Managing variable type nullability

One day you may find yourself with a custom function for Drill that’s very particular about the kind of variables that it accepts. In particular, it may hold strong opinions about whether or not a variable is allowed to express a NULL value. In fact it may even be you who wrote this unavoidably fussy function (SPOILER: This is exactly what happened to me earlier this week).

Currently Drill lacks built-in functions to add or strip nullability from variables, but luckily it’s very easy to whip up a couple UDFs which do exactly that. Today I’ll be showcasing two such functions which respectively add and remove nullability from Drill FLOAT variables. As usual you should perform the necessary incantations and summoning rituals for creating a custom function project in Maven (see the section “Downloading Maven and starting a new project” in this article for a refresher).

Once you’re ready to start thinking about source code, the class associated with the function to add nullability looks like:

package com.yourgroupidentifier.udf;

import org.apache.drill.exec.expr.DrillSimpleFunc;
import org.apache.drill.exec.expr.holders.NullableFloat8Holder;
import org.apache.drill.exec.expr.holders.Float8Holder;

import org.apache.drill.exec.expr.annotations.FunctionTemplate;
import org.apache.drill.exec.expr.annotations.Output;
import org.apache.drill.exec.expr.annotations.Param;

@FunctionTemplate(
        name = "add_null_float",
        scope = FunctionTemplate.FunctionScope.SIMPLE,
        nulls = FunctionTemplate.NullHandling.INTERNAL
)

public class addNullFloat implements DrillSimpleFunc {

    @Param
    Float8Holder input;

    @Output
    NullableFloat8Holder output;

    public void setup() {
    }

    public void eval() {
        output.isSet = 1;
        output.value = input.value;
    }
}

while the one for removing nullability is as follows:

package com.yourgroupidentifier.udf;

import org.apache.drill.exec.expr.DrillSimpleFunc;
import org.apache.drill.exec.expr.holders.NullableFloat8Holder;
import org.apache.drill.exec.expr.holders.Float8Holder;

import org.apache.drill.exec.expr.annotations.FunctionTemplate;
import org.apache.drill.exec.expr.annotations.Output;
import org.apache.drill.exec.expr.annotations.Param;

@FunctionTemplate(
        name = "remove_null_float",
        scope = FunctionTemplate.FunctionScope.SIMPLE,
        nulls = FunctionTemplate.NullHandling.INTERNAL
)

public class removeNullFloat implements DrillSimpleFunc {

    @Param
    NullableFloat8Holder input;

    @Output
    Float8Holder output;

    public void setup() {
    }

    public void eval() {
        output.value = input.value;
    }
}

Remember back in this post when I thought I was presenting the simplest UDFs I’d ever seen demonstrated? Well, these guys are definitely setting a new world record. They’re just about the smallest custom functions you can code for Drill. But that doesn’t mean they’re not useful! ADD_NULL_FLOAT() and REMOVE_NULL_FLOAT() make valuable additions to a Drill power user’s toolbox, and I’ll be putting one of them to work in a post later this week.