--- title: "dayofweek" slug: "dev-database-reference-sql-functions-date-time-dayofweek" updated: 2026-10-30T18:19:01Z published: 2026-10-30T18:19:01Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-date-time-dayofweek" --- > ## 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. # dayofweek *Applies to VAST Cluster 5.5.* *[Functions](/develop/docs/dev-database-reference-sql-functions-index) › [Date and time functions](/develop/docs/dev-database-reference-sql-functions-date-time) › Weeks and days of the week.* Returns the day of the week of a date or timestamp, from 0 for Sunday to 6 for Saturday. `weekday` is the same function. ## Syntax ``` dayofweek(value) weekday(value) ``` | Argument | Type | Description | | --- | --- | --- | | `value` | `DATE` or `TIMESTAMP` (also `TIMESTAMP_NS`, `TIMESTAMP_MS`, `TIMESTAMP_S`) | The value to take the day of the week of. | **Returns:** `BIGINT`. **Aliases:** `weekday`. **Same as:** `date_part('dow', value)`, also written `date_part('dayofweek', value)` or `date_part('weekday', value)`; see [date_part](/develop/docs/dev-database-reference-sql-functions-date-time-date-part). ## 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_date_time_dayofweek`. 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 seven 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, row 6 is the last moment of 1969, and row 7 is a Sunday early in January 2027: ``` 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_dayofweek"' 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}.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 '2024-02-29', TIMESTAMP '2024-02-29 00:00:00', TIME '00:00:00', " "CAST('2024-02-29 00:00:00' AS TIMESTAMP_NS)), " "(6, 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)), " "(7, DATE '2027-01-03', TIMESTAMP '2027-01-03 00:00:00.5', TIME '00:00:00.000001', " "CAST('2027-01-03 00:00:00.5' AS TIMESTAMP_NS))") ``` ``` [] [] [(7,)] ``` ## What it does `dayofweek` numbers the days of the week from Sunday: | Day | `dayofweek` | Example | Result | | --- | --- | --- | --- | | Sunday | 0 | `dayofweek(DATE '2026-03-01')` | `0` | | Monday | 1 | `dayofweek(DATE '2026-03-02')` | `1` | | Tuesday to Friday | 2 to 5 | | | | Saturday | 6 | `dayofweek(DATE '2026-02-28')` | `6` | To number the days from Monday (1) to Sunday (7), as ISO 8601 does, use [isodow](/develop/docs/dev-database-reference-sql-functions-date-time-isodow). For the day's name, use [dayname](/develop/docs/dev-database-reference-sql-functions-date-time-dayname). ## NULLs and edge cases | Input | Result | | --- | --- | | A NULL value (row 4) | `NULL` | | `DATE 'infinity'`, `TIMESTAMP '-infinity'` | `NULL` | | `DATE '0001-01-01'` | `1` (a Monday) | | `DATE '9999-12-31'` | `5` (a Friday) | | `TIMESTAMP '1969-12-31 23:59:59.999999'` | `3` (a Wednesday) | | `weekday(DATE '2026-03-01')` | `0` | | An untyped `NULL`: `dayofweek(NULL)` | Error: `Could not choose a best candidate function for the function call "dayofweek("NULL")"` | | An `INTERVAL` | Error: `interval units "dow" not recognized` | | A `TIME` | Error: `No function matches the given name and argument types 'dayofweek(TIME)'` | | A string | Error: `Could not choose a best candidate function`; cast it to `DATE` first | | A number, boolean, `BLOB` or list | Error: `No function matches the given name and argument types`, for example `'dayofweek(INTEGER_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-date-time-dayofweek#set-up-the-examples): ``` run(f"SELECT id, dayofweek(ts) FROM {TABLE} WHERE id = 4") run("SELECT dayofweek(DATE '2026-03-01'), dayofweek(DATE '2026-03-02'), dayofweek(DATE '2026-02-28'), " "weekday(DATE '2026-03-01')") run("SELECT dayofweek(DATE 'infinity'), dayofweek(TIMESTAMP '-infinity'), dayofweek(DATE '0001-01-01'), " "dayofweek(DATE '9999-12-31'), dayofweek(TIMESTAMP '1969-12-31 23:59:59.999999')") run("SELECT dayofweek(NULL)") run("SELECT dayofweek(INTERVAL '1 year 2 months 3 days')") run("SELECT dayofweek(TIME '13:45:30')") run("SELECT dayofweek('2026-03-01')") run("SELECT dayofweek(123)") ``` ``` [(4, None)] [(0, 1, 6, 0)] [(None, None, 1, 5, 3)] InternalError: Binder Error: Could not choose a best candidate function for the function call "dayofweek("NULL")". In order to select one, please add explicit type casts. OSError: interval units "dow" not recognized InternalError: Binder Error: No function matches the given name and argument types 'dayofweek(TIME)'. You might need to add explicit type casts. InternalError: Binder Error: Could not choose a best candidate function for the function call "dayofweek(STRING_LITERAL)". In order to select one, please add explicit type casts. InternalError: Binder Error: No function matches the given name and argument types 'dayofweek(INTEGER_LITERAL)'. You might need to