Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/17902#discussion_r141080917
--- Diff:
common/kvstore/src/main/java/org/apache/spark/kvstore/LevelDBTypeInfo.java ---
@@ -0,0 +1,516 @@
+/*
+ * 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.spark.kvstore;
+
+import java.lang.reflect.Array;
+import java.lang.reflect.Field;
+import java.lang.reflect.Method;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.util.Collection;
+import java.util.HashMap;
+import java.util.Map;
+import static java.nio.charset.StandardCharsets.UTF_8;
+
+import com.google.common.base.Preconditions;
+import com.google.common.base.Throwables;
+import org.iq80.leveldb.WriteBatch;
+
+/**
+ * Holds metadata about app-specific types stored in LevelDB. Serves as a
cache for data collected
+ * via reflection, to make it cheaper to access it multiple times.
+ *
+ * <p>
+ * The hierarchy of keys stored in LevelDB looks roughly like the
following. This hierarchy ensures
+ * that iteration over indices is easy, and that updating values in the
store is not overly
+ * expensive. Of note, indices choose using more disk space (one value per
key) instead of keeping
+ * lists of pointers, which would be more expensive to update at runtime.
+ * </p>
+ *
+ * <p>
+ * Indentation defines when a sub-key lives under a parent key. In
LevelDB, this means the full
+ * key would be the concatenation of everything up to that point in the
hierarchy, with each
+ * component separated by a NULL byte.
+ * </p>
+ *
+ * <pre>
+ * +TYPE_NAME
+ * NATURAL_INDEX
+ * +NATURAL_KEY
+ * -
+ * -NATURAL_INDEX
+ * INDEX_NAME
+ * +INDEX_VALUE
+ * +NATURAL_KEY
--- End diff --
is this the so-called pointer? For secondary index the value is natural
key, which can be used to get the actual value/app-data.
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