% (modulo)

Applies to VAST Cluster 5.5.

Functions › Operators and predicates › Arithmetic operators. a % b is the remainder of a divided by b, and mod(a, b) is the same function. The remainder has the sign of a: 7 % 3 is 1 and -7 % 3 is -1. It works on integers, DECIMAL values (exactly) and floating-point values.

Syntax

a % b
mod(a, b)

Returns: for two integers of the same type, that type: INTEGER % INTEGER is an INTEGER. DECIMAL arguments give an exact DECIMAL; FLOAT gives a FLOAT and DOUBLE a DOUBLE.

Aliases: mod(a, b).

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_operators_modulo. 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 three tickets: a ticket number (INTEGER, negative for a refund), an amount (DECIMAL(10,2)) and a compass bearing in degrees (DOUBLE). 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_operators_modulo"'

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}.tickets"
run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}")
run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, num INTEGER, amount DECIMAL(10,2), bearing DOUBLE)")
run(f"INSERT INTO {TABLE} VALUES "
    "(1, 17, 7.50, 370.5), "
    "(2, -17, -7.50, -30.0), "
    "(3, 0, 0.00, 0.0), "
    "(4, NULL, NULL, NULL)")

[]
[]
[(4,)]

What it does

a % b is what's left of a after taking out as many whole b as fit, counting toward zero. It goes with // (integer division): a = (a // b) * b + a % b.

The remainder has the sign of a, whatever the sign of b:

So n % 2 = 1 is true only for positive odd numbers: -17 % 2 is -1. Test for odd numbers with n % 2 <> 0. For a remainder with the sign of b instead, use fmod: fmod(-7, 2) is 1.0.

DECIMAL remainders are exact: 2.45 % 0.7 is Decimal('0.35'), while the same on DOUBLE values is 0.3500000000000003. 7.5 % 2 is Decimal('1.5').

By zero, the result depends on the type: an integer or DECIMAL remainder by 0 is NULL; a FLOAT or DOUBLE remainder by 0 is NaN. Neither is an error.

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 NULL % 2, 2 % NULL, 7 % 3, -7 % 3, 7 % -3, -7 % -3, mod(-7, 3), mod(7, -3)")
run("SELECT 7.5 % 2, typeof(7.5 % 2), 2.45 % 0.7, 2.45::DOUBLE % 0.7")
run("SELECT 1 % 0, 2.45 % 0, 1.0::DOUBLE % 0")
run("SELECT 'inf'::DOUBLE % 1, 1 % 'inf'::DOUBLE, 'nan'::DOUBLE % 1")
run("SELECT (-128)::TINYINT % (-1)::TINYINT")
run(f"SELECT 'abc' % num FROM {TABLE} WHERE id = 1")
run("SELECT 7 MOD 2")
run("SELECT true % 2")

[(None, None, 1, -1, 1, -1, -1, 1)]
[(Decimal('1.5'), 'DECIMAL(11,1)', Decimal('0.35'), 0.3500000000000003)]
[(None, None, nan)]
[(nan, 1.0, nan)]
OSError: Overflow in division of -128 / -1
OSError: Could not convert string 'abc' to INT32
InternalError: Parse query: 'SELECT 7 MOD 2' failed with: Parser Error: syntax error at or near "2"
InternalError: Binder Error: No function matches the given name and argument types '%(BOOLEAN, INTEGER_LITERAL)'. You might need to add explicit type casts.

