--- title: "list_first" slug: "dev-database-reference-sql-functions-lists-list-first" updated: 2026-10-30T18:19:06Z published: 2026-10-30T18:19:06Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-lists-list-first" --- > ## 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_first *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) › Aggregate a list.* Returns the first element of a list, even when that element is NULL. To skip NULL elements, use [list_any_value](/develop/docs/dev-database-reference-sql-functions-lists-list-any-value). ## Syntax ``` list_first(l) ``` | Argument | Type | Description | | --- | --- | --- | | `l` | a list of any type | The list whose first element is returned. | **Returns:** the element type of the list (`INTEGER` for `[1, 2]`, `DOUBLE` for a `DOUBLE[]`). ## 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_first`. 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, flags 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_first"' 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[], " "flags BOOLEAN[], nested BIGINT[][])") run(f"INSERT INTO {TABLE} VALUES " "(1, [1, 2, 3], ['a', 'b', 'c'], [0.5, 2.5, -1.0], [true, false, true], [[1, 2], [3]]), " "(2, [3, NULL, 3], ['b', NULL, 'B'], [2.0, NULL], [NULL, true], [[], NULL, [4]]), " "(3, [], [], [], [], []), " "(4, NULL, NULL, NULL, NULL, NULL), " "(5, [-5, 0, -5, 10], ['\u00e9', 'Zo\u00eb', '\U0001F1EB\U0001F1F7', ''], [1e308, 1e308, -1e-308], " "[false, false], [[5, NULL], [], [6, 7, 8]])") ``` ``` [] [] [(5,)] ``` ## What it does `list_first(l)` returns the element at the first position of `l`, whatever its value: - `list_first([1, 2])` is `1`. - `list_first([NULL, 2])` is NULL: the first element is NULL, and `list_first` doesn't skip it. - An empty list and a NULL list return NULL. - In a list of lists, the result is the first inner list: `list_first([[1], [2]])` is `[1]`. ## NULLs and edge cases | Input | Result | | --- | --- | | `NULL` | `NULL` | | `[]` | `NULL` | | `[NULL, 2]` | `NULL` | | `[1, 2]` | `1` | | `[[1], [2]]` | `[1]` | | A number, string, `BLOB`, date or boolean | Error: `No function matches the given name and argument types`, for example `'list_aggr(INTEGER_LITERAL, STRING_LITERAL)'` | 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-first#set-up-the-examples): ``` run("SELECT list_first(NULL), list_first([]::BIGINT[]), list_first([NULL, 2]), list_first([1, 2]), list_first([[1], [2]])") run("SELECT list_first(123)") ``` ``` [(None, None, None, 1, [1])] InternalError: Binder Error: No function matches the given name and argument types 'list_aggr(INTEGER_LITERAL, STRING_LITERAL)'. You might need to add explicit type casts. ``` ## Examples The first element of each list in the table. Row 2's first inner list is empty, so `list_first(nested)` returns `[]` for it: ``` run(f"SELECT id, list_first(nums), list_first(words), list_first(vals), list_first(nested) " f"FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, 1, 'a', 0.5, [1, 2]), (2, 3, 'b', 2.0, []), (3, None, None, None, None), (4, None, None, None, None), (5, -5, 'é', 1e+308, [5, None])] ``` The rows whose list of numbers has a first element: ``` run(f"SELECT id, nums FROM {TABLE} WHERE list_first(nums) IS NOT NULL ORDER BY id LIMIT 10") ``` ``` [(1, [1, 2, 3]), (2, [3, None, 3]), (5, [-5, 0, -5, 10])] ``` ## 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_first()` always runs in the Query Engine: - **In the `SELECT` list,** a projection step computes `list_first()` for each row the scan returns. - **In `WHERE`,** the scan sends the list for every row, and a `FILTER` step computes `list_first()` 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 list_first(nums) IS NOT NULL") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #1 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ (list_aggr(nums, 'first') │ │ IS NOT NULL) │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ 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 `list_first()` 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 │ │ │ └───────────────────────────┘ ``` - **Store the first element if you filter on it often.** Keep it in its own column when you write the rows: `first_num = 1` is pushed down, `list_first(nums) = 1` isn't. ## Limitations and errors - **Lists only.** Any other type fails with `No function matches the given name and argument types`. The error names `list_aggr`, the function `list_first` calls, and its accepted form `list_aggr(ANY[], VARCHAR, [ANY...]) -> ANY`. - **No `array_first`.** It fails with `Scalar Function with name array_first does not exist!`. Use `list_first`. - **`ORDER BY` needs a `LIMIT`.** Without one, the query fails with `The operator 'ORDER_BY' type=1 is currently not supported for this usage`. ``` run("SELECT array_first([1, 2])") ``` ``` InternalError: Catalog Error: Scalar Function with name array_first does not exist! ``` ## Similar functions | Function | What it returns | Result for `[3, 1, NULL, 3, 10]` | | --- | --- | --- | | `list_first` | the first element, even if NULL | `3` | | [list_any_value](/develop/docs/dev-database-reference-sql-functions-lists-list-any-value) | the first element that isn't NULL | `3` | | [list_last](/develop/docs/dev-database-reference-sql-functions-lists-list-last) | the last element, even if NULL | `10` | | `l[-1]` ([list_extract](/develop/docs/dev-database-reference-sql-functions-lists-list-extract)) | the last element, by negative index | `10` | On `[NULL, 2]`, `list_first` returns NULL and `list_any_value` returns `2`: ``` run("SELECT list_first(l), list_any_value(l), list_last(l), l[-1] FROM (SELECT [3, 1, NULL, 3, 10] AS l)") run("SELECT list_first([NULL, 2]), list_any_value([NULL, 2])") ``` ``` [(3, 3, 10, 10)] [(None, 2)] ``` 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. - [list_extract](/develop/docs/dev-database-reference-sql-functions-lists-list-extract): the element at any position. - [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.