--- title: "map" slug: "dev-database-reference-sql-functions-maps-map" updated: 2026-10-30T18:18:46Z published: 2026-10-30T18:18:46Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-maps-map" --- > ## 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 *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) › Build a map.* Builds a map from a list of keys and a list of values of the same length. The literal `MAP {key: value, ...}` builds one from pairs written in the query. ## Syntax ``` map(keys, values) map() MAP {key: value [, ...]} MAP {} ``` | Argument | Type | Description | | --- | --- | --- | | `keys` | a list of any type that can be compared | The keys. They must be unique and not NULL. | | `values` | a list of any type | The values, in the same order as the keys. NULL values are allowed. | **Returns:** `MAP(key type, value type)`, such as `MAP(VARCHAR, INTEGER)`. An empty map is `MAP("NULL", "NULL")`. ## 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`. 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, the word count of each translation, and the language a reader asked for. Article 2 has a translation with no word count yet and was asked for in a language it doesn't have, 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"' 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[], lang VARCHAR)") run(f"INSERT INTO {TABLE} VALUES " "(1, ['en', 'de'], [120, 135], 'de'), " "(2, ['fr', 'es'], [NULL, 140], 'it'), " "(3, [], [], ''), " "(4, NULL, NULL, NULL)") ``` ``` [] [] [(4,)] ``` ## What it does `map(keys, values)` pairs the first key with the first value, the second key with the second value, and so on. `MAP {'a': 1, 'b': 2}` and `map(['a', 'b'], [1, 2])` are the same map. Python receives a map as a list of `(key, value)` tuples: `[('a', 1), ('b', 2)]`. - **Order is kept.** The map holds its pairs in the order you give them: `map(['b', 'a'], [2, 1])` is `[('b', 2), ('a', 1)]`. Two maps with the same pairs in a different order aren't equal. - **Keys are compared exactly.** `'A'` and `'a'` are two keys, and so are `é` written as one code point and `e` followed by a combining accent (U+0301). `0.0` and `-0.0` are the same key, and so are two `NaN` values. - **Keys and values can be of any type.** Numbers, dates, booleans, lists and structs work as keys; values can be lists, structs or maps. - **All keys have one type, and all values have one type.** A value that can't be converted to the type of the first one fails, such as `MAP {'a': 1, 'b': 'x'}`. Cast a map to change its types: `MAP {'a': 1}::MAP(VARCHAR, DOUBLE)`. - **As text,** a map cast to `VARCHAR` is `{a=1}`; [to_json](/develop/docs/dev-database-reference-sql-functions-json-to-json) turns it into a JSON object. ## NULLs and edge cases | Input | Result | | --- | --- | | `map(NULL, NULL)`, `map(['a'], NULL)` | `NULL` | | `map()`, `map([], [])`, `MAP {}` | an empty map, `[]` | | `map(['a'], [NULL])`, `MAP {'a': NULL}` | `[('a', None)]`: NULL values are allowed | | `map(['A', 'a'], [1, 2])` | two keys, `[('A', 1), ('a', 2)]` | | `map(['\u00e9', 'e\u0301'], [1, 2])` | two keys: the code points differ | | `MAP {'a': 1, 'b': 2} = MAP {'b': 2, 'a': 1}` | `false` | | `map(['a', 'a'], [1, 2])`, `map([0.0, -0.0], [1, 2])` | Error: `Map keys must be unique.` | | `map(['a'], [1, 2])` | Error: `The map key list does not align with the map value list.` | | `map([NULL], [1])`, `MAP {NULL: 1}` | Error: `Map keys can not be NULL.` | | `MAP {'a': 1, 'b': 'x'}` | Error: `Could not convert string 'x' to INT32` | | `map('a', 1)`, `map(['a'])` | Error: `Invalid map argument(s).