--- title: "list_zip" slug: "dev-database-reference-sql-functions-lists-list-zip" updated: 2026-10-30T18:19:06Z published: 2026-10-30T18:19:06Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-lists-list-zip" --- > ## 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_zip *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.* Pairs up the elements of two or more lists, position by position, and returns a list of structs: one struct per position, one field per list. `array_zip` is the same function. ## Syntax ``` list_zip(list1, list2 [, ...] [, truncate]) array_zip(list1, list2 [, ...] [, truncate]) ``` | Argument | Type | Description | | --- | --- | --- | | `list1`, `list2`, ... | lists of any element types | The lists to pair up. One list is allowed. | | `truncate` | `BOOLEAN`, optional, last | `true`: the result is as long as the shortest list. `false` (the default): it's as long as the longest list, and the missing elements are NULL. | **Returns:** a list of structs with one unnamed field per list, such as `STRUCT(INTEGER, VARCHAR)[]`. **Aliases:** `array_zip`. ## 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_zip`. 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 weather stations, each with a list of sensor names and a list of readings. Station 2 has more sensors than readings, station 3 has empty lists, and the last row has no values. `run()` also catches `ValueError`: that's how the Python driver reports a result it can't convert to Python objects, such as the unnamed struct fields that `list_zip` returns: ``` 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_zip"' 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, ValueError) 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}.stations" run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}") run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, sensors VARCHAR[], readings DOUBLE[])") run(f"INSERT INTO {TABLE} VALUES " "(1, ['t1', 't2'], [20.5, 21.0]), " "(2, ['t1', 't2', 't3'], [19.0]), " "(3, [], []), " "(4, NULL, NULL)") ``` ``` [] [] [(4,)] ``` ## What it does `list_zip` builds one struct per position: the first struct holds the first element of each list, the second struct the second elements, and so on. `list_zip([1, 2], ['a', 'b'])` is `[(1, a), (2, b)]`, of type `STRUCT(INTEGER, VARCHAR)[]`. - **Lists of different lengths.** By default the result is as long as the longest list, and a shorter list contributes NULL: `list_zip([1, 2], ['a'])` is `[(1, a), (2, NULL)]`. With `true` as the last argument, the result stops at the shortest list: `[(1, a)]`. - **The struct fields have no names.** Read them by position inside the query: `z[1]`, `z[2]`, or `struct_extract(z, 1)`. The Python driver can't convert a struct with unnamed fields to Python objects, so a query that returns the result of `list_zip` as it is ends with `ValueError: Converting to Python dictionary is not supported when duplicate field names are present`, although the query ran on the cluster. Return the fields you need, or cast the result to `VARCHAR` or convert it with [to_json](/develop/docs/dev-database-reference-sql-functions-json-to-json). ## NULLs and edge cases | Input | Result | | --- | --- | | `list_zip(NULL, NULL)` | `[]` | | `list_zip([1], NULL)` (a NULL list) | `[(1, NULL)]`: a NULL list counts as an empty one | | `list_zip([], [])` | `[]` | | `list_zip([1, 2], ['a'])` | `[(1, a), (2, NULL)]` | | `list_zip([1, 2], ['a'], true)` | `[(1, a)]` | | `list_zip([1, 2], ['a'], false)` | `[(1, a), (2, NULL)]` | | `list_zip([1], ['a'], [true])` (three lists) | `[(1, a, true)]` | | `list_zip([1, 2])` (one list) | `[(1), (2)]` | | `list_zip()` | Error: `Provide at least one argument to list_zip` | | `list_zip(1, 2)` (not lists) | Error: `Parameter type needs to be List` | The same cases, as the cluster returns them, cast to `VARCHAR` so the driver can read them. They use the table and helpers from [Set up the examples](/develop/docs/dev-database-reference-sql-functions-lists-list-zip#set-up-the-examples): ``` run("SELECT CAST(list_zip(NULL, NULL) AS VARCHAR), CAST(list_zip([1], NULL) AS VARCHAR), " "CAST(list_zip([], []) AS VARCHAR)") run("SELECT CAST(list_zip([1, 2], ['a']) AS VARCHAR), CAST(list_zip([1, 2], ['a'], true) AS VARCHAR), " "CAST(list_zip([1, 2], ['a'], false) AS VARCHAR)") run("SELECT CAST(list_zip([1], ['a'], [true]) AS VARCHAR), CAST(list_zip([1, 2]) AS VARCHAR)") run("SELECT list_zip()") run("SELECT list_zip(1, 