--- title: "list_extract" slug: "dev-database-reference-sql-functions-lists-list-extract" updated: 2026-10-30T18:19:06Z published: 2026-10-30T18:19:06Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-lists-list-extract" --- > ## 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_extract *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 element of a list at a position, counting from 1; a negative position counts from the end. `list_element` and `array_extract` are the same function, and `l[i]` is the bracket form of it. On a `VARCHAR`, it returns one character. ## Syntax ``` list_extract(list, index) list_element(list, index) array_extract(list, index) list[index] ``` | Argument | Type | Description | | --- | --- | --- | | `list` | a list (`ANY[]`), a fixed-size array or a `VARCHAR` | The list, or the string, to take an element from. | | `index` | `BIGINT` (any integer) | The position. `1` is the first element, `-1` the last. | **Returns:** the list's element type; `VARCHAR` for a string. **Aliases:** `list_element`, `array_extract`, `list[index]`. ## 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_extract`. 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 orders: the items, their quantities, a position to pick and a code. The second order has a NULL item, the third 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_list_extract"' 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}.orders" run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}") run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, items VARCHAR[], qty BIGINT[], pick BIGINT, code VARCHAR)") run(f"INSERT INTO {TABLE} VALUES " "(1, ['pen', 'ink', 'pad'], [2, 1, 5], 2, 'AB7'), " "(2, ['cup', NULL], [1, NULL], -1, 'Q'), " "(3, [], [], 1, ''), " "(4, NULL, NULL, NULL, NULL)") ``` ``` [] [] [(4,)] ``` ## What it does `list_extract(l, i)` returns the element at position `i` of `l`. Positions count from 1, and a negative position counts from the end: | Call | Result | | --- | --- | | `list_extract([1, 2, 3], 1)`, `[1, 2, 3][1]` | `1` | | `list_extract([1, 2, 3], -1)` | `3` | | `list_extract([1, 2, 3], -3)` | `1` | | `list_extract([1, 2, 3], 0)`, `(.., 4)`, `(.., -4)` | `NULL` (no such position) | | `[[1, 2], [3]][2][1]` | `3` (the first element of the second list) | A position that doesn't exist returns NULL, not an error. So does a position that holds a NULL element: `list_extract([1, NULL, 3], 2)` is NULL as well. The index can be a column: `items[pick]` takes a different position in each row. **On a string,** `list_extract(s, i)` returns the character at position `i` as a `VARCHAR`: `list_extract('abc', 2)` is `'b'` and `list_extract('abc', -1)` is `'c'`. A position that doesn't exist returns an empty string `''`, not NULL. For more than one character, use [substring](/develop/docs/dev-database-reference-sql-functions-string-substring). ## NULLs and edge cases | Input | Result | | --- | --- | | `NULL` list | `NULL` | | `NULL` index | `NULL` | | `[]` at any position | `NULL` | | `[1, NULL, 3]` at 2 | `NULL` | | A position past either end, or `0` | `NULL` (on a string: `''`) | | Index `'2'` (a numeric string) | Converted: the second element | | Index `1.5` (a `DECIMAL`) | Error: `No function matches the given name and argument types 'list_extract(INTEGER[], DECIMAL(2,1))'` | | A number, `BLOB`, date or boolean | Error: `No function matches the given name and argument types` | | A `MAP` or a `STRUCT` | Error: `No function matches the given name and argument types` | 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-list-extract#set-up-the-examples): ``` run("SELECT list_extract(NULL, 2), list_extract([1, 2], NULL), list_extract([], 1), list_extract([1, NULL, 3], 2)") run("SELECT list_extract([1, 2, 3], 0), list_extract([1, 2, 3], 4), list_extract([1, 2, 3], -4), list_extract([1, 2, 3], '2')") run("SELECT list_extract('abc', 2), list_extract('abc', -1), list_extract('abc', 0), list_extract('abc', 9)") run("SELECT list_extract([1, 2, 3], 1.5)") run("SELECT list_extract(123, 2)") ``` ``` [(None, None, None, None)] [(None, None, None, 2)] [('b', 'c', '', '')] InternalError: Binder Error: No function matches the given name and argument types 'list_extract(INTEGER[], DECIMAL(2,1))'. You might need to add explicit type casts. InternalError: Binder Error: No function matches the given name and argument types 'list_extract(INTEGER_LITERAL, INTEGER_LITERAL)'. You might need to add explicit type casts. ``` ## Examples The first and the last item of each order, with each of the four forms: ``` run(f"SELECT id, items[1], list_extract(items, -1), list_element(items, 1), array_extract(items, -1) " f"FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, 'pen', 