Examples

The remainder by 2 and by 5, with % and mod(), and the bearing reduced to one turn. Ticket 2 is negative, so its remainders are negative too:

run(f"SELECT id, num % 2, mod(num, 5), amount % 2, bearing % 360 FROM {TABLE} ORDER BY id LIMIT 10")

[(1, 1, 2, Decimal('1.50'), 10.5), (2, -1, -2, Decimal('-1.50'), -30.0), (3, 0, 0, Decimal('0.00'), 0.0), (4, None, None, None, None)]

A remainder from 0 to 4 for every ticket, negative or not: add the divisor and take the remainder again, or use fmod, which returns a DOUBLE:

run(f"SELECT id, ((num % 5) + 5) % 5, fmod(num, 5) FROM {TABLE} ORDER BY id LIMIT 10")

[(1, 2, 2.0), (2, 3, 3.0), (3, 0, 0.0), (4, None, None)]

Keep only the odd ticket numbers. num % 2 = 1 misses ticket 2, because -17 % 2 is -1; num % 2 <> 0 finds both:

run(f"SELECT id, num FROM {TABLE} WHERE num % 2 = 1 ORDER BY id LIMIT 10")
run(f"SELECT id, num FROM {TABLE} WHERE num % 2 <> 0 ORDER BY id LIMIT 10")

[(1, 17)]
[(1, 17), (2, -17)]

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 remainder of a column, so num % 2 is computed by the Query Engine:

• In the SELECT list, the scan reads the column and the Query Engine computes the remainder for each row it returns.
• In WHERE, the scan sends the column for every row, and a FILTER step in the Query Engine computes the remainder and keeps the rows that match. No rows are skipped in storage. mod(num, 2) runs the same way.

The plan of the WHERE example shows the FILTER step above the scan:

plan(f"SELECT id FROM {TABLE} WHERE num % 2 <> 0")

┌───────────────────────────┐
│         PROJECTION        │
│    ────────────────────   │
│             #1            │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│           FILTER          │
│    ────────────────────   │
│      ((num % 2) != 0)     │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│         VAST_SCAN         │
│    ────────────────────   │
│    Function: VAST_SCAN    │
│                           │
│        Projections:       │
│            num            │
│             id            │
│                           │
└───────────────────────────┘

Make it faster

• Store the bucket you filter on. A remainder is often a bucket: odd or even, a shard number, a day of a cycle. If you filter on one often, compute it when you write the rows and keep it in its own column: shard = 3 is pushed down to VAST DataBase, num % 8 = 3 isn't. The plan for a raw column compared with a constant has no FILTER step:

plan(f"SELECT id FROM {TABLE} WHERE num = 17")

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

• Combine it with a pushed-down condition. In id > 100 AND num % 2 <> 0, id > 100 is still evaluated in VAST DataBase, so the remainder is computed only on the rows that pass it.

Limitations and errors

• The smallest value of a signed integer type % -1 stops the query with Overflow in division of -128 / -1 (for a TINYINT), although the remainder would be 0. Cast to a wider type first.
• No MOD operator. 7 MOD 2 is a syntax error. Use % or mod().
• A string that isn't a number fails with Could not convert string '…' to INT32 (or the type of the other side). A numeric string, such as '12', is converted.
• Booleans, dates, INTERVAL values, BLOB values and lists fail with No function matches the given name and argument types.

run("SELECT CAST((-128)::TINYINT AS INTEGER) % -1")
run(f"SELECT '12' % num FROM {TABLE} WHERE id = 1")

[(0,)]
[(12,)]

Similar functions

run("SELECT -7 % 2, mod(-7, 2), fmod(-7, 2), -7 // 2, fdiv(-7, 2), gcd(-7, 2)")
run("SELECT 7 % -2, mod(7, -2), fmod(7, -2), 7 // -2, fdiv(7, -2), gcd(7, -2)")
run("SELECT 1 % 0, mod(1, 0), fmod(1, 0), 1 // 0, fdiv(1, 0), gcd(1, 0)")

[(-1, -1, 1.0, -3, -4.0, 1)]
[(1, 1, -1.0, -3, -4.0, 1)]
[(None, None, nan, None, inf, 1)]

Clean up:

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

[]
[]

See also

• Operators and predicates: all the arithmetic operators.
• // (integer division), fmod and fdiv.
• Where filters run and pushdowns.
• EXPLAIN: how to read a plan.