--- title: "cardinality" slug: "dev-database-reference-sql-functions-maps-cardinality" updated: 2026-10-30T18:18:46Z published: 2026-10-30T18:18:46Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-maps-cardinality" --- > ## 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. # cardinality *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) › Size.* Returns the number of key-value pairs in a map. It works on maps only; for a list, use [array_length](/develop/docs/dev-database-reference-sql-functions-lists-array-length). ## Syntax ``` cardinality(map) ``` | Argument | Type | Description | | --- | --- | --- | | `map` | `MAP(K, V)` | The map to measure. | **Returns:** `UBIGINT`. ## 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_cardinality`. 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. A table can't return a `MAP` column, so the examples keep each map as two list columns, `codes` (the keys) and `counts` (the values), and build the map in the query with [map](/develop/docs/dev-database-reference-sql-functions-maps-map)`(codes, counts)`. This block connects, defines the `run()` and `plan()` helpers, and creates a table of five rows. Row 3 has empty lists (an empty map) and row 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_maps_cardinality"' 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}.langs" run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}") run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, codes VARCHAR[], counts BIGINT[])") run(f"INSERT INTO {TABLE} VALUES " "(1, ['en', 'de'], [1, 2]), " "(2, ['\u00e9', '\u00fc'], [NULL, 3]), " "(3, [], []), " "(4, NULL, NULL), " "(5, ['k1', 'k2', 'k3'], [10, 20, 30])") ``` ``` [] [] [(5,)] ``` ## What it does `cardinality` counts the keys of a map, which is the number of key-value pairs, because a map's keys are unique: - **Every pair counts,** including a key whose value is NULL: `cardinality(MAP {'a': NULL})` is 1. - **An empty map** (`MAP {}` or `map()`) has cardinality 0. - **Merged maps count each key once.** [map_concat](/develop/docs/dev-database-reference-sql-functions-maps-map-concat)`(MAP {'a': 1}, MAP {'a': 2})` keeps one `'a'`, so its cardinality is 1. ## NULLs and edge cases | Input | Result | | --- | --- | | `cardinality(NULL)` | `NULL` | | A NULL map: `cardinality(NULL::MAP(VARCHAR, BIGINT))` | `NULL` | | `cardinality(MAP {})`, `cardinality(map())` | `0` | | `cardinality(MAP {'a': NULL})` | `1` | | `cardinality(MAP {'en': 'English', 'de': 'German'})` | `2` | | `cardinality(map_concat(MAP {'a': 1}, MAP {'a': 2}))` | `1` | | `cardinality([1, 2, 3])` (a list) | Error: `Cardinality can only operate on MAPs` | 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-cardinality#set-up-the-examples): ``` run("SELECT cardinality(NULL), cardinality(NULL::MAP(VARCHAR, BIGINT)), cardinality(MAP {}), cardinality(map()), " "cardinality(MAP {'a': NULL})") run("SELECT cardinality(MAP {'en': 'English', 'de': 'German'}), cardinality(map_concat(MAP {'a': 1}, MAP {'a': 2}))") run("SELECT cardinality([1, 2, 3])") ``` ``` [(None, None, 0, 0, 1)] [(2, 1)] InternalError: Binder Error: Cardinality can only operate on MAPs ``` ## Examples The number of pairs in each row's map. Row 3's map is empty; row 4's lists are NULL, so its map and its count are NULL: ``` run(f"SELECT id, cardinality(map(codes, counts)) FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, 2), (2, 2), (3, 0), (4, None), (5, 3)] ``` Keep only the rows whose map has more than one pair: ``` run(f"SELECT id FROM {TABLE} WHERE cardinality(map(codes, counts)) > 1 ORDER BY id LIMIT 10") ``` ``` [(1,), (2,), (5,)] ``` ## 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`). A map exists only inside the query: the scan sends the list columns, and the Query Engine builds the map and counts its pairs, row by row: - **In the `SELECT` list,** the Query Engine builds the map and computes `cardinality()` for each row it returns. - **In `WHERE`,** the scan sends the lists for every row, and a `FILTER` step in the Query Engine builds the map, counts it 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 cardinality(map(codes, counts)) > 1") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #2 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ (cardinality("map"(codes, │ │ counts)) > 1) │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ codes │ │ counts │ │ id │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Count the key list instead.** When the keys are in a list column, `len(codes) > 1` returns the same rows without building the map. It still runs in a `FILTER` step: ``` run(f"SELECT id FROM {TABLE} WHERE len(codes) > 1 ORDER BY id LIMIT 10") plan(f"SELECT id FROM {TABLE} WHERE len(codes) > 1") ``` ``` [(1,), (2,), (5,)] ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #1 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ (len(codes) > 1) │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ codes │ │ id │ │ │ └───────────────────────────┘ ``` - **Store the count if you filter on it often.** Compute it when you write the rows and keep it in its own column: a comparison of that column with a constant is evaluated in VAST DataBase, `cardinality(map(...)) > 1` isn't. ## Limitations and errors - **Maps only.** A list, string, number, date or struct fails with `Cardinality can only operate on MAPs`. For a list, use [array_length](/develop/docs/dev-database-reference-sql-functions-lists-array-length) or `len`. - **One argument.** Two maps fail with `Cardinality must have exactly one arguments`; none fails with `No function matches the given name and argument types 'cardinality()'`. - **A map can't hold a key twice.** Building a map with a repeated key fails at run time with `Map keys must be unique.`, so there is nothing to count. Remove the duplicates first, or merge maps with [map_concat](/develop/docs/dev-database-reference-sql-functions-maps-map-concat), which keeps one value per key. - **Other SQL dialects' names aren't available.** `map_size` fails with `Scalar Function with name map_size does not exist!`. Use `cardinality`. ``` run("SELECT cardinality(MAP {'a': 1}, MAP {'b': 2})") run("SELECT cardinality(map(['a', 'a'], [1, 2]))") run("SELECT map_size(MAP {'a': 1})") ``` ``` InternalError: Binder Error: Cardinality must have exactly one arguments OSError: Map keys must be unique. InternalError: Catalog Error: Scalar Function with name map_size does not exist! ``` ## Similar functions | Function | Counts | Result for `MAP {'en': 'English', 'de': 'German'}` | Result for `[1, 2, 3]` | | --- | --- | --- | --- | | `cardinality` | pairs of a map | `2` | Error: maps only | | `len(`[map_keys](/develop/docs/dev-database-reference-sql-functions-maps-map-keys)`(m))` | keys of a map, as a list | `2` | (maps only) | | [array_length](/develop/docs/dev-database-reference-sql-functions-lists-array-length), `len`, [length](/develop/docs/dev-database-reference-sql-functions-string-length) | elements of a list | (lists only) | `3` | ``` run("SELECT cardinality(MAP {'en': 'English', 'de': 'German'}), len(map_keys(MAP {'en': 'English', 'de': 'German'}))") run("SELECT array_length([1, 2, 3]), len([1, 2, 3]), length([1, 2, 3])") ``` ``` [(2, 2)] [(3, 3, 3)] ``` 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 from two lists. - [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.