'pad', 'pen', 'pad'), (2, 'cup', None, 'cup', None), (3, None, None, None, None), (4, None, None, None, None)] ``` The item and the quantity at the position in `pick`, and the first character of the code. Order 2 picks position `-1`, its last item, which is NULL: ``` run(f"SELECT id, items[pick], qty[pick], list_extract(code, 1) FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, 'ink', 1, 'A'), (2, None, None, 'Q'), (3, None, None, ''), (4, None, None, None)] ``` Keep the orders whose first item is a pen: ``` run(f"SELECT id FROM {TABLE} WHERE items[1] = 'pen' ORDER BY id LIMIT 10") ``` ``` [(1,)] ``` ## 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 `list_extract()` and `l[i]` are always computed there: - **In the `SELECT` list,** the scan reads the list and the Query Engine takes the element for each row it returns. - **In `WHERE`,** the scan sends the whole list for every row, and a `FILTER` step in the Query Engine takes the element 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. The plan writes `items[1]` as `array_extract(items, 1)`: ``` plan(f"SELECT id FROM {TABLE} WHERE items[1] = 'pen'") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #1 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ (array_extract(items, 1) =│ │ 'pen') │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ items │ │ id │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Store an element you filter on in its own column.** If queries often test the first item, keep it in a column such as `first_item` when you write the rows: `first_item = 'pen'` is pushed down to VAST DataBase, so rows that don't match never leave storage, while `items[1] = 'pen'` runs in a `FILTER` step. The plan of a plain column compared with a constant, `code = 'AB7'`, has no `FILTER` step: ``` plan(f"SELECT id FROM {TABLE} WHERE code = 'AB7'") ``` ``` ┌───────────────────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ Projections: id │ │ │ └───────────────────────────┘ ``` - **Combine it with a pushed-down condition.** In `id > 100 AND items[1] = 'pen'`, `id > 100` is still evaluated in VAST DataBase, so the element is taken only from the rows that pass it. ## Limitations and errors - **Positions must be integers.** A `DECIMAL` such as `1.5` fails with `No function matches the given name and argument types`; a string that isn't a number fails with `Could not convert string 'a' to INT64`. Cast it to an integer first: `list_extract(l, CAST(x AS BIGINT))`. - **NULL means missing or NULL.** A position that doesn't exist and a NULL element both return NULL. Compare the position with [array_length](/develop/docs/dev-database-reference-sql-functions-lists-array-length) if you need to tell them apart. - **Not for maps or structs.** Use [map_extract](/develop/docs/dev-database-reference-sql-functions-maps-map-extract) or `m[key]` for a map, and [struct_extract](/develop/docs/dev-database-reference-sql-functions-structs-struct-extract) or `s.field` for a struct. `element_at` works only on maps: on a list it fails with `Binder Error: 'element_at' can only operate on MAPs`. ``` run("SELECT list_extract([1, 2, 3], 'a')") run("SELECT element_at([1, 2, 3], 1)") ``` ``` OSError: Could not convert string 'a' to INT64 InternalError: Binder Error: 'element_at' can only operate on MAPs ``` ## Similar functions | Function | Returns | Result for `[3, 1, NULL, 3, 10]` | | --- | --- | --- | | `l[2]`, `list_extract(l, 2)`, `list_element(l, 2)`, `array_extract(l, 2)` | the element at a position | `1` | | `l[-1]` | the last element | `10` | | [list_slice](/develop/docs/dev-database-reference-sql-functions-lists-list-slice) `(l, 2, 2)`, `l[2:2]` | a list of the elements in a range of positions | `[1]` | | [list_select](/develop/docs/dev-database-reference-sql-functions-lists-list-select) `(l, [2])` | a list of the elements at a list of positions | `[1]` | | [list_first](/develop/docs/dev-database-reference-sql-functions-lists-list-first) | the first element | `3` | | [list_last](/develop/docs/dev-database-reference-sql-functions-lists-list-last) | the last element | `10` | ``` run("SELECT l[2], list_extract(l, 2), list_element(l, 2), array_extract(l, 2), l[-1], l[2:2], list_slice(l, 2, 2), " "list_select(l, [2]), list_first(l), list_last(l) FROM (SELECT [3, 1, NULL, 3, 10] AS l)") ``` ``` [(1, 1, 1, 1, 10, [1], [1], [1], 3, 10)] ``` 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_slice](/develop/docs/dev-database-reference-sql-functions-lists-list-slice), [array_length](/develop/docs/dev-database-reference-sql-functions-lists-array-length) and [substring](/develop/docs/dev-database-reference-sql-functions-string-substring). - [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.