2)") ``` ``` [('[]', '[(1, NULL)]', '[]')] [('[(1, a), (2, NULL)]', '[(1, a)]', '[(1, a), (2, NULL)]')] [('[(1, a, true)]', '[(1), (2)]')] InternalError: Binder Error: Provide at least one argument to list_zip InternalError: Binder Error: Parameter type needs to be List ``` ## Examples The result as it is can't be read by the Python driver; the same query cast to `VARCHAR` can. Station 2 has three sensors and one reading, so the last two pairs have a NULL reading; the NULL lists of station 4 give an empty list: ``` run(f"SELECT id, list_zip(sensors, readings) FROM {TABLE} ORDER BY id LIMIT 10") run(f"SELECT id, CAST(list_zip(sensors, readings) AS VARCHAR) FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` ValueError: Converting to Python dictionary is not supported when duplicate field names are present [(1, '[(t1, 20.5), (t2, 21.0)]'), (2, '[(t1, 19.0), (t2, NULL), (t3, NULL)]'), (3, '[]'), (4, '[]')] ``` Read the fields by position. `z[1]` is the sensor and `z[2]` the reading; with `true`, the pairs stop at the shorter list: ``` run(f"SELECT id, list_transform(list_zip(sensors, readings, true), z -> z[1] || '=' || z[2]) " f"FROM {TABLE} ORDER BY id LIMIT 10") run("SELECT list_zip([1, 2], ['a', 'b'])[2][2], struct_extract(list_zip([1, 2], ['a', 'b'])[1], 1)") ``` ``` [(1, ['t1=20.5', 't2=21.0']), (2, ['t1=19.0']), (3, []), (4, [])] [('b', 1)] ``` As JSON, the fields have empty names: ``` run("SELECT to_json(list_zip([1, 2], ['a', 'b']))") ``` ``` [('[{"":1,"":"a"},{"":2,"":"b"}]',)] ``` ## 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 list columns whole, and `list_zip()` is always computed by the Query Engine: - **In the `SELECT` list,** the scan sends `sensors` and `readings`, and the Query Engine builds the structs for each row it returns. - **In `WHERE`,** a `FILTER` step in the Query Engine computes `list_zip()` for every row the scan sends. No rows are skipped in storage. The plan of a condition on the result shows the `FILTER` step above the scan: ``` plan(f"SELECT id FROM {TABLE} WHERE len(list_zip(sensors, readings)) > 2") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #2 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ (len(list_zip(sensors, │ │ readings)) > 2) │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ sensors │ │ readings │ │ id │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Return only what you need.** Rather than the whole list of structs, return the values you use, such as `list_transform(list_zip(a, b), z -> z[2])`: the result is a plain list, which the driver reads directly, and there's no cast to text. - **Pair a condition on the result with a pushed-down one.** A comparison of a plain column with a constant, such as `id > 1`, is evaluated in VAST DataBase, so the rows it rules out never reach the `FILTER` step. ``` run(f"SELECT id, list_transform(list_zip(sensors, readings), z -> z[2]) FROM {TABLE} WHERE id > 1 ORDER BY id LIMIT 10") ``` ``` [(2, [19.0, None, None]), (3, []), (4, [])] ``` ## Limitations and errors - **The Python driver can't read the result as it is.** See [What it does](/develop/docs/dev-database-reference-sql-functions-lists-list-zip#what-it-does). This is a conversion error in the client, after the query ran. - **Lists only.** Arguments that aren't lists fail with `Parameter type needs to be List`; a call with no argument fails with `Provide at least one argument to list_zip`. - **No field names.** To get named fields, build the structs yourself with a lambda, as in the next section. ## Similar functions | Function | What it does | Result for `[1, 2]` and `['a', 'b']`, as text | | --- | --- | --- | | `list_zip` | pairs the lists as structs with unnamed fields | `[(1, a), (2, b)]` | | [list_transform](/develop/docs/dev-database-reference-sql-functions-lists-list-transform) with a struct literal | builds structs with named fields | `[{'n': 1, 's': a}, {'n': 2, 's': b}]` | | [map](/develop/docs/dev-database-reference-sql-functions-maps-map) | pairs keys with values, as a map | `{1=a, 2=b}` | ``` run("SELECT CAST(list_zip([1, 2], ['a', 'b']) AS VARCHAR), " "CAST(list_transform(range(1, 3), i -> {'n': [1, 2][i], 's': ['a', 'b'][i]}) AS VARCHAR), " "CAST(map([1, 2], ['a', 'b']) AS VARCHAR)") ``` ``` [('[(1, a), (2, b)]', "[{'n': 1, 's': a}, {'n': 2, 's': b}]", '{1=a, 2=b}')] ``` 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. - [struct_extract](/develop/docs/dev-database-reference-sql-functions-structs-struct-extract): read a struct field. - [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.