--- title: "map_keys" slug: "dev-database-reference-sql-functions-maps-map-keys" updated: 2026-10-30T18:18:46Z published: 2026-10-30T18:18:46Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-maps-map-keys" --- > ## 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. # map_keys *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) › Keys, values and entries.* Returns the keys of a map as a list, in the order the map holds them. ## Syntax ``` map_keys(m) ``` | Argument | Type | Description | | --- | --- | --- | | `m` | a map | The map whose keys you want. | **Returns:** a list of the map's key type, such as `VARCHAR[]`. ## 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_maps_map_keys`. 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 articles: the languages each one is translated into and the word count of each translation. Article 2 has a translation with no word count yet, article 3 has no translations, and the last row has no values. A table can't return a `MAP` column, so the keys and values are stored as two lists: ``` 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_maps_map_keys"' 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}.articles" run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}") run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, langs VARCHAR[], words BIGINT[])") run(f"INSERT INTO {TABLE} VALUES " "(1, ['en', 'de'], [120, 135]), " "(2, ['fr', 'es'], [NULL, 140]), " "(3, [], []), " "(4, NULL, NULL)") ``` ``` [] [] [(4,)] ``` ## What it does `map_keys(m)` lists the keys of `m` in the map's own order: the order the pairs were given in when the map was built. `map_keys(MAP {'b': 1, 'a': 2})` is `['b', 'a']`. When [map_concat](/develop/docs/dev-database-reference-sql-functions-maps-map-concat) replaces a key's value, the key keeps its place. To get the keys in sorted order, wrap the result in [list_sort](/develop/docs/dev-database-reference-sql-functions-lists-list-sort). The list has one element per pair, so `len(map_keys(m))` is the number of pairs, the same as [cardinality](/develop/docs/dev-database-reference-sql-functions-maps-cardinality). The values come in the same order from [map_values](/develop/docs/dev-database-reference-sql-functions-maps-map-values). ## NULLs and edge cases | Input | Result | | --- | --- | | `map_keys(NULL::MAP(VARCHAR, BIGINT))`, `map_keys(NULL)` | `NULL` | | `map_keys(MAP {})`, `map_keys(map())` | `[]` | | `map_keys(MAP {'b': 1, 'a': 2})` | `['b', 'a']` | | `map_keys(map_concat(MAP {'b': 1}, MAP {'a': 2, 'b': 3}))` | `['b', 'a']` | | `map_keys(MAP {1: 'x', 2: 'y'})` | `[1, 2]` | | `map_keys([1, 2])` or a string, number, struct or date | Error: `The provided argument is not a map` | | `map_keys(MAP {'a': 1}, MAP {'b': 2})`, `map_keys()` | Error: `Too many arguments provided, only expecting a single map` | 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-maps-map-keys#set-up-the-examples): ``` run("SELECT map_keys(NULL::MAP(VARCHAR, BIGINT)), map_keys(NULL), map_keys(MAP {}), map_keys(map())") run("SELECT map_keys(MAP {'b': 1, 'a': 2}), map_keys(map_concat(MAP {'b': 1}, MAP {'a': 2, 'b': 3})), " "map_keys(MAP {1: 'x', 2: 'y'})") run("SELECT map_keys([1, 2])") run("SELECT map_keys(MAP {'a': 1}, MAP {'b': 2})") run("SELECT map_keys()") ``` ``` [(None, None, [], [])] [(['b', 'a'], ['b', 'a'], [1, 2])] InternalError: Invalid Input Error: The provided argument is not a map InternalError: Invalid Input Error: Too many arguments provided, only expecting a single map InternalError: Invalid Input Error: Too many arguments provided, only expecting a single map ``` ## Examples The keys of a literal map, and sorted: ``` run("SELECT map_keys(MAP {'en': 'English', 'de': 'German'}), list_sort(map_keys(MAP {'en': 'English', 'de': 'German'}))") ``` ``` [(['en', 'de'], ['de', 'en'])] ``` The keys of each article's map. Article 3's empty map gives `[]` and article 4's NULL map gives NULL: ``` run(f"SELECT