[GitHub] [druid] gianm commented on a change in pull request #9111: Add HashJoinSegment, a virtual segment for joins.

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

gianm commented on a change in pull request #9111: Add HashJoinSegment, a virtual segment for joins.
URL: https://github.com/apache/druid/pull/9111#discussion_r365384423
 
 

 ##########
 File path: processing/src/main/java/org/apache/druid/segment/join/Joinable.java
 ##########
 @@ -0,0 +1,74 @@
+/*
+ * 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.join;
+
+import org.apache.druid.segment.ColumnSelectorFactory;
+import org.apache.druid.segment.column.ColumnCapabilities;
+
+import javax.annotation.Nullable;
+import java.util.List;
+
+/**
+ * Represents something that can be the right-hand side of a join.
+ *
+ * This class's most important method is {@link #makeJoinMatcher}. Its main user is
+ * {@link HashJoinEngine#makeJoinCursor}.
+ */
+public interface Joinable
+{
+  int CARDINALITY_UNKNOWN = -1;
+
+  /**
+   * Returns the list of columns that this Joinable offers.
+   */
+  List<String> getAvailableColumns();
+
+  /**
+   * Returns the cardinality of "columnName", or {@link #CARDINALITY_UNKNOWN} if not known. May be used at query
+   * time to trigger optimizations.
+   */
+  int getCardinality(String columnName);
+
+  /**
+   * Returns capabilities for one of this Joinable's columns.
+   *
+   * @param columnName column name
+   *
+   * @return capabilities, or null if the columnName is not one of this Joinable's columns
+   */
+  @Nullable
+  ColumnCapabilities getColumnCapabilities(String columnName);
 
 Review comment:
   I made this one nullable since it mirrors a lot of other methods with identical signature in other interfaces. I didn't want to change them all and I thought it'd be nice for them to echo each other.
   
   I still think changing them all in this PR is a bad idea, but maybe the echoing is _also_ a bad idea, and the right thing would be to make this one an Optional.
   
   What do you think?

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