--- title: "map_extract_value" slug: "dev-database-reference-sql-functions-maps-map-extract-value" updated: 2026-10-30T18:18:46Z published: 2026-10-30T18:18:46Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-maps-map-extract-value" --- > ## 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_extract_value *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) › Look up a key.* Looks up a key in a map and returns its value, or NULL if the key isn't in the map. The subscript `m[key]` is the same. ## Syntax ``` map_extract_value(m, key) m[key] ``` | Argument | Type | Description | | --- | --- | --- | | `m` | a map | The map to look in. | | `key` | the map's key type, or a value that can be cast to it | The key to look up. | **Returns:** the map's value type, such as `VARCHAR`. **Aliases:** the subscript `m[key]`. ## 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_extract_value`. 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_extract_value"' 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_extract_value(m, key)` and `m[key]` return the value stored under `key`. The result is NULL in three cases: the key isn't in the map, its value is NULL, or the map is NULL. To tell these apart, use [map_extract](/develop/docs/dev-database-reference-sql-functions-maps-map-extract), which returns `[]` for a missing key and `[None]` for a NULL value, or [map_contains](/develop/docs/dev-database-reference-sql-functions-maps-map-contains). - **Keys are compared exactly.** `'DE'` doesn't find `'de'`. - **The key is cast to the map's key type.** In a map with `INTEGER` keys, `'1'` finds the key `1`. A key that doesn't match, such as `1` in a map whose keys are strings like `'a'`, gives NULL. - **The result can be used directly:** in arithmetic (`MAP {'a': 1}['a'] + 1` is `2`), with another subscript for a nested map or a list (`MAP {'a': [5, 6]}['a'][2]` is `6`), or with `.field` for a struct value. ## NULLs and edge cases | Input | Result | | --- | --- | | `map_extract_value(MAP {'a': 1}, NULL)`, `(MAP {'a': 1})[NULL]` | `NULL` | | `map_extract_value(NULL::MAP(VARCHAR, BIGINT), 'a')` | `NULL` | | `map_extract_value(MAP {'a': NULL::BIGINT}, 'a')` | `NULL` | | `map_extract_value(MAP {}, 'a')` | `NULL` | | `(MAP {'en': 'English', 'de': 'German'})['DE']` | `NULL` | | `MAP {'a': 1}[1]` | `NULL` | | `MAP {1: 'x'}['1']` | `'x'` | | `MAP {'a': MAP {'b': 2}}['a']['b']` | `2` | | `MAP {'a': 1}['a':'b']` (a slice) | Error: `ARRAY_SLICE can only operate on LISTs and VARCHARs` | | `map_extract_value([1, 2], 1)` or a string, number, struct or date | Error: `'map_extract_value' can only operate on MAPs` | | `map_extract_value(MAP {'a': 1}, 'a', 'b')` | Error: `MAP_EXTRACT must have exactly two arguments` | 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-extract-value#set-up-the-examples): ``` run("SELECT map_extract_value(MAP {'a': 1}, NULL), (MAP {'a': 1})[NULL], " "map_extract_value(NULL::MAP(VARCHAR, BIGINT), 'a'), map_extract_value(MAP {'a': NULL::BIGINT}, 'a'), " "map_extract_value(MAP {}, 'a')") run("SELECT (MAP {'en': 'English', 'de': 'German'})['DE'], MAP {'a': 1}[1], MAP {1: 'x'}['1'], " "MAP {'a': MAP {'b': 2}}['a']['b']") run("SELECT MAP {'a': 1}['a':'b']") run("SELECT map_extract_value([1, 2], 1)") run("SELECT map_extract_value(MAP {'a': 1}, 'a', 'b')") ``` ``` [(None, None, None, None, None)] [(None, None, 'x', 2)] InternalError: Binder Error: ARRAY_SLICE can only operate on LISTs and VARCHARs InternalError: Binder Error: 'map_extract_value' can only operate on MAPs InternalError: Binder Error: MAP_EXTRACT must have exactly two arguments ``` ## Examples The function and the subscript give the same value; a missing key gives NULL: ``` run("SELECT map_extract_value(MAP {'en': 'English', 'de': 'German'}, 'de'), " "(MAP {'en': 'English', 'de': 'German'})['de'], (MAP {'en': 'English', 'de': 'German'})['fr']") ``` ``` [('German', 'German', None)] ``` Look up the language each reader asked for. Only article 1 has it; the others give NULL: ``` run(f"SELECT id, lang, map(langs, words)[lang], map_extract_value(map(langs, words), lang) " f"FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, 