Functions¶
Plugin Implementation¶
The function framework is used to implement SQL functions. Presto includes a
number of built-in functions. In order to implement new functions, you can
write a plugin that returns one more functions from getFunctions()
:
public class ExampleFunctionsPlugin
implements Plugin
{
@Override
public Set<Class<?>> getFunctions()
{
return ImmutableSet.<Class<?>>builder()
.add(ExampleNullFunction.class)
.add(IsNullFunction.class)
.add(IsEqualOrNullFunction.class)
.add(ExampleStringFunction.class)
.add(ExampleAverageFunction.class)
.build();
}
}
Note that the ImmutableSet
class is a utility class from Guava.
The getFunctions()
method contains all of the classes for the functions
that we will implement below in this tutorial.
For a full example in the codebase, see either the presto-ml
module for machine
learning functions or the presto-teradata-functions
module for Teradata-compatible
functions, both in the root of the Presto source.
Scalar Function Implementation¶
The function framework uses annotations to indicate relevant information
about functions, including name, description, return type and parameter
types. Below is a sample function which implements is_null
:
public class ExampleNullFunction
{
@ScalarFunction("is_null", calledOnNullInput = true)
@Description("Returns TRUE if the argument is NULL")
@SqlType(StandardTypes.BOOLEAN)
public static boolean isNull(@SqlNullable @SqlType(StandardTypes.VARCHAR) Slice string)
{
return (string == null);
}
}
The function is_null
takes a single VARCHAR
argument and returns a
BOOLEAN
indicating if the argument was NULL
. Note that the argument to
the function is of type Slice
. VARCHAR
uses Slice
, which is essentially
a wrapper around byte[]
, rather than String
for its native container type.
@SqlType
:The
@SqlType
annotation is used to declare the return type and the argument types. Note that the return type and arguments of the Java code must match the native container types of the corresponding annotations.@SqlNullable
:The
@SqlNullable
annotation indicates that the argument may beNULL
. Without this annotation the framework assumes that all functions returnNULL
if any of their arguments areNULL
. When working with aType
that has a primitive native container type, such asBigintType
, use the object wrapper for the native container type when using@SqlNullable
. The method must be annotated with@SqlNullable
if it can returnNULL
when the arguments are non-null.
Parametric Scalar Functions¶
Scalar functions that have type parameters have some additional complexity. To make our previous example work with any type we need the following:
@ScalarFunction(name = "is_null", calledOnNullInput = true)
@Description("Returns TRUE if the argument is NULL")
public final class IsNullFunction
{
@TypeParameter("T")
@SqlType(StandardTypes.BOOLEAN)
public static boolean isNullSlice(@SqlNullable @SqlType("T") Slice value)
{
return (value == null);
}
@TypeParameter("T")
@SqlType(StandardTypes.BOOLEAN)
public static boolean isNullLong(@SqlNullable @SqlType("T") Long value)
{
return (value == null);
}
@TypeParameter("T")
@SqlType(StandardTypes.BOOLEAN)
public static boolean isNullDouble(@SqlNullable @SqlType("T") Double value)
{
return (value == null);
}
// ...and so on for each native container type
}
@TypeParameter
:The
@TypeParameter
annotation is used to declare a type parameter which can be used in the argument types@SqlType
annotation, or return type of the function. It can also be used to annotate a parameter of typeType
. At runtime, the engine will bind the concrete type to this parameter. Optionally, the type parameter can be constrained to descendants of a particular type by providing aboundedBy
type class to@TypeParameter
.@OperatorDependency