id, map_keys(map(langs, words)) FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, ['en', 'de']), (2, ['fr', 'es']), (3, []), (4, None)] ``` The keys of a merged map, and one row per key with `unnest`: ``` run(f"SELECT id, map_keys(map_concat(map(langs, words), MAP {{'la': 50}}::MAP(VARCHAR, BIGINT))) " f"FROM {TABLE} ORDER BY id LIMIT 10") run("SELECT unnest(map_keys(MAP {'a': 1, 'b': 2})) AS k") ``` ``` [(1, ['en', 'de', 'la']), (2, ['fr', 'es', 'la']), (3, ['la']), (4, ['la'])] [('a',), ('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`). `map_keys()` is always computed by the Query Engine: - **In the `SELECT` list,** the scan sends the list columns, and the Query Engine builds the map and lists its keys for each row it returns. - **In `WHERE`,** a `FILTER` step in the Query Engine does the same for every row the scan sends. No rows are skipped in storage. The plan of a condition on the keys shows the `FILTER` step above the scan: ``` plan(f"SELECT id FROM {TABLE} WHERE list_contains(map_keys(map(langs, words)), 'de')") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #2 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ list_contains(map_keys( │ │"map"(langs, words)), 'de')│ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ langs │ │ words │ │ id │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Use the key column itself.** When the map is built from a list of keys, `map_keys(map(langs, words))` is `langs`: test `langs` directly, as in `list_contains(langs, 'de')`, and the Query Engine doesn't build a map. The condition on `langs` still runs in a `FILTER` step, as the plan below shows; the `id > 1` condition doesn't, because it is evaluated in VAST DataBase. - **Pair a list condition 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 FROM {TABLE} WHERE list_contains(langs, 'de') ORDER BY id LIMIT 10") plan(f"SELECT id FROM {TABLE} WHERE id > 1 AND list_contains(langs, 'de')") ``` ``` [(1,)] ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #0 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ list_contains(langs, 'de')│ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ id │ │ langs │ │ │ └───────────────────────────┘ ``` ## Limitations and errors - **Maps only.** A list, a struct or any other value fails with `The provided argument is not a map`. For the field names of a struct, convert it to JSON and use [json_keys](/develop/docs/dev-database-reference-sql-functions-json-json-keys). - **One argument.** Two arguments, or none, fail with `Too many arguments provided, only expecting a single map`. - **A table can't return a `MAP` column**, so the maps on this page are built from list columns ([data types in SQL](/develop/docs/dev-database-reference-sql-data-types)). ``` run("SELECT map_keys({'a': 1})") run("SELECT json_keys(to_json({'a': 1}))") ``` ``` InternalError: Invalid Input Error: The provided argument is not a map [(['a'],)] ``` ## Similar functions | Function | Result for `MAP {'en': 'English', 'de': 'German'}` | | --- | --- | | `map_keys` | `['en', 'de']` | | [map_values](/develop/docs/dev-database-reference-sql-functions-maps-map-values) | `['English', 'German']` | | [map_entries](/develop/docs/dev-database-reference-sql-functions-maps-map-entries) | `[{'key': 'en', 'value': 'English'}, {'key': 'de', 'value': 'German'}]` | | [cardinality](/develop/docs/dev-database-reference-sql-functions-maps-cardinality) | `2` | ``` run("SELECT map_keys(MAP {'en': 'English', 'de': 'German'}), map_values(MAP {'en': 'English', 'de': 'German'}), " "map_entries(MAP {'en': 'English', 'de': 'German'}), cardinality(MAP {'en': 'English', 'de': 'German'})") ``` ``` [(['en', 'de'], ['English', 'German'], [{'key': 'en', 'value': 'English'}, {'key': 'de', 'value': 'German'}], 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 map functions. - [map](/develop/docs/dev-database-reference-sql-functions-maps-map): build a map. - [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.