` | 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#set-up-the-examples): ``` run("SELECT map(NULL, NULL), map(['a'], NULL), map(), map([], []), MAP {}") run("SELECT map(['a'], [NULL]), MAP {'a': NULL}, map(['A', 'a'], [1, 2]), map(['\u00e9', 'e\u0301'], [1, 2])") run("SELECT MAP {'a': 1, 'b': 2} = MAP {'b': 2, 'a': 1}, typeof(map()), typeof(MAP {'a': 1, 'b': 2})") run("SELECT map(['a', 'a'], [1, 2])") run("SELECT map([0.0, -0.0], [1, 2])") run("SELECT map(['a'], [1, 2])") run("SELECT MAP {NULL: 1}") run("SELECT MAP {'a': 1, 'b': 'x'}") run("SELECT map('a', 1)") ``` ``` [(None, None, [], [], [])] [([('a', None)], [('a', None)], [('A', 1), ('a', 2)], [('é', 1), ('é', 2)])] [(False, 'MAP("NULL", "NULL")', 'MAP(VARCHAR, INTEGER)')] OSError: Map keys must be unique. OSError: Map keys must be unique. OSError: The map key list does not align with the map value list. OSError: Map keys can not be NULL. OSError: Could not convert string 'x' to INT32 InternalError: Invalid Input Error: Invalid map argument(s). Valid map arguments are a list of key-value pairs (MAP {'key1': 'val1', ...}), two lists (MAP ([1, 2], [10, 11])), or no arguments. ``` ## Examples Build a map per article from its two list columns. Article 2's French translation has a NULL word count, article 3's lists are empty, so its map is empty, and the NULL lists of article 4 give NULL: ``` run(f"SELECT id, map(langs, words) FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, [('en', 120), ('de', 135)]), (2, [('fr', None), ('es', 140)]), (3, []), (4, None)] ``` Look up the language each reader asked for, with `m[key]` ([map_extract_value](/develop/docs/dev-database-reference-sql-functions-maps-map-extract-value)). A key that isn't in the map gives NULL: ``` run(f"SELECT id, lang, map(langs, words)[lang] FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, 'de', 135), (2, 'it', None), (3, '', None), (4, None, None)] ``` Keep the articles whose German translation has 135 words: ``` run(f"SELECT id FROM {TABLE} WHERE map(langs, words)['de'] = 135 ORDER BY id LIMIT 10") ``` ``` [(1,)] ``` Literals, keys and values of other types, and a map as text and as JSON: ``` run("SELECT MAP {'en': 'English', 'de': 'German'}, map([1, 2], ['x', 'y']), map(['k'], [[1, 2]])") run("SELECT map([DATE '2026-10-16'], ['d']), map([true, false], [1, 0]), MAP {'a': 1}::MAP(VARCHAR, DOUBLE)") run("SELECT CAST(MAP {'a': 1} AS VARCHAR), to_json(MAP {'a': 1, 'b': 2})") ``` ``` [([('en', 'English'), ('de', 'German')], [(1, 'x'), (2, 'y')], [('k', [1, 2])])] [([(datetime.date(2026, 10, 16), 'd')], [(True, 1), (False, 0)], [('a', 1.0)])] [('{a=1}', '{"a":1,"b":2}')] ``` ## 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()` is computed by the Query Engine: - **In the `SELECT` list,** the scan sends `langs` and `words`, and the Query Engine builds the map for each row it returns. - **In `WHERE`,** a `FILTER` step in the Query Engine builds the map and tests it for every row the scan sends. 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 map(langs, words)['de'] = 135") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #2 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ (map_extract_value("map" │ │ (langs, words), 'de') = │ │ 135) │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ langs │ │ words │ │ id │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Keep a value you filter on in its own column.** A comparison of a plain column with a constant, such as `lang = 'de'`, is evaluated in VAST DataBase, so it has no `FILTER` step; a test on a map built in the query always has one. If queries often filter on one key, store its value in a column when you write the rows. - **Filter on plain columns where you can.** A condition on a plain column, such as `lang = 'de'` above, is evaluated in VAST DataBase. The second statement returns the articles with `id > 1`, each with its map. ``` plan(f"SELECT id FROM {TABLE} WHERE lang = 'de'") run(f"SELECT id, map(langs, words) FROM {TABLE} WHERE id > 1 ORDER BY id LIMIT 10") ``` ``` ┌───────────────────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ Projections: id │ │ │ └───────────────────────────┘ [(2, [('fr', None), ('es', 140)]), (3, []), (4, None)] ``` ## Limitations and errors - **A table can't return a `MAP` column.