may be used to declare that an additional function for operating on the given type parameter is needed. For example, the following function will only bind to types which have an equals function defined:
@ScalarFunction(name = "is_equal_or_null", calledOnNullInput = true)
@Description("Returns TRUE if arguments are equal or both NULL")
public final class IsEqualOrNullFunction
{
@TypeParameter("T")
@SqlType(StandardTypes.BOOLEAN)
public static boolean isEqualOrNullSlice(
@OperatorDependency(operator = OperatorType.EQUAL, returnType = StandardTypes.BOOLEAN, argumentTypes = {"T", "T"}) MethodHandle equals,
@SqlNullable @SqlType("T") Slice value1,
@SqlNullable @SqlType("T") Slice value2)
{
if (value1 == null && value2 == null) {
return true;
}
if (value1 == null || value2 == null) {
return false;
}
return (boolean) equals.invokeExact(value1, value2);
}
// ...and so on for each native container type
}
Another Scalar Function Example¶
The lowercaser
function takes a single VARCHAR
argument and returns a
VARCHAR
, which is the argument converted to lower case:
public class ExampleStringFunction
{
@ScalarFunction("lowercaser")
@Description("converts the string to alternating case")
@SqlType(StandardTypes.VARCHAR)
public static Slice lowercaser(@SqlType(StandardTypes.VARCHAR) Slice slice)
{
String argument = slice.toStringUtf8();
return Slices.utf8Slice(argument.toLowerCase());
}
}
Note that for most common string functions, including converting a string to
lower case, the Slice library also provides implementations that work directly
on the underlying byte[]
, which have much better performance. This function
has no @SqlNullable
annotations, meaning that if the argument is NULL
,
the result will automatically be NULL
(the function will not be called).
Codegen Scalar Function Implementation¶
Scalar functions can also be implemented in bytecode, allowing us to specialize
and optimize functions according to the @TypeParameter
@CodegenScalarFunction
:The
@CodegenScalarFunction
annotation is used to declare a scalar function which is implemented in bytecode.@SqlType
annotation is used to declare the return type. It takesType
as parameters which have@SqlType
annotation as well. Return type isMethodHandle
which is codegen function method.
public class CodegenArrayLengthFunction
{
@CodegenScalarFunction("array_length", calledOnNullInput = true)
@SqlType(StandardTypes.INTEGER)
@TypeParameter("K")
public static MethodHandle arrayLength(@SqlType("array(K)") Type arr)
{
CallSiteBinder binder = new CallSiteBinder();
ClassDefinition classDefinition = new ClassDefinition(a(Access.PUBLIC, FINAL), makeClassName("ArrayLength"), type(Object.class));
classDefinition.declareDefaultConstructor(a(PRIVATE));
Parameter inputBlock = arg("inputBlock", Block.class);
MethodDefinition method = classDefinition.declareMethod(a(Access.PUBLIC, STATIC), "array_length", type(Block.class), ImmutableList.of(inputBlock));
BytecodeBlock body = method.getBody();
body.append(inputBlock.invoke("getPositionCount", int.class).ret());
Class<?> clazz = defineClass(classDefinition, Object.class, binder.getBindings(), CodegenArrayLengthFunction.class.getClassLoader());
return new methodHandle(clazz, "array_length", Block.class), Optional.of();
}
}
Aggregation Function Implementation¶
Aggregation functions use a similar framework to scalar functions, but are a bit more complex.
AccumulatorState
:All aggregation functions accumulate input rows into a state object; this object must implement
AccumulatorState
. For simple aggregations, just extendAccumulatorState
into a new interface with the getters and setters you want, and the framework will generate all the implementations and serializers for you. If you need a more complex state object, you will need to implementAccumulatorStateFactory
andAccumulatorStateSerializer
and provide these via theAccumulatorStateMetadata
annotation.
