition of INT64 (9223372036854775807 + 1)! InternalError: Binder Error: failed to bind function, either: Invalid LIST argument during lambda function binding! ``` The examples from [What it does](/develop/docs/dev-database-reference-sql-functions-lists-list-reduce#what-it-does): ``` run("SELECT list_reduce([1, 2, 3], (acc, x) -> acc + x), list_reduce(['a', 'b', 'c'], (acc, x) -> acc || x), " "list_reduce(['a', 'b', 'c'], (acc, x) -> x || acc), list_reduce([1, 2, 3], (acc, x, i) -> acc + x * i)") run("SELECT list_reduce([1, 2, 3], (acc, x) -> acc * 1.5), list_reduce([1, 2, 3], (acc, x) -> acc::VARCHAR || x::VARCHAR)") ``` ``` [(6, 'abc', 'cba', 14)] [(3, 123)] ``` ## Examples On the whole table, `list_reduce` fails, because row 3's lists are empty: ``` run(f"SELECT id, list_reduce(nums, (acc, x) -> acc + x) FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` OSError: Cannot perform list_reduce on an empty input list ``` Skip the empty lists with `WHERE len(...) > 0`. Join the words with `-`, and concatenate the lists of lists into one list: ``` run(f"SELECT id, list_reduce(nums, (acc, x) -> acc + x), reduce(words, (acc, x) -> acc || '-' || x), " f"array_reduce(nested, (acc, x) -> list_concat(acc, x)) FROM {TABLE} WHERE len(nums) > 0 ORDER BY id LIMIT 10") ``` ``` [(1, 6, 'a-b-c', [1, 2, 3]), (2, None, None, [4]), (5, 0, 'é-Zoë-🇫🇷-', [5, None, 6, 7, 8])] ``` Keep every row, with a guard or a starting value. With `CASE`, an empty or NULL list returns NULL; with `list_prepend(0, ...)`, it returns `0`: ``` run(f"SELECT id, CASE WHEN len(nums) > 0 THEN list_reduce(nums, (acc, x) -> acc + x) END, " f"list_reduce(list_prepend(0, nums), (acc, x) -> acc + x) FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, 6, 6), (2, None, None), (3, None, 0), (4, None, 0), (5, 0, 0)] ``` ## 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_reduce()` and its lambda always run in the Query Engine: - **In the `SELECT` list,** a projection step folds each row's list after the scan. - **In `WHERE`,** the scan sends the list for every row, and a `FILTER` step computes the condition. `len(nums) > 0` is such a condition: it runs in the Query Engine too, before the projection. The plan of the `WHERE len(...) > 0` example shows the `FILTER` step above the scan: ``` plan(f"SELECT id, list_reduce(nums, (acc, x) -> acc + x) FROM {TABLE} WHERE len(nums) > 0") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ id │ │ list_reduce(nums) │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ (len(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 │ │ │ └───────────────────────────┘ ``` - **Use a list aggregate for sums and the like.** [list_sum](/develop/docs/dev-database-reference-sql-functions-lists-list-sum), and [list_aggregate](/develop/docs/dev-database-reference-sql-functions-lists-list-aggregate) skip NULL elements and return NULL for an empty list instead of failing, so they need no guard. ``` run(f"SELECT id, list_sum(nums), list_aggregate(nums, 'max') FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, Decimal('6'), 3), (2, Decimal('6'), 3), (3, None, None), (4, None, None), (5, Decimal('0'), 10)] ``` ## Limitations and errors - **No initial-value argument.** A third argument fails with `Invalid number of function arguments!`. Use `list_prepend(initial, l)`, as in [What it does](/develop/docs/dev-database-reference-sql-functions-lists-list-reduce#what-it-does). - **Empty lists stop the query.** `Cannot perform list_reduce on an empty input list`; guard the call as shown in [Examples](/develop/docs/dev-database-reference-sql-functions-lists-list-reduce#examples). - **Two or three lambda parameters.** One fails with `list_reduce expects a function with 2 or 3 arguments`, four with `This lambda function only supports up to three lambda parameters!`. ## Similar functions | Function | Result for `[3, 1, NULL, 3, 10]` | | --- | --- | | `list_reduce(l, (acc, x) -> acc + x)` | `NULL`: the NULL element makes the sum NULL | | `list_reduce(list_filter(l, x -> x IS NOT NULL), (acc, x) -> acc + x)` | `17` | | [list_sum](/develop/docs/dev-database-reference-sql-functions-lists-list-sum) | `17` | | [list_aggregate](/develop/docs/dev-database-reference-sql-functions-lists-list-aggregate)`(l, 'sum')` | `17` | | [list_transform](/develop/docs/dev-database-reference-sql-functions-lists-list-transform) | a list of results, one per element, instead of one value | ``` run("SELECT list_reduce(l, (acc, x) -> acc + x), list_reduce(list_filter(l, x -> x IS NOT NULL), (acc, x) -> acc + x), " "list_sum(l), list_aggregate(l, 'sum') FROM (SELECT [3, 1, NULL, 3, 10] AS l)") ``` ``` [(None, 17, Decimal('17'), Decimal('17'))] ``` 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_filter](/develop/docs/dev-database-reference-sql-functions-lists-list-filter): 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.