--- title: "array_pop_front" slug: "dev-database-reference-sql-functions-lists-array-pop-front" updated: 2026-10-30T18:19:06Z published: 2026-10-30T18:19:06Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-lists-array-pop-front" --- > ## 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_pop_front *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) › Combine and change lists.* Returns a list without its first element. ## Syntax ``` array_pop_front(list) ``` | Argument | Type | Description | | --- | --- | --- | | `list` | A list of any type (also takes a `VARCHAR` or a `BLOB`) | The list to shorten. | **Returns:** a value of the same type as `list`: a list, or a `VARCHAR` or `BLOB` for those. ## 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_pop_front`. 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 sensors, each with a list of readings (`readings`) and a list of status codes (`codes`). Sensor 2 has a NULL reading, sensor 3 has one reading and an empty list of codes, and sensor 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_array_pop_front"' 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}.sensors" run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}") run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, readings DOUBLE[], codes VARCHAR[])") run(f"INSERT INTO {TABLE} VALUES " "(1, [0.5, 2.5, -1.0], ['ok', 'ok', 'warn']), " "(2, [2.0, NULL], ['ok', NULL]), " "(3, [1.5], []), " "(4, NULL, NULL)") ``` ``` [] [] [(4,)] ``` ## What it does `array_pop_front` returns the list without its first element: `array_pop_front([1, 2, 3])` is `[2, 3]`. A list of one element gives an empty list, and an empty list stays empty. On a list of lists it removes the first inner list: `array_pop_front([[1, 2], [3]])` is `[[3]]`. To remove more than one element, call it again (`array_pop_front(array_pop_front([1, 2, 3]))` is `[3]`) or take a slice with [list_slice](/develop/docs/dev-database-reference-sql-functions-lists-list-slice). It also takes a `VARCHAR`, and removes the first character (`array_pop_front('abc')` is `'bc'`), and a `BLOB`: the one-byte `BLOB` `'\xAA'::BLOB` gives an empty `BLOB`. ## NULLs and edge cases | Input | Result | | --- | --- | | `NULL` (untyped) | `NULL` | | `NULL::BIGINT[]` | `NULL` | | `[]` | `[]` | | `[]::BIGINT[]` | `[]` | | `[1]` | `[]` | | `[NULL, NULL]` | `[NULL]` | | `[[1, 2], [3]]` | `[[3]]` | | `array_pop_front(array_pop_front([1, 2, 3]))` | `[3]` | | `'abc'` (a `VARCHAR`) | `'bc'` | | `'\xAA'::BLOB` (one byte) | an empty `BLOB` | | A number, date or boolean | Error: `ARRAY_SLICE can only operate on LISTs and VARCHARs` | 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-array-pop-front#set-up-the-examples): ``` run("SELECT array_pop_front(NULL), array_pop_front(NULL::BIGINT[]), array_pop_front([]), array_pop_front([]::BIGINT[]), " "array_pop_front([1]), array_pop_front([NULL, NULL])") run("SELECT array_pop_front([[1, 2], [3]]), array_pop_front(array_pop_front([1, 2, 3])), array_pop_front('abc'), " "array_pop_front('\\xAA'::BLOB)") run("SELECT array_pop_front(123)") ``` ``` [(None, None, [], [], [], [None])] [([[3]], [3], 'bc', b'')] InternalError: Binder Error: ARRAY_SLICE can only operate on LISTs and VARCHARs ``` ## Examples Drop the first reading and the first code of each sensor. Sensor 3's single reading leaves an empty list, and sensor 4's NULL lists stay NULL: ``` run(f"SELECT id, array_pop_front(readings), array_pop_front(codes) FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, [2.5, -1.0], ['ok', 'warn']), (2, [None], [None]), (3, [], []), (4, None, None)] ``` Keep the sensors that reported `'warn'` after their first code: ``` run(f"SELECT id FROM {TABLE} WHERE list_contains(array_pop_front(codes), 'warn') 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`). The scan reads a list column whole, and VAST DataBase doesn't evaluate list functions, so `array_pop_front()` is always computed by the Query Engine: - **In the `SELECT` list,** the scan reads the list column and the Query Engine computes `array_pop_front()` for each row it returns. - **In `WHERE`,** the scan sends the list for every row, and a `FILTER` step in the Query Engine computes the condition 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_contains(array_pop_front(codes), 'warn')") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #1 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ list_contains(array_slice │ │ (codes, 2, []), 'warn') │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ codes │ │ id │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Pair a list condition with a pushed-down condition.** In `id > 1 AND list_contains(array_pop_front(codes), 'warn')`, `id > 1` is evaluated in VAST DataBase, so the list condition runs only on the rows that pass it. The `FILTER` step in the plan holds only the list condition: ``` plan(f"SELECT id FROM {TABLE} WHERE id > 1 AND list_contains(array_pop_front(codes), 'warn')") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #0 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ list_contains(array_slice │ │ (codes, 2, []), 'warn') │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ id │ │ codes │ │ │ └───────────────────────────┘ ``` - **Keep values you filter on outside lists.** If queries often test the first reading, store it in its own column when you write the rows, so the filter can be pushed down. ## Limitations and errors - **Takes a list, a `VARCHAR` or a `BLOB`.** A number, date or boolean fails with `ARRAY_SLICE can only operate on LISTs and VARCHARs`: `array_pop_front` is defined with a slice. - **One element at a time.** It takes no count. Nest the calls, or use [list_slice](/develop/docs/dev-database-reference-sql-functions-lists-list-slice) to skip the first *n* elements. - **A NULL list returns NULL; an empty list returns an empty list.** Test with `IS NULL` or `len()` if you need to tell them apart. ## Similar functions | Function | Result for `[3, 1, NULL, 3, 10]` | | --- | --- | | `array_pop_front(l)` | `[1, NULL, 3, 10]` | | [array_pop_back](/develop/docs/dev-database-reference-sql-functions-lists-array-pop-back): removes the last element | `[3, 1, NULL, 3]` | | [list_slice](/develop/docs/dev-database-reference-sql-functions-lists-list-slice): `list_slice(l, 2, 3)`, elements 2 to 3 | `[1, NULL]` | | `l[2:3]`: the same slice | `[1, NULL]` | | [list_first](/develop/docs/dev-database-reference-sql-functions-lists-list-first): the first element, not a list | `3` | ``` run("SELECT array_pop_front(l), array_pop_back(l), list_slice(l, 2, 3), l[2:3], list_first(l) " "FROM (SELECT [3, 1, NULL, 3, 10] AS l)") ``` ``` [([1, None, 3, 10], [3, 1, None, 3], [1, None], [1, None], 3)] ``` Clean up: ``` run(f"DROP TABLE IF EXISTS {TABLE}") run(f"DROP SCHEMA IF EXISTS {SCHEMA}") conn.close() ``` ``` [] [] ``` ## See also - [array_pop_back](/develop/docs/dev-database-reference-sql-functions-lists-array-pop-back): remove the last element. - [list_prepend](/develop/docs/dev-database-reference-sql-functions-lists-list-prepend): add an element at the start. - [List, struct and map functions](/develop/docs/dev-database-reference-sql-functions-lists-and-structs): all the list 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.