Add an elementwise operation

This notebook will show you how to add a new elementwise operation to an existing backend.

We are going to add julianday, a function supported by the SQLite database, to the SQLite Ibis backend.

The Julian day of a date, is the number of days since January 1st, 4713 BC. For more information check the Julian day Wikipedia page.

Step 1: Define the Operation

Let’s define the julianday operation as a function that takes one string input argument and returns a float.

def julianday(date: str) -> float:
    """Return the Julian day from a date."""
import ibis.expr.datatypes as dt
import ibis.expr.rules as rlz
import ibis.expr.datashape as ds

from ibis.expr.operations import Value

class JulianDay(Value):
    arg: Value[dt.String, ds.Any]

    dtype = dt.float32
    shape = rlz.shape_like('arg')

We just defined a JulianDay class that takes one argument of type string or binary, and returns a float.

Step 2: Define the API

Because we know the output type of the operation, to make an expression out of JulianDay we can construct it and call its ibis.expr.types.Node.to_expr method.

We still need to add a method to StringValue (this needs to work on both scalars and columns).

When you add a method to any of the expression classes whose name matches *Value both the scalar and column child classes will pick it up, making it easy to define operations for both scalars and columns in one place.

We can do this by defining a function and assigning it to the appropriate class of expressions.

from ibis.expr.types import StringValue

def julianday(string_value):
    return JulianDay(string_value).to_expr()

StringValue.julianday = julianday

Interlude: Create some expressions with julianday

import ibis

t = ibis.table(dict(string_col="string"), name="t")

r0 := UnboundTable: t
  string_col string

JulianDay(string_col): JulianDay(r0.string_col)

Step 3: Turn the Expression into SQL

import sqlalchemy as sa

def _julianday(translator, expr):
    # pull out the arguments to the expression
    (arg,) = expr.args

    # compile the argument
    compiled_arg = translator.translate(arg)

    # return a SQLAlchemy expression that calls into the SQLite julianday function
    return sa.func.julianday(compiled_arg)

Step 4: Putting it all Together

Download the geography database.

!curl -LsS -o geography.db ''

con = ibis.sqlite.connect("geography.db")

Create and execute a julianday expression

ind = con.table("independence")
DatabaseTable: independence
  country_code      string
  independence_date date
  independence_from string
day = ind.independence_date.cast("string")
r0 := DatabaseTable: independence
  country_code      string
  independence_date date
  independence_from string

Cast(independence_date, string): Cast(r0.independence_date, to=string)
jday_expr = day.julianday().name("jday")
r0 := DatabaseTable: independence
  country_code      string
  independence_date date
  independence_from string

jday: JulianDay(Cast(r0.independence_date, to=string))
  JULIANDAY(CAST(t0.independence_date AS TEXT)) AS jday
FROM independence AS t0

Because we’ve defined our operation on StringValue, and not just on StringColumn we get operations on both string scalars and string columns for free.

jday = ibis.literal("2010-03-14").julianday()
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