add explicit type casts. ``` ## Examples The day of the week of each event, with its name: ``` run(f"SELECT id, d, dayofweek(d), weekday(ts), dayname(d) FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, datetime.date(2026, 2, 28), 6, 6, 'Saturday'), (2, datetime.date(2026, 3, 1), 0, 0, 'Sunday'), (3, datetime.date(2024, 12, 30), 1, 1, 'Monday'), (4, None, None, None, None), (5, datetime.date(2024, 2, 29), 4, 4, 'Thursday'), (6, datetime.date(1969, 12, 31), 3, 3, 'Wednesday'), (7, datetime.date(2027, 1, 3), 0, 0, 'Sunday')] ``` Keep only the events on a weekend (Sunday is 0, Saturday is 6): ``` run(f"SELECT id FROM {TABLE} WHERE dayofweek(d) IN (0, 6) ORDER BY id LIMIT 10") ``` ``` [(1,), (2,), (7,)] ``` ## 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 `dayofweek()` is always computed by the Query Engine: - **In the `SELECT` list,** the scan reads the column and the Query Engine computes `dayofweek()` for each row it returns. - **In `WHERE`,** the scan sends the column for every row, and a `FILTER` step in the Query Engine computes `dayofweek()` and keeps the rows that match. No rows are skipped in storage. The plan of the weekend example shows the `FILTER` step above the scan: ``` plan(f"SELECT id FROM {TABLE} WHERE dayofweek(d) IN (0, 6)") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #1 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ ((dayofweek(d) = 0) OR │ │ (dayofweek(d) = 6)) │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ d │ │ id │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Store the day of the week if you filter on it often.** Weekends recur every week, so no single range of dates selects them. Compute `dayofweek` when you write the rows and keep it in its own column: a comparison of that column with a constant is pushed down to VAST DataBase, `dayofweek(d) IN (0, 6)` isn't. - **Narrow the dates first.** In `d >= DATE '2026-01-01' AND dayofweek(d) IN (0, 6)`, the range on `d` is still evaluated in VAST DataBase, so `dayofweek()` runs only on the rows that pass it. ## Limitations and errors - **No time-zone support.** On a `TIMESTAMP WITH TIME ZONE` value, such as `now()`, `dayofweek` fails with an `Extension Autoloading Error`. Cast the value to `TIMESTAMP` first; see [time zones](/develop/docs/dev-database-reference-sql-functions-date-time-time-zones). - **Not for `TIME` or `INTERVAL`.** The error for an `INTERVAL` lists `dayofweek(INTERVAL)` among the candidates, but running it fails with `interval units "dow" not recognized`. - **`TIMESTAMP_NS` before 1970.** Row 6's `TIMESTAMP_NS` value `1969-12-31 23:59:59.999999999` gives `4` (Thursday), while the `TIMESTAMP` value `1969-12-31 23:59:59.999999` gives `3` (Wednesday). If you need the day of nanosecond values before 1970 that are close to midnight, check the result or store the value as `TIMESTAMP`. - **Other SQL dialects' names aren't available.** `day_of_week` and `dow` fail with `Scalar Function with name ... does not exist!`. Use `dayofweek`, or `isodow` for the ISO numbering. ``` run(f"SELECT id, dayofweek(tns), dayofweek(ts) FROM {TABLE} WHERE id = 6") run("SELECT day_of_week(DATE '2026-03-01')") run("SELECT dow(DATE '2026-03-01')") ``` ``` [(6, 4, 3)] InternalError: Catalog Error: Scalar Function with name day_of_week does not exist! InternalError: Catalog Error: Scalar Function with name dow does not exist! ``` ## Similar functions | Function | Returns | Result for `DATE '2026-03-01'`, a Sunday (`dayofweek` gives `0`) | | --- | --- | --- | | [isodow](/develop/docs/dev-database-reference-sql-functions-date-time-isodow) | The ISO day of the week, 1 (Monday) to 7 (Sunday) | `7` | | [dayname](/develop/docs/dev-database-reference-sql-functions-date-time-dayname) | The day's name | `'Sunday'` | | [strftime](/develop/docs/dev-database-reference-sql-functions-date-time-strftime) with `'%w'` | The same number as `dayofweek`, as text | `'0'` | | [date_part](/develop/docs/dev-database-reference-sql-functions-date-time-date-part) with `'dow'` | The same as `dayofweek` | `0` | ``` run("SELECT dayofweek(DATE '2026-03-01'), isodow(DATE '2026-03-01'), dayname(DATE '2026-03-01'), " "strftime(DATE '2026-03-01', '%w'), date_part('dow', DATE '2026-03-01')") ``` ``` [(0, 7, 'Sunday', '0', 0)] ``` Clean up: ``` run(f"DROP TABLE IF EXISTS {TABLE}") run(f"DROP SCHEMA IF EXISTS {SCHEMA}") conn.close() ``` ``` [] [] ``` ## See also - [isodow](/develop/docs/dev-database-reference-sql-functions-date-time-isodow) and [dayname](/develop/docs/dev-database-reference-sql-functions-date-time-dayname): other ways to get the day of the week. - [Date and time functions](/develop/docs/dev-database-reference-sql-functions-date-time): all the date and time functions. - [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.