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A module that is everything required to understands Druid Segments
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/*
 * 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.java.util.common.io.Closer;
import org.apache.druid.query.filter.InDimFilter;
import org.apache.druid.segment.ColumnSelectorFactory;
import org.apache.druid.segment.ReferenceCountedObject;
import org.apache.druid.segment.column.ColumnCapabilities;

import javax.annotation.Nullable;
import java.util.List;
import java.util.Optional;
import java.util.Set;

/**
 * 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 extends ReferenceCountedObject
{
  int CARDINALITY_UNKNOWN = -1;

  /**
   * Returns the list of columns that this Joinable offers.
   */
  List getAvailableColumns();

  /**
   * Returns the cardinality of "columnName", or {@link #CARDINALITY_UNKNOWN} if not known. May be used at query
   * time to trigger optimizations.
   *
   * If not {@link #CARDINALITY_UNKNOWN}, this must match the cardinality of selectors returned by the
   * {@link ColumnSelectorFactory#makeDimensionSelector} method of this joinable's
   * {@link JoinMatcher#getColumnSelectorFactory()} .
   */
  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);

  /**
   * Creates a JoinMatcher that can be used to implement a join onto this Joinable.
   *
   * @param leftColumnSelectorFactory column selector factory that allows access to the left-hand side of the join
   * @param condition                 join condition for the matcher
   * @param remainderNeeded           whether or not {@link JoinMatcher#matchRemainder()} will ever be called on the
   *                                  matcher. If we know it will not, additional optimizations are often possible.
   * @param descending                true if join cursor is iterated in descending order
   * @param closer                    closer that will run after join cursor has completed to clean up any per query
   *                                  resources the joinable uses
   *
   * @return the matcher
   */
  JoinMatcher makeJoinMatcher(
      ColumnSelectorFactory leftColumnSelectorFactory,
      JoinConditionAnalysis condition,
      boolean remainderNeeded,
      boolean descending,
      Closer closer
  );

  /**
   * Returns all matchable values from a particular column along with a flag to tell if they are all unique in the column.
   * If the matchable values are greater than "maxNumValues" or if the column doesn't exists or doesn't supports this
   * operation, returns an object with empty set for column values and false for uniqueness flag.
   * The uniqueness flag will only be true if we've collected all matchable values in the column and found that they're
   * all unique. In all other cases it will be false.
   *
   * The returned set may be passed to {@link InDimFilter}. For efficiency,
   * implementations should prefer creating the returned set with
   * {@code new TreeSet(Comparators.naturalNullsFirst()}}. This avoids a copy in the filter's constructor.
   *
   * @param columnName   name of the column
   * @param includeNull  whether null should be considered a matchable value. If true, this method returns all values
   *                     that are present in the column. If false, this method returns all non-null values.
   * @param maxNumValues maximum number of values to return. If exceeded, returns an empty set with the "allUnique"
   *                     flag set to false.
   */
  ColumnValuesWithUniqueFlag getMatchableColumnValues(String columnName, boolean includeNull, int maxNumValues);

  /**
   * Searches a column from this Joinable for a particular value, finds rows that match,
   * and returns values of a second column for those rows.
   *
   * The returned set may be passed to {@link InDimFilter}. For efficiency,
   * implementations should prefer creating the returned set with
   * {@code new TreeSet(Comparators.naturalNullsFirst()}}. This avoids a copy in the filter's constructor.
   *
   * @param searchColumnName        Name of the search column. This is the column that is being used in the filter
   * @param searchColumnValue       Target value of the search column. This is the value that is being filtered on.
   * @param retrievalColumnName     The column to retrieve values from. This is the column that is being joined against.
   * @param maxCorrelationSetSize   Maximum number of values to retrieve. If we detect that more values would be
   *                                returned than this limit, return absent.
   * @param allowNonKeyColumnSearch If true, allow searchs on non-key columns. If this is false,
   *                                a search on a non-key column returns absent.
   *
   * @return The set of correlated column values. If we cannot determine correlated values, return absent.
   *
   * In case either the search or retrieval column names are not found, this will return absent.
   */
  Optional getCorrelatedColumnValues(
      String searchColumnName,
      String searchColumnValue,
      String retrievalColumnName,
      long maxCorrelationSetSize,
      boolean allowNonKeyColumnSearch
  );


  class ColumnValuesWithUniqueFlag
  {
    final Set columnValues;
    final boolean allUnique;

    public ColumnValuesWithUniqueFlag(Set columnValues, boolean allUnique)
    {
      this.columnValues = columnValues;
      this.allUnique = allUnique;
    }

    public Set getColumnValues()
    {
      return columnValues;
    }

    public boolean isAllUnique()
    {
      return allUnique;
    }
  }
}