day

Applies to VAST Cluster 5.5.

Functions › Date and time functions › Parts of a date or time. Returns the day of the month of a date or timestamp, from 1 to 31, or the days field of an interval. dayofmonth is the same function.

Syntax

day(value)
dayofmonth(value)

Returns: BIGINT.

Aliases: dayofmonth. Also the same as date_part('day', value) and extract(day FROM value); see date_part.

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_date_time_day. 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 events. Each has a date (d), a timestamp (ts), a time of day (t) and a timestamp with nanoseconds (tns). Row 4 has no values, and row 5 is the last moment of 1969:

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_date_time_day"'

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}.events"
run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}")
run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, d DATE, ts TIMESTAMP, t TIME, tns TIMESTAMP_NS)")
run(f"INSERT INTO {TABLE} VALUES "
    "(1, DATE '2026-02-28', TIMESTAMP '2026-02-28 23:15:00', TIME '23:15:00', "
    "CAST('2026-02-28 23:15:00.123456789' AS TIMESTAMP_NS)), "
    "(2, DATE '2026-03-01', TIMESTAMP '2026-03-01 08:30:45.123456', TIME '08:30:45.123456', "
    "CAST('2026-03-01 08:30:45.123456' AS TIMESTAMP_NS)), "
    "(3, DATE '2024-12-30', TIMESTAMP '2024-12-30 12:00:00', TIME '12:00:00', "
    "CAST('2024-12-30 12:00:00' AS TIMESTAMP_NS)), "
    "(4, NULL, NULL, NULL, NULL), "
    "(5, DATE '1969-12-31', TIMESTAMP '1969-12-31 23:59:59.999999', TIME '23:59:59.999999', "
    "CAST('1969-12-31 23:59:59.999999999' AS TIMESTAMP_NS))")

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[(5,)]

What it does

day returns the day of the month of a date or timestamp: day(DATE '2024-02-29') is 29. It works on every timestamp precision (TIMESTAMP, TIMESTAMP_NS, TIMESTAMP_MS, TIMESTAMP_S). dayofmonth returns the same values.

On an INTERVAL, day returns its days field as written. It doesn't turn hours into days or days into months:

To split an interval into months, days and time the usual way first, use normalized_interval: normalized_interval(INTERVAL '40 days 30 hours') is 1 month, 11 days and 6 hours, so its day is 11.

NULLs and edge cases

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

run(f"SELECT id, day(ts), dayofmonth(ts) FROM {TABLE} WHERE id = 4")
run("SELECT day(DATE 'infinity'), day(TIMESTAMP '-infinity'), day(DATE '0001-01-01'), day(DATE '9999-12-31'), "
    "day(CAST('0044-03-15 (BC)' AS DATE))")
run("SELECT day(INTERVAL '1 year 2 months 3 days 04:05:06.789'), day(INTERVAL '40 days'), day(INTERVAL '36 hours'), "
    "day(INTERVAL '1 month'), day(INTERVAL '-1 year -2 months -3 days -04:05:06.789'), "
    "day(INTERVAL '25 months 40 days 30 hours')")
run("SELECT normalized_interval(INTERVAL '40 days 30 hours'), day(normalized_interval(INTERVAL '40 days 30 hours'))")
run("SELECT day(NULL)")
run("SELECT day(TIME '13:45:30')")
run("SELECT dayofmonth('2024-02-29')")
run("SELECT day(123)")

[(4, None, None)]
[(None, None, 1, 31, 15)]
[(3, 40, 0, 0, -3, 40)]
[(MonthDayNano(months=1, days=11, nanoseconds=21600000000000), 11)]
InternalError: Binder Error: Could not choose a best candidate function for the function call "day("NULL")". In order to select one, please add explicit type casts.
InternalError: Binder Error: No function matches the given name and argument types 'day(TIME)'. You might need to add explicit type casts.
InternalError: Binder Error: Could not choose a best candidate function for the function call "dayofmonth(STRING_LITERAL)". In order to select one, please add explicit type casts.
InternalError: Binder Error: No function matches the given name and argument types 'day(INTEGER_LITERAL)'. You might need to add explicit type casts.

Examples

The day of the month of each event, with day and dayofmonth:

run(f"SELECT id, day(d), day(ts), dayofmonth(ts) FROM {TABLE} ORDER BY id LIMIT 10")

[(1, 28, 28, 28), (2, 1, 1, 1), (3, 30, 30, 30), (4, None, None, None), (5, 31, 31, 31)]

Keep only the events on the first day of a month:

run(f"SELECT id FROM {TABLE} WHERE day(ts) = 1 ORDER BY id LIMIT 10")

[(2,)]

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). VAST DataBase can evaluate a comparison of a column with a constant, but not a function of a column, so day() is always computed by the Query Engine:

• In the SELECT list, the scan reads the column and the Query Engine computes day() for each row it returns.
• In WHERE, the scan sends the column for every row, and a FILTER step in the Query Engine computes day() 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 day(ts) = 1")

┌───────────────────────────┐
│         PROJECTION        │
│    ────────────────────   │
│             #1            │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│           FILTER          │
│    ────────────────────   │
│      ("day"(ts) = 1)      │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│         VAST_SCAN         │
│    ────────────────────   │
│    Function: VAST_SCAN    │
│                           │
│        Projections:       │
│             ts            │
│             id            │
│                           │
└───────────────────────────┘

Make it faster

• For one particular day, filter on a range of the raw column. ts >= TIMESTAMP '2026-03-01 00:00:00' AND ts < TIMESTAMP '2026-03-02 00:00:00' is 1 March 2026, and it's pushed down to VAST DataBase, so rows from other days never leave storage. On a DATE column, compare it with the date: d = DATE '2026-03-01'. The plan for the range has no FILTER step:

plan(f"SELECT id FROM {TABLE} WHERE ts >= TIMESTAMP '2026-03-01 00:00:00' AND ts < TIMESTAMP '2026-03-02 00:00:00'")

┌───────────────────────────┐
│         VAST_SCAN         │
│    ────────────────────   │
│    Function: VAST_SCAN    │
│      Projections: id      │
│                           │
└───────────────────────────┘

• Store the day if you filter on it across months. day(ts) = 1 for every month has no single range. Compute it when you write the rows and keep it in its own column: event_day = 1 is pushed down, day(ts) = 1 isn't.

Limitations and errors

• No time-zone support. On a TIMESTAMP WITH TIME ZONE value, such as now(), day fails with an Extension Autoloading Error. Cast the value to TIMESTAMP first; see time zones.
• Not for TIME. day(TIME ...) fails with No function matches the given name and argument types 'day(TIME)'.
• TIMESTAMP_NS just before 1970. Row 5's tns, 1969-12-31 23:59:59.999999999, gives 1, not 31. Store such values as TIMESTAMP if you need their day.

run(f"SELECT id, day(tns), day(ts) FROM {TABLE} ORDER BY id LIMIT 10")

[(1, 28, 28), (2, 1, 1), (3, 30, 30), (4, None, None), (5, 1, 31)]

Similar functions

run("SELECT day(DATE '2024-02-29'), dayofweek(DATE '2024-02-29'), isodow(DATE '2024-02-29'), "
    "dayofyear(DATE '2024-02-29'), dayname(DATE '2024-02-29'), last_day(DATE '2024-02-29')")

[(29, 4, 4, 60, 'Thursday', datetime.date(2024, 2, 29))]

Clean up:

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

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See also

• date_part: any part of a value, by name.
• Date and time functions: all the date and time functions.
• Where filters run and pushdowns.
• EXPLAIN: how to read a plan.