[GitHub] [calcite] jacques-n commented on a change in pull request #2603: [CALCITE-4882] Introduce new Lambda-based Metadata framework

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jacques-n commented on a change in pull request #2603:
URL: https://github.com/apache/calcite/pull/2603#discussion_r747023794



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File path: core/src/main/java/org/apache/calcite/rel/metadata/lambda/ReflectionToLambdaProvider.java
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@@ -0,0 +1,359 @@
+/*
+ * 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.calcite.rel.metadata.lambda;
+
+import org.apache.calcite.rel.RelNode;
+import org.apache.calcite.rel.metadata.RelMdAllPredicates;
+import org.apache.calcite.rel.metadata.RelMdCollation;
+import org.apache.calcite.rel.metadata.RelMdColumnOrigins;
+import org.apache.calcite.rel.metadata.RelMdColumnUniqueness;
+import org.apache.calcite.rel.metadata.RelMdDistinctRowCount;
+import org.apache.calcite.rel.metadata.RelMdDistribution;
+import org.apache.calcite.rel.metadata.RelMdExplainVisibility;
+import org.apache.calcite.rel.metadata.RelMdExpressionLineage;
+import org.apache.calcite.rel.metadata.RelMdLowerBoundCost;
+import org.apache.calcite.rel.metadata.RelMdMaxRowCount;
+import org.apache.calcite.rel.metadata.RelMdMemory;
+import org.apache.calcite.rel.metadata.RelMdMinRowCount;
+import org.apache.calcite.rel.metadata.RelMdNodeTypes;
+import org.apache.calcite.rel.metadata.RelMdParallelism;
+import org.apache.calcite.rel.metadata.RelMdPercentageOriginalRows;
+import org.apache.calcite.rel.metadata.RelMdPopulationSize;
+import org.apache.calcite.rel.metadata.RelMdPredicates;
+import org.apache.calcite.rel.metadata.RelMdRowCount;
+import org.apache.calcite.rel.metadata.RelMdSelectivity;
+import org.apache.calcite.rel.metadata.RelMdSize;
+import org.apache.calcite.rel.metadata.RelMdTableReferences;
+import org.apache.calcite.rel.metadata.RelMdUniqueKeys;
+import org.apache.calcite.rel.metadata.RelMetadataQuery;
+import org.apache.calcite.util.BuiltInMethod;
+
+import com.google.common.cache.CacheBuilder;
+import com.google.common.cache.CacheLoader;
+import com.google.common.cache.LoadingCache;
+import com.google.common.collect.HashBasedTable;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.Table;
+import com.google.common.primitives.Primitives;
+
+import java.lang.invoke.CallSite;
+import java.lang.invoke.LambdaMetafactory;
+import java.lang.invoke.MethodHandle;
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.MethodType;
+import java.lang.reflect.Constructor;
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Locale;
+import java.util.Map;
+import java.util.concurrent.ExecutionException;
+import java.util.stream.Collectors;
+import javax.annotation.concurrent.ThreadSafe;
+
+/**
+ * Create a set of lambda handlers via reflection patterns.
+ *
+ * This does reflection to lambda conversion based on a set of singleton objects
+ * with appropriate signatures, similar to how ReflectiveRelMetadataProvider works.
+ * Any class that can be discovered using ReflectiveRelMetadataProvider should also be consumable
+ * using this class. Ultimately, the goal may be to move to direct lambda registration as opposed to
+ * the old system of partial reflection discovery.
+ */
+@ThreadSafe
+public class ReflectionToLambdaProvider implements LambdaProvider {
+
+  public static final ImmutableList<Source> DEFAULT_SOURCES = ImmutableList.<Source>builder()

Review comment:
       This is effectively a reimagining of `MetadataHandler` as a functional interface with a more useful hierarchy/generic definition. Could we convert MetdataHandler to provide this functionality: yes, we probably could. 
   
   I haven't spent time trying to untangle the way we would get there. I would expect that it would take many releases to try minimize disruption while slowly converting all the interfaces/impls/etc.
   
   
   




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