list_count

Applies to VAST Cluster 5.5.

Functions › List, struct and map functions › Aggregate a list. Returns the number of elements in a list that aren't NULL. To count every element, NULLs included, use len.

Syntax

list_count(l)

Returns: BIGINT.

Set up the examples

The examples connect with the ADBC driver, as in VAST Query Engine (ADBC), and work in a schema called docs_sql_fn_lists_list_count. Set QE_ENDPOINT, AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY and VASTDB_BUCKET (SQL reference), 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: numbers, words, measurements, flags and lists of lists. Row 3 has empty lists 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_lists_list_count"'

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, "<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[], words VARCHAR[], vals DOUBLE[], "
    "flags BOOLEAN[], nested BIGINT[][])")
run(f"INSERT INTO {TABLE} VALUES "
    "(1, [1, 2, 3], ['a', 'b', 'c'], [0.5, 2.5, -1.0], [true, false, true], [[1, 2], [3]]), "
    "(2, [3, NULL, 3], ['b', NULL, 'B'], [2.0, NULL], [NULL, true], [[], NULL, [4]]), "
    "(3, [], [], [], [], []), "
    "(4, NULL, NULL, NULL, NULL, NULL), "
    "(5, [-5, 0, -5, 10], ['\u00e9', 'Zo\u00eb', '\U0001F1EB\U0001F1F7', ''], [1e308, 1e308, -1e-308], "
    "[false, false], [[5, NULL], [], [6, 7, 8]])")

[]
[]
[(5,)]

What it does

list_count(l) counts the elements of l that aren't NULL:

• [1, NULL, 3] has 2, and [NULL] has 0.
• An empty list has 0. A NULL list returns NULL, not 0.
• In a list of lists, an empty inner list is an element like any other: list_count([[], NULL]) is 1.
• Duplicates count each time. To count distinct values, use list_unique.

list_count(l) gives the same result as list_aggregate(l, 'count').

NULLs and edge cases

The same cases, as the cluster returns them. They use the helpers from Set up the examples:

run("SELECT list_count(NULL), list_count([]::BIGINT[]), list_count([NULL]::BIGINT[]), list_count([1, NULL, 3])")
run("SELECT list_count([[], NULL]), typeof(list_count([1]))")
run("SELECT list_count(123)")

[(None, 0, 0, 2)]
[(1, 'BIGINT')]
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 number of non-NULL elements in each list of the table. Row 2's NULL elements aren't counted; row 3's empty lists count 0 and row 4's NULL lists return NULL:

run(f"SELECT id, list_count(nums), list_count(words), list_count(vals), list_count(flags), list_count(nested) "
    f"FROM {TABLE} ORDER BY id LIMIT 10")

[(1, 3, 3, 3, 3, 2), (2, 2, 2, 1, 1, 2), (3, 0, 0, 0, 0, 0), (4, None, None, None, None, None), (5, 4, 4, 3, 2, 3)]

The rows whose numbers include a NULL: there, list_count is smaller than len:

run(f"SELECT id, nums FROM {TABLE} WHERE list_count(nums) < len(nums) ORDER BY id LIMIT 10")

[(2, [3, None, 3])]

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_count() always runs in the Query Engine:

• In the SELECT list, a projection step computes list_count() for each row the scan returns.
• In WHERE, the scan sends the list for every row, and a FILTER step computes list_count() and keeps the rows that match. No rows are skipped in storage.

The plan of a WHERE condition on list_count() shows the FILTER step above the scan:

plan(f"SELECT id FROM {TABLE} WHERE list_count(nums) IS NOT NULL")

┌───────────────────────────┐
│         PROJECTION        │
│    ────────────────────   │
│             #1            │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│           FILTER          │
│    ────────────────────   │
│ (list_aggr(nums, 'count') │
│        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_count() 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 count if you filter on it often. Compute it when you write the rows and keep it in its own column: num_count > 2 is pushed down, list_count(nums) > 2 isn't.

Limitations and errors

• Lists only. Any other type fails with No function matches the given name and argument types. The error names list_aggr, the function list_count calls, 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

run("SELECT list_count(l), list_aggregate(l, 'count'), len(l), list_unique(l) FROM (SELECT [3, 1, NULL, 3, 10] AS l)")

[(4, 4, 5, 3)]

Clean up:

run(f"DROP TABLE IF EXISTS {TABLE}")
run(f"DROP SCHEMA IF EXISTS {SCHEMA}")
conn.close()

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See also

• List, struct and map functions: all the list functions.
• Where filters run and pushdowns.
• EXPLAIN: how to read a plan.