--- title: "list_histogram" slug: "dev-database-reference-sql-functions-lists-list-histogram" updated: 2026-10-30T18:19:11Z published: 2026-10-30T18:19:11Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-lists-list-histogram" --- > ## 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_histogram *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) › Aggregate a list.* Counts how often each distinct value occurs in a list and returns the counts as a map from value to count. NULL elements aren't counted. ## Syntax ``` list_histogram(l) ``` | Argument | Type | Description | | --- | --- | --- | | `l` | a list of any type whose values can be compared | The values to count. | **Returns:** `MAP(element type, UBIGINT)`: each distinct value of `l` is a key, and its count (an unsigned 64-bit integer) is the value. For example `typeof(list_histogram([1, 2]))` is `'MAP(INTEGER, 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_lists_list_histogram`. 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 five rows of lists: whole numbers (`nums`), measurements (`vals`) and words (`words`). Row 2 has NULL elements, row 3 has empty lists, row 4 has no values, and row 5 has non-ASCII words and an empty string: ``` 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_histogram"' 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}.samples" run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}") run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, nums BIGINT[], vals DOUBLE[], words VARCHAR[])") run(f"INSERT INTO {TABLE} VALUES " "(1, [1, 2, 3], [0.5, 2.5, -1.0], ['a', 'b', 'c']), " "(2, [3, NULL, 3], [2.0, NULL], ['b', NULL, 'B']), " "(3, [], [], []), " "(4, NULL, NULL, NULL), " "(5, [-5, 0, -5, 10], [1e308, 1e308, -1e-308], ['\u00e9', 'Zo\u00eb', '\U0001F1EB\U0001F1F7', ''])") ``` ``` [] [] [(5,)] ``` ## What it does `list_histogram(l)` returns one map entry per distinct non-NULL value of `l`, holding how many times it occurs: - `[1, 2, 2]` gives `{1: 1, 2: 2}`, and `['b', 'a', 'b']` gives `{'a': 1, 'b': 2}`. - In every result returned for this page's cases, the keys come in ascending order: numbers by value, text by character code. - Text is compared exactly: `'b'` and `'B'` are counted separately. - A list of lists counts each inner list as a whole: `[[1], [1]]` gives `{[1]: 2}`. Python receives a map as a list of `(key, count)` tuples, for example `[(1, 1), (2, 2)]`. Use the map functions on the result. `list_histogram(l)[v]` returns the count of the value `v`, and [map_keys](/develop/docs/dev-database-reference-sql-functions-maps-map-keys) returns the distinct values in key order. ## NULLs and edge cases | Input | Result | | --- | --- | | `NULL`, or `NULL::BIGINT[]` | `NULL` | | `[]::BIGINT[]` (empty list) | `NULL`, not an empty map | | `[NULL, 1, NULL]` | `{1: 1}` (NULL elements aren't counted) | | `[[1], [1]]` | `{[1]: 2}` | | `list_histogram([1, 2, 2])[2]` | `2` | | `map_keys(list_histogram(['b', 'a', 'b']))` | `['a', 'b']` | | `typeof(list_histogram([1.5::DOUBLE]))` | `'MAP(DOUBLE, UBIGINT)'` | | A number, string, `BLOB`, date or boolean | Error: `No function matches the given name and argument types`, for example `'list_aggr(INTEGER_LITERAL, STRING_LITERAL)'` | 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-lists-list-histogram#set-up-the-examples): ``` run("SELECT list_histogram(NULL), list_histogram(NULL::BIGINT[]), list_histogram([]::BIGINT[]), list_histogram([NULL, 1, NULL])") run("SELECT list_histogram([[1], [1]]), list_histogram([1, 2, 2])[2], map_keys(list_histogram(['b', 'a', 'b'])), " "typeof(list_histogram([1.5::DOUBLE]))") run(f"SELECT list_histogram(123) FROM {TABLE} LIMIT 1") ``` ``` [(None, None, None, [(1, 1)])] [([([1], 2)], 2, ['a', 'b'], 'MAP(DOUBLE, UBIGINT)')] InternalError: Binder Error: No function matches the given name and argument types 'list_aggr(INTEGER_LITERAL, STRING_LITERAL)'. You might need to add explicit type casts. ``` ## Examples The counts in each row's numbers and words. Row 5's numbers `[-5, 0, -5, 10]` give `-5` a count of 2. Row 2's words `['b', NULL, 'B']` give `'B'` and `'b'` one each and skip the NULL. The empty lists of row 3 and the NULL lists of row 4 give NULL: ``` run(f"SELECT id, list_histogram(nums), list_histogram(words) FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, [(1, 1), (2, 1), (3, 1)], [('a', 1), ('b', 1), ('c', 1)]), (2, [(3, 2)], [('B', 1), ('b', 1)]), (3, None, None), (4, None, None), (5, [(-5, 2), (0, 1), (10, 1)], [('', 1), ('Zoë', 1), ('é', 1), ('🇫🇷', 1)])] ``` How many times `3` occurs in each row's numbers: ``` run(f"SELECT id, nums, list_histogram(nums)[3] FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, [1, 2, 3], 1), (2, [3, None, 3], 2), (3, [], None), (4, None, None), (5, [-5, 0, -5, 10], None)] ``` The rows whose numbers aren't NULL or empty: ``` run(f"SELECT id FROM {TABLE} WHERE list_histogram(nums) IS NOT NULL 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`). The scan reads a list column whole, and `list_histogram()` always runs in the Query Engine: - **In the `SELECT` list,** a projection step computes `list_histogram()` for each row the scan returns. - **In `WHERE`,** the scan sends the list for every row, and a `FILTER` step computes `list_histogram()` and keeps the rows that match. No rows are skipped in storage. The plan of the last example shows the `FILTER` step above the scan: ``` plan(f"SELECT id FROM {TABLE} WHERE list_histogram(nums) IS NOT NULL") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #1 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ (list_aggr(nums, │ │ 'histogram') IS NOT NULL) │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ nums │ │ id │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Add a condition on a plain column.** A comparison of a column with a constant, such as `id > 1`, is evaluated in VAST DataBase, so `list_histogram()` runs only on the rows that pass it. The plan for `id > 1` alone has no `FILTER` step: ``` plan(f"SELECT id FROM {TABLE} WHERE id > 1") ``` ``` ┌───────────────────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ Projections: id │ │ │ └───────────────────────────┘ ``` - **Store the counts you filter on.** If you often filter on how many times a value occurs, compute that count when you write the rows and keep it in its own column: `threes > 1` is pushed down, `list_histogram(nums)[3] > 1` isn't. ## Limitations and errors - **An empty list gives NULL, not an empty map.** You can't tell an empty list from a NULL list by the result; test `len(l) = 0` if you need to. - **Lists only.** Any other type fails with `No function matches the given name and argument types`. The error names `list_aggr`, the list aggregate that `list_histogram` runs on, and its accepted form `list_aggr(ANY[], VARCHAR, [ANY...]) -> ANY`. - **`ORDER BY` needs a `LIMIT`.** Without one, the query fails with `The operator 'ORDER_BY' type=1 is currently not supported for this usage`. ## Similar functions | Function | What it returns | Result for `[3, 1, NULL, 3, 10]` | | --- | --- | --- | | `list_histogram` | the count of each distinct value, as a map | `{1: 1, 3: 2, 10: 1}` | | [list_distinct](/develop/docs/dev-database-reference-sql-functions-lists-list-distinct) | the distinct values, as a list in no set order | `[10, 3, 1]` | | [list_unique](/develop/docs/dev-database-reference-sql-functions-lists-list-unique) | the number of distinct non-NULL values | `3` | | [list_entropy](/develop/docs/dev-database-reference-sql-functions-lists-list-entropy) | entropy of the value counts, in bits | `1.5` | ``` run("SELECT list_histogram(l), list_distinct(l), list_unique(l), list_entropy(l) FROM (SELECT [3, 1, NULL, 3, 10] AS l)") ``` ``` [([(1, 1), (3, 2), (10, 1)], [10, 3, 1], 3, 1.5)] ``` 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. - [map_keys](/develop/docs/dev-database-reference-sql-functions-maps-map-keys), [map_values](/develop/docs/dev-database-reference-sql-functions-maps-map-values) and [map_extract_value](/develop/docs/dev-database-reference-sql-functions-maps-map-extract-value): work with the map it returns. - [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.