[GitHub] leventov commented on a change in pull request #4551: Fixes and improvements to SQL metadata caching.

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

leventov commented on a change in pull request #4551: Fixes and improvements to SQL metadata caching.
URL: https://github.com/apache/incubator-druid/pull/4551#discussion_r246337020
 
 

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 File path: sql/src/main/java/io/druid/sql/calcite/schema/DruidSchema.java
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 @@ -59,42 +58,66 @@
 import io.druid.sql.calcite.view.DruidViewMacro;
 import io.druid.sql.calcite.view.ViewManager;
 import io.druid.timeline.DataSegment;
-import org.apache.calcite.schema.Function;
 import org.apache.calcite.schema.Table;
 import org.apache.calcite.schema.impl.AbstractSchema;
 import org.joda.time.DateTime;
 
+import java.io.IOException;
+import java.util.Comparator;
 import java.util.EnumSet;
-import java.util.List;
+import java.util.HashMap;
+import java.util.HashSet;
 import java.util.Map;
 import java.util.Set;
+import java.util.TreeMap;
+import java.util.TreeSet;
 import java.util.concurrent.ConcurrentMap;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutorService;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import java.util.stream.StreamSupport;
 
 @ManageLifecycle
 public class DruidSchema extends AbstractSchema
 {
+  // Newest segments first, so they override older ones.
 
 Review comment:
   I fail to see where exactly this order is used and why inner maps in `segmentSignatures` need to be TreeMaps, @gianm could you please explain?

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