org.apache.flink.table.api.functions.ScalarFunction Maven / Gradle / Ivy
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*
* http://www.apache.org/licenses/LICENSE-2.0
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package org.apache.flink.table.api.functions;
import org.apache.flink.table.api.types.DataType;
import org.apache.flink.table.expressions.Expression;
import org.apache.flink.table.expressions.ScalarFunctionCall;
import scala.collection.Seq;
/**
* Base class for a user-defined scalar function. A user-defined scalar functions maps zero, one,
* or multiple scalar values to a new scalar value.
*
* The behavior of a {@link ScalarFunction} can be defined by implementing a custom evaluation
* method. An evaluation method must be declared publicly and named "eval". Evaluation methods
* can also be overloaded by implementing multiple methods named "eval".
*
*
User-defined functions must have a default constructor and must be instantiable during
* runtime.
*
*
By default the result type of an evaluation method is determined by Flink's type extraction
* facilities. This is sufficient for basic types or simple POJOs but might be wrong for more
* complex, custom, or composite types. In these cases {@link DataType} of the result type
* can be manually defined by overriding {@link #getResultType}.
*
*
Internally, the Table/SQL API code generation works with primitive values as much as possible.
* If a user-defined scalar function should not introduce much overhead during runtime, it is
* recommended to declare parameters and result types as primitive types instead of their boxed
* classes. DATE/TIME is equal to int, TIMESTAMP is equal to long.
*/
public abstract class ScalarFunction extends CustomTypeDefinedFunction {
/**
* Creates a call to a {@link ScalarFunction} in Scala Table API.
*
* @param params actual parameters of function
* @return {@link Expression} in form of a {@link ScalarFunctionCall}
*/
public final Expression apply(Expression... params) {
return ScalarFunctionCall.apply(this, params);
}
public final Expression apply(Seq params) {
return new ScalarFunctionCall(this, params);
}
}