/*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.facebook.presto.spi.function;
import com.facebook.presto.spi.block.Block;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;
import static java.lang.annotation.ElementType.METHOD;
import static java.lang.annotation.RetentionPolicy.RUNTIME;
/**
* A function annotated with @CombineFunction is one that will be used
* to combine two states into one in aggregation functions.
* <p>
* The function should take two arguments. And it should merge both arguments
* into the first one.
* <p>
* The second argument is guaranteed to be the output of
* {@link AccumulatorStateSerializer#deserialize(Block, int, Object)}.
* As a result, the implementation of {@code deserialize} method likely
* provides some application-specific guarantees to the second argument.
*/
@Retention(RUNTIME)
@Target(METHOD)
public @interface CombineFunction
{
}