list_max

Applies to VAST Cluster 5.5.

Functions › List, struct and map functions › Aggregate a list. Returns the largest element of a list, ignoring NULL elements. It works on lists of numbers, text, dates, booleans and lists.

Syntax

list_max(l)

Returns: the element type of the list (INTEGER for [1, 2], DOUBLE for a DOUBLE[]).

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_max. 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_max"'

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_max(l) compares the elements of l and returns the largest. NULL elements are skipped, so list_max([NULL, 2, 1]) is 2. An empty list, a list of only NULLs and a NULL list return NULL.

How elements compare depends on their type:

• Numbers by value. NaN is larger than every other value, inf included: list_max(['NaN'::DOUBLE, 1.0, 'inf'::DOUBLE]) is NaN.
• Text exactly, not by a language's alphabetical order: list_max(['b', 'A', 'a']) is 'b' and list_max(['é', 'e', 'f']) is 'é'.
• Dates by time: the later one is larger.
• Booleans: true is larger than false.
• Lists element by element, from the first: list_max([[2], [1, 9]]) is [2].

list_max(l) gives the same result as list_aggregate(l, 'max').

NULLs and edge cases

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

run("SELECT list_max(NULL), list_max([]::BIGINT[]), list_max([NULL, 2, 1]), list_max(['b', 'A', 'a'])")
run("SELECT list_max(['NaN'::DOUBLE, 1.0, 'inf'::DOUBLE]), list_max([[2], [1, 9]]), list_max([true, false])")
run("SELECT list_max(123)")

[(None, None, 2, 'b')]
[(nan, [2], True)]
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 largest element of each list in the table. Rows 3 and 4 return NULL: their lists are empty or NULL. Row 2 skips its NULL element:

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

[(1, 3, 2.5, True, [3]), (2, 3, 2.0, True, [4]), (3, None, None, None, None), (4, None, None, None, None), (5, 10, 1e+308, False, [6, 7, 8])]

The largest word compares text exactly, so row 2's 'b' is larger than 'B':

run(f"SELECT id, list_max(words) FROM {TABLE} WHERE id < 3 ORDER BY id LIMIT 10")

[(1, 'c'), (2, 'b')]

The rows whose largest number is above 3:

run(f"SELECT id, nums FROM {TABLE} WHERE list_max(nums) > 3 ORDER BY id LIMIT 10")

[(5, [-5, 0, -5, 10])]

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_max() always runs in the Query Engine:

• In the SELECT list, a projection step computes list_max() for each row the scan returns.
• In WHERE, the scan sends the list for every row, and a FILTER step computes list_max() 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 list_max(nums) > 3")

┌───────────────────────────┐
│         PROJECTION        │
│    ────────────────────   │
│             #1            │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│           FILTER          │
│    ────────────────────   │
│ (list_aggr(nums, 'max') > │
│             3)            │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│         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_max() 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 maximum if you filter on it often. Compute it when you write the rows and keep it in its own column: max_num > 3 is pushed down, list_max(nums) > 3 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_max calls, and its accepted form list_aggr(ANY[], VARCHAR, [ANY...]) -> ANY.
• No array_max. It fails with Scalar Function with name array_max does not exist!. Use list_max.
• ORDER BY needs a LIMIT. Without one, the query fails with The operator 'ORDER_BY' type=1 is currently not supported for this usage.

run("SELECT array_max([1, 2, 3])")

InternalError: Catalog Error: Scalar Function with name array_max does not exist!

Similar functions

run("SELECT list_max(l), list_min(l), list_aggregate(l, 'max'), list_last(l), list_first(l) "
    "FROM (SELECT [3, 1, NULL, 3, 10] AS l)")

[(10, 1, 10, 10, 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.
• list_sort: the elements in order.
• Where filters run and pushdowns.
• EXPLAIN: how to read a plan.