** A `MAP` column can be created and written, but a query that returns it fails with `Unsupported column type`, and `CREATE TABLE ... AS SELECT` with a map fails with `Not implemented Error: not supported` ([data types in SQL](/develop/docs/dev-database-reference-sql-data-types)). Store the keys and values as two list columns, as on this page, or the pairs as a list of structs ([map_entries](/develop/docs/dev-database-reference-sql-functions-maps-map-entries)). - **One bad row fails the whole query.** Duplicate keys, a NULL key or lists of different lengths in any row stop the query. Here `lang` is NULL in the last row; leave such rows out with `WHERE`. - **No aggregate builds a map.** `map(list(k), list(v))` over a table fails, because `list()` needs a grouping step that isn't available ([aggregate functions](/develop/docs/dev-database-reference-sql-functions-aggregate)), and there's no `map_agg`. Build the map in your code from the rows. - **Not available:** `map_from_arrays` (use `map(keys, values)`) and `str_to_map` (split the string, then use [map_from_entries](/develop/docs/dev-database-reference-sql-functions-maps-map-from-entries)). ``` run(f"CREATE TABLE IF NOT EXISTS {SCHEMA}.attrs (id BIGINT, m MAP(VARCHAR, BIGINT))") run(f"INSERT INTO {SCHEMA}.attrs VALUES (1, MAP {{'a': 1}})") run(f"SELECT * FROM {SCHEMA}.attrs") run(f"SELECT id, map([lang], [id]) FROM {TABLE} ORDER BY id LIMIT 10") run(f"SELECT id, map([lang], [id]) FROM {TABLE} WHERE lang IS NOT NULL ORDER BY id LIMIT 10") run(f"SELECT map(list(lang), list(id)) FROM {TABLE}") run("SELECT map_from_arrays(['a'], [1])") ``` ``` [] [(1,)] InternalError: Unsupported column type 'MAP(VARCHAR, BIGINT)' of column 'm' in 'VAST_SCAN '. OSError: Map keys can not be NULL. [(1, [('de', 1)]), (2, [('it', 2)]), (3, [('', 3)])] InternalError: The operator 'HASH_GROUP_BY' type=9 is currently not supported. InternalError: Catalog Error: Scalar Function with name map_from_arrays does not exist! ``` ## Similar functions | Function | Builds a map from | Example | | --- | --- | --- | | `map` | a list of keys and a list of values | `map(['a', 'b'], [1, 2])` | | `MAP {...}` | pairs written in the query | `MAP {'a': 1, 'b': 2}` | | [map_from_entries](/develop/docs/dev-database-reference-sql-functions-maps-map-from-entries) | a list of two-field structs | `map_from_entries([{'k': 'a', 'v': 1}, {'k': 'b', 'v': 2}])` | | [map_concat](/develop/docs/dev-database-reference-sql-functions-maps-map-concat) | two or more maps | `map_concat(MAP {'a': 1}, MAP {'b': 2})` | | [list_histogram](/develop/docs/dev-database-reference-sql-functions-lists-list-histogram) | the values of a list and how often each occurs | `list_histogram(['a', 'b', 'a'])` | | [json_transform](/develop/docs/dev-database-reference-sql-functions-json-json-transform) | a JSON object | `json_transform('{"a": 1, "b": 2}', '"MAP(VARCHAR, BIGINT)"')` | The first four give the same map: ``` run("SELECT map(['a', 'b'], [1, 2]), MAP {'a': 1, 'b': 2}, " "map_from_entries([{'k': 'a', 'v': 1}, {'k': 'b', 'v': 2}]), map_concat(MAP {'a': 1}, MAP {'b': 2})") run("SELECT list_histogram(['a', 'b', 'a']), json_transform('{\"a\": 1, \"b\": 2}', '\"MAP(VARCHAR, BIGINT)\"')") ``` ``` [([('a', 1), ('b', 2)], [('a', 1), ('b', 2)], [('a', 1), ('b', 2)], [('a', 1), ('b', 2)])] [([('a', 2), ('b', 1)], [('a', 1), ('b', 2)])] ``` Clean up: ``` run(f"DROP TABLE IF EXISTS {TABLE}") run(f"DROP TABLE IF EXISTS {SCHEMA}.attrs") 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. - [Data types in SQL](/develop/docs/dev-database-reference-sql-data-types): which column types a table can return. - [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.