fmod

Applies to VAST Cluster 5.5.

Functions › Numeric and math functions › Arithmetic. Returns the remainder of a divided by b when the quotient is rounded down, as a DOUBLE. The remainder has the sign of b, so fmod(-5.5, 2) is 0.5. It goes with fdiv: for ordinary values, a = fdiv(a, b) * b + fmod(a, b).

Syntax

fmod(a, b)

Returns: DOUBLE, for every numeric input type.

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_numeric_fmod. 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 seven pairs of numbers a and b. Rows 4 and 5 are compass headings with b = 360; the last row 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_numeric_fmod"'

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}.pairs"
run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}")
run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, a DOUBLE, b DOUBLE)")
run(f"INSERT INTO {TABLE} VALUES "
    "(1, 5.5, 2), "
    "(2, -5.5, 2), "
    "(3, 5.5, -2), "
    "(4, 370, 360), "
    "(5, -90, 360), "
    "(6, 7, 0), "
    "(7, NULL, NULL)")

[]
[]
[(7,)]

What it does

fmod(a, b) divides a by b, rounds the quotient down (towards minus infinity), and returns what's left. For finite values the result lies between 0 and b, so it has the sign of b:

This differs from % and mod(), whose remainder has the sign of a: -5.5 % 2 is -1.5. fmod is the one to use to bring a value into a range that starts at 0, such as a heading in degrees into 0 to 360.

NULLs and edge cases

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

run("SELECT fmod(NULL, 2), fmod(1, NULL), fmod(5, 0), fmod(5.0, 0.0), fmod(0, 0)")
run("SELECT fmod('inf'::DOUBLE, 1), fmod(1, 'inf'::DOUBLE), fmod('NaN'::DOUBLE, 1), fmod(1e308, 1e-308)")
run("SELECT fmod(2.5, 1.5), fmod(7::DECIMAL(4,1), 2), fmod(-9223372036854775808, -1), typeof(fmod(5, 2))")
run("SELECT fmod('7', 2)")

[(None, None, nan, nan, nan)]
[(nan, nan, nan, -inf)]
[(1.0, 1.0, 0.0, 'DOUBLE')]
InternalError: Binder Error: No function matches the given name and argument types 'floor(INTERVAL)'. You might need to add explicit type casts.

Examples

The remainder for each pair, and the matching quotient from fdiv. Row 5 turns a heading of -90 degrees into 270, and row 6 divides by zero:

run(f"SELECT id, a, b, fmod(a, b), fdiv(a, b) FROM {TABLE} ORDER BY id LIMIT 10")

[(1, 5.5, 2.0, 1.5, 2.0), (2, -5.5, 2.0, 0.5, -3.0), (3, 5.5, -2.0, -0.5, -3.0), (4, 370.0, 360.0, 10.0, 1.0), (5, -90.0, 360.0, 270.0, -1.0), (6, 7.0, 0.0, nan, inf), (7, None, None, None, None)]

The rows with an even id:

run(f"SELECT id FROM {TABLE} WHERE fmod(id, 2) = 0 ORDER BY id LIMIT 10")

[(2,), (4,), (6,)]

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 fmod() is always computed by the Query Engine:

• In the SELECT list, the scan reads the columns and the Query Engine computes fmod() for each row it returns.
• In WHERE, the scan sends the columns for every row, and a FILTER step in the Query Engine computes fmod() 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 fmod(id, 2) = 0")

┌───────────────────────────┐
│           FILTER          │
│    ────────────────────   │
│ ((CAST(id AS DOUBLE) - (2 │
│   .0 * floor((CAST(id AS  │
│  DOUBLE) / 2.0)))) = 0.0) │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│         VAST_SCAN         │
│    ────────────────────   │
│    Function: VAST_SCAN    │
│      Projections: id      │
│                           │
└───────────────────────────┘

Make it faster

• Combine it with a pushed-down condition. In id > 3 AND fmod(a, b) > 0, id > 3 is a comparison of a raw column with a constant, which VAST DataBase evaluates, so fmod() runs only on the rows that pass it. Compare its plan with the one above:

plan(f"SELECT id FROM {TABLE} WHERE id > 3 AND fmod(a, b) > 0")

┌───────────────────────────┐
│         PROJECTION        │
│    ────────────────────   │
│             #0            │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│           FILTER          │
│    ────────────────────   │
│ ((a - (b * floor((a / b)))│
│          ) > 0.0)         │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│         VAST_SCAN         │
│    ────────────────────   │
│    Function: VAST_SCAN    │
│                           │
│        Projections:       │
│             id            │
│             a             │
│             b             │
│                           │
└───────────────────────────┘

• Store the remainder if you filter on it often. Compute it when you write the rows and keep it in its own column, which a comparison with a constant can then filter in storage.

Limitations and errors

• The result is always a DOUBLE, also for integers: fmod(5, 3) is 2.0. Cast it if you need an integer: CAST(fmod(5, 3) AS BIGINT).
• Division by zero returns nan, not an error and not NULL. Test for it with isnan(), or guard the divisor: fmod(a, NULLIF(b, 0)) returns NULL instead.
• Numbers only. A date, boolean, list or BLOB fails with an error that names the division fmod performs, for example No function matches the given name and argument types '/(DATE, INTEGER_LITERAL)'. A string, even a numeric one, fails with 'floor(INTERVAL)'. Cast strings to a number first.

run("SELECT CAST(fmod(5, 3) AS BIGINT), isnan(fmod(1, 0)), fmod(1, NULLIF(0, 0))")
run("SELECT fmod(DATE '2026-10-16', 2)")
run("SELECT fmod(CAST('7' AS DOUBLE), 2)")

[(2, True, None)]
InternalError: Binder Error: No function matches the given name and argument types '/(DATE, INTEGER_LITERAL)'. You might need to add explicit type casts.
[(1.0,)]

Similar functions

run("SELECT fmod(-5.5, 2), -5.5 % 2, mod(-5.5, 2), fdiv(-5.5, 2)")
run("SELECT fmod(5, 3), 5 % 3, mod(5, 3), fdiv(5, 3), 5 // 3")

[(0.5, Decimal('-1.5'), Decimal('-1.5'), -3.0)]
[(2.0, 2, 2, 1.0, 1)]

Clean up:

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

[]
[]

See also

• Numeric and math functions: all the numeric functions.
• fdiv, % and mod and floor.
• Where filters run and pushdowns.
• EXPLAIN: how to read a plan.