The following code implements the aggregation function avg_double
which computes the
average of a DOUBLE
column:
@AggregationFunction("avg_double")
public class AverageAggregation
{
@InputFunction
public static void input(LongAndDoubleState state, @SqlType(StandardTypes.DOUBLE) double value)
{
state.setLong(state.getLong() + 1);
state.setDouble(state.getDouble() + value);
}
@CombineFunction
public static void combine(LongAndDoubleState state, LongAndDoubleState otherState)
{
state.setLong(state.getLong() + otherState.getLong());
state.setDouble(state.getDouble() + otherState.getDouble());
}
@OutputFunction(StandardTypes.DOUBLE)
public static void output(LongAndDoubleState state, BlockBuilder out)
{
long count = state.getLong();
if (count == 0) {
out.appendNull();
}
else {
double value = state.getDouble();
DOUBLE.writeDouble(out, value / count);
}
}
}
The average has two parts: the sum of the DOUBLE
in each row of the column
and the LONG
count of the number of rows seen. LongAndDoubleState
is an interface
which extends AccumulatorState
:
public interface LongAndDoubleState
extends AccumulatorState
{
long getLong();
void setLong(long value);
double getDouble();
void setDouble(double value);
}
As stated above, for simple AccumulatorState
objects, it is sufficient to
just to define the interface with the getters and setters, and the framework
will generate the implementation for you.
An in-depth look at the various annotations relevant to writing an aggregation function follows:
@InputFunction
:The
@InputFunction
annotation declares the function which accepts input rows and stores them in theAccumulatorState
. Similar to scalar functions you must annotate the arguments with@SqlType
. Note that, unlike in the above scalar example whereSlice
is used to holdVARCHAR
, the primitivedouble
type is used for the argument to input. In this example, the input function simply keeps track of the running count of rows (viasetLong()
) and the running sum (viasetDouble()
).@CombineFunction
:The
@CombineFunction
annotation declares the function used to combine two state objects. This function is used to merge all the partial aggregation states. It takes two state objects, and merges the results into the first one (in the above example, just by adding them together).@OutputFunction
:The
@OutputFunction
is the last function called when computing an aggregation. It takes the final state object (the result of merging all partial states) and writes the result to aBlockBuilder
.Where does serialization happen, and what is
GroupedAccumulatorState
?The
@InputFunction
is usually run on a different worker from the@CombineFunction
, so the state objects are serialized and transported between these workers by the aggregation framework.GroupedAccumulatorState
is used when performing aGROUP BY
aggregation, and an implementation will be automatically generated for you, if you don’t specify aAccumulatorStateFactory
Advanced Use Cases¶
Raw Block Inputs¶
Both scalar and aggregation function annotations allow you to define methods
which operate on native types. In Java, these native types are boolean
,
Slice
, and long
. For parameterized implementations or parametric types,
the standard Java types can’t be used as they aren’t able to represent the input
data.
To define a method handle which can accept any types, use @BlockPosition
in conjunction with the @BlockIndex
parameters. Similar to the
@SqlNullable
annotation, use the @NullablePosition
annotation to denote
that the function should be called when the block position is NULL
.
This works for both scalar and aggregation function implementations.
@ScalarFunction("example")
public static Block exampleFunction(
@BlockPosition @NullablePosition @SqlType("array(int)") Block block,
@BlockIndex int index) { /* ...implementation */ }
Applying Generic Types with @BlockPosition
¶
Function signatures that use the @BlockPosition
syntax are able to operate
over generic types when the function is defined with a @TypeParameter
annotation. Augment the @BlockPosition
argument with an additional
@SqlType("T")
annotation to denote that it accepts an argument corresponding
to the generic type. This works for both scalar and aggregation function
implementations.
@ScalarFunction("example")
@TypeParameter("T")
public static Block exampleFunction(
@BlockPosition @SqlType("T") Block block,
@BlockIndex int index) { /* ...implementation */ }
Retrieving the Generic Type with @TypeParameter
¶
Add the @TypeParameter
annotation at the beginning of a function’s argument
list to allow the implementation to perform type-specific logic. Add a
Type
-typed argument annotated with @TypeParameter
as the first argument
of the function signature to get access to the Type
. This works for both
scalar and aggregation functions.
@ScalarFunction("example")
@TypeParameter("T")
public static Block exampleFunction(
@TypeParameter("T") Type type,
@BlockPosition @SqlType("T") Block block,
@BlockIndex int index) { /* ...implementation */ }