Put some Numba in your SQLite

Fair Warning

This library does unsafe things like pass around function pointer addresses as integers. Use at your own risk.

If you're unfamiliar with why passing function pointers' addresses around as integers might be unsafe, then you shouldn't use this library.

Requirements

• Python >=3.7
• numba

Use nix-shell from the repository to avoid dependency hell.

Installation

• poetry install

Examples

Scalar Functions

These are almost the same as decorating a Python function with numba.jit.

from typing import Optional

from numbsql import sqlite_udf


@sqlite_udf
def add_one(x: Optional[int]) -> Optional[int]:
    """Add one to `x` if `x` is not NULL."""

    if x is not None:
        return x + 1
    return None

Aggregate Functions

These follow the API of the Python standard library's sqlite3.Connection.create_aggregate method. The difference with numbsql aggregates is that they require two decorators: numba.experimental.jit_class and numbsql.sqlite_udaf. Let's define the avg (arithmetic mean) function for 64-bit floating point numbers.

from typing import Optional

from numba.experimental import jitclass

from numbsql import sqlite_udaf


@sqlite_udaf
@jitclass
class Avg:
    total: float
    count: int

    def __init__(self):
        self.total = 0.0
        self.count = 0

    def step(self, value: Optional[float]) -> None:
        if value is not None:
            self.total += value
            self.count += 1

    def finalize(self) -> Optional[float]:
        if not self.count:
            return None
        return self.total / self.count

Window Functions

You can also define window functions for use with SQLite's OVER construct:

from typing import Optional

from numba.experimental import jitclass

from numbsql import sqlite_udaf


@sqlite_udaf
@jitclass
class WinAvg:  # pragma: no cover
    total: float
    count: int

    def __init__(self) -> None:
        self.total = 0.0
        self.count = 0

    def step(self, value: Optional[float]) -> None:
        if value is not None:
            self.total += value
            self.count += 1

    def finalize(self) -> Optional[float]:
        count = self.count
        if count:
            return self.total / count
        return None

    def value(self) -> Optional[float]:
        return self.finalize()

    def inverse(self, value: Optional[float]) -> None:
        if value is not None:
            self.total -= value
            self.count -= 1

Calling your aggregate function

Similar to scalar functions, we register the function with a sqlite3.Connection object:

>>> import sqlite3
>>> from numbsql import create_aggregate, create_function
>>> con = sqlite3.connect(":memory:")
>>> create_function(con, "add_one", 1, add_one)
>>> con.execute("SELECT add_one(1)").fetchall()
[(2,)]