[GitHub] [druid] cheddar commented on a change in pull request #11853: complex typed expressions

[GitBox <gi...@apache.org>]

cheddar commented on a change in pull request #11853:
URL: https://github.com/apache/druid/pull/11853#discussion_r745243478



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File path: core/src/main/java/org/apache/druid/segment/column/ObjectByteStrategy.java
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@@ -0,0 +1,58 @@
+/*
+ * 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.druid.segment.column;
+
+import javax.annotation.Nullable;
+import java.nio.ByteBuffer;
+import java.util.Comparator;
+
+/**
+ * Naming is hard. This is the core interface extracted from another interface called ObjectStrategy that lives in
+ * 'druid-processing'. It provides basic methods for handling converting some type of object to a binary form, reading
+ * the binary form back into an object from a {@link ByteBuffer}, and mechanism to perform comparisons between objects.
+ *
+ * Complex types register one of these in {@link Types#registerStrategy}, which can be retrieved by the complex
+ * type name to convert values to and from binary format, and compare them.
+ *
+ * This could be recombined with 'ObjectStrategy' should these two modules be combined.
+ */
+public interface ObjectByteStrategy<T> extends Comparator<T>
+{
+  Class<? extends T> getClazz();
+
+  /**
+   * Convert values from their underlying byte representation.
+   *
+   * Implementations of this method <i>may</i> change the given buffer's mark, or limit, and position.
+   *
+   * Implementations of this method <i>may not</i> store the given buffer in a field of the "deserialized" object,
+   * need to use {@link ByteBuffer#slice()}, {@link ByteBuffer#asReadOnlyBuffer()} or {@link ByteBuffer#duplicate()} in
+   * this case.
+   *
+   * @param buffer buffer to read value from
+   * @param numBytes number of bytes used to store the value, starting at buffer.position()
+   * @return an object created from the given byte buffer representation
+   */
+  @Nullable
+  T fromByteBuffer(ByteBuffer buffer, int numBytes);
+
+  @Nullable
+  byte[] toBytes(@Nullable T val);

Review comment:
       If you do that, please name it `ObjectRowStrategy` to indicate that it's primary usage should be when building row-oriented data sets.  One of the fundamental problems with the `ObjectStrategy` that exists here is that the interface forces you to serialize the object in a row-oriented format.  This is bad when serializing columns as it doesn't allow you to take advantage of commonalities between values in the same column to achieve smaller sizing and that is fundamentally why the `ObjectStrategy` method of serializing and deserializing is deprecated.  This interface definitely does make sense for result sets where the data is primarily row-oriented, but we should do everything possible to push people away from using it for column persistence.  




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