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Subject [GitHub] [flink] dawidwys commented on a change in pull request #14378: [FLINK-20522][table] Make implementing a built-in function straightforward
Date Thu, 07 Jan 2021 11:26:43 GMT

dawidwys commented on a change in pull request #14378:
URL: https://github.com/apache/flink/pull/14378#discussion_r553270306



##########
File path: flink-table/flink-table-common/src/main/java/org/apache/flink/table/functions/SpecializedFunction.java
##########
@@ -0,0 +1,73 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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 org.apache.flink.table.functions;
+
+import org.apache.flink.annotation.PublicEvolving;
+import org.apache.flink.configuration.ReadableConfig;
+import org.apache.flink.table.api.TableException;
+import org.apache.flink.table.catalog.DataTypeFactory;
+import org.apache.flink.table.types.inference.CallContext;
+
+/**
+ * A {@link FunctionDefinition} that can provide a runtime implementation (i.e. the function's
body)
+ * that is specialized for the given call and session.
+ *
+ * <p>The planner tries to defer the specialization until shortly before code generation,
where the
+ * information given by a {@link FunctionDefinition} is not enough anymore and a subclass
of
+ * {@link UserDefinedFunction} is required for runtime.
+ *
+ * <p>This interface is useful when the runtime code should know about information
that is only available
+ * after planning (e.g. local session time zone or precision/scale of decimal return type).
+ *
+ * <p>A {@link UserDefinedFunction} that is registered in the API is implicitly specialized
but can
+ * also implement this interface to reconfigure itself before runtime.
+ */
+@PublicEvolving
+public interface SpecializedFunction extends FunctionDefinition {
+
+	/**
+	 * Provides a runtime implementation that is specialized for the given call and session.
+	 *
+	 * <p>The method must return an instance of {@link UserDefinedFunction} or throw a
{@link TableException}
+	 * if the given call is not supported. The returned instance must have the same {@link FunctionDefinition}
+	 * semantics but can have a different {@link #getTypeInference(DataTypeFactory)} implementation.

Review comment:
       Sorry, my example was wrong, in a respect that I did not use a literal and therefore
never ended up with `new NumberScalarFunction(true)`. Because of the example I assumed we
do not have such a check, but obviously we do. Again sorry for the confusion. Nice that we
have that check!
   
   Still it won't harm to have a slightly more prominent warning.




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