'de', 135, 135), (2, 'it', None, None), (3, '', None, None), (4, None, None, None)] ``` Keep the articles whose German translation has more than 100 words, and use the value in arithmetic: ``` run(f"SELECT id, map(langs, words)['de'] - map(langs, words)['en'] FROM {TABLE} " "WHERE map(langs, words)['de'] > 100 ORDER BY id LIMIT 10") ``` ``` [(1, 15)] ``` Values that are lists or structs: ``` run("SELECT map_extract_value(MAP {'a': [1, 2]}, 'a'), MAP {'a': [5, 6]}['a'][2], " "map_extract_value(MAP {'a': {'x': 1}}, 'a').x") ``` ``` [([1, 2], 6, 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 lookup 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 looks up the key 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 lookup in `WHERE` 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 and has no `FILTER` step; a lookup in 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. - **Pair a map condition with a pushed-down one,** such as `id > 1`, so the rows it rules out never reach the `FILTER` step. ``` plan(f"SELECT id FROM {TABLE} WHERE lang = 'de'") run(f"SELECT id, map(langs, words)['es'] FROM {TABLE} WHERE id > 1 ORDER BY id LIMIT 10") ``` ``` ┌───────────────────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ Projections: id │ │ │ └───────────────────────────┘ [(2, 140), (3, None), (4, None)] ``` ## Limitations and errors - **NULL doesn't say why.** A missing key, a NULL value and a NULL map all give NULL; see [What it does](/develop/docs/dev-database-reference-sql-functions-maps-map-extract-value#what-it-does). - **One key at a time.** `m['a', 'b']` is a syntax error, and a slice `m['a':'b']` fails with `ARRAY_SLICE can only operate on LISTs and VARCHARs`. - **The subscript works on other types too.** On a list, a string or a struct, `x[...]` is a list, string or struct lookup, with that type's errors: a string key on a list or a date fails with `No function matches the given name and argument types 'array_extract(...)'`, and a key from a column on a struct fails with `Key name for struct_extract needs to be a constant string`. - **Not available:** `map_get`; use `map_extract_value` or `m[key]`. - **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(f"SELECT ([1, 2])[lang] FROM {TABLE} LIMIT 1") run(f"SELECT ({{'a': 1}})[lang] FROM {TABLE} LIMIT 1") run("SELECT map_get(MAP {'a': 1}, 'a')") ``` ``` InternalError: Binder Error: No function matches the given name and argument types 'array_extract(INTEGER[], VARCHAR)'. You might need to add explicit type casts. InternalError: Binder Error: Key name for struct_extract needs to be a constant string InternalError: Catalog Error: Scalar Function with name map_get does not exist! ``` ## Similar functions | Function | Result for `'de'` | Result for `'fr'` (not in the map) | | --- | --- | --- | | `map_extract_value`, `m[key]` | `'German'` | `NULL` | | [map_extract](/develop/docs/dev-database-reference-sql-functions-maps-map-extract), `element_at` | `['German']` | `[]` | | [map_contains](/develop/docs/dev-database-reference-sql-functions-maps-map-contains) | `true` | `false` | On a struct, the same subscript reads a field ([struct_extract](/develop/docs/dev-database-reference-sql-functions-structs-struct-extract)), and the field name must be a constant. The first two lines are for `MAP {'en': 'English', 'de': 'German'}`; the last compares a map and a struct: ``` run("SELECT map_extract_value(MAP {'en': 'English', 'de': 'German'}, 'de'), " "map_extract(MAP {'en': 'English', 'de': 'German'}, 'de'), map_contains(MAP {'en': 'English', 'de': 'German'}, 'de')") run("SELECT map_extract_value(MAP {'en': 'English', 'de': 'German'}, 'fr'), " "map_extract(MAP {'en': 'English', 'de': 'German'}, 'fr'), map_contains(MAP {'en': 'English', 'de': 'German'}, 'fr')") run("SELECT MAP {'a': 1, 'b': 2}['a'], {'a': 1, 'b': 2}['a'], typeof(MAP {'a': 1, 'b': 2}), typeof({'a': 1, 'b': 2})") ``` ``` [('German', ['German'], True)] [(None, [], False)] [(1, 1, 'MAP(VARCHAR, INTEGER)', 'STRUCT(a INTEGER, b INTEGER)')] ``` 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.