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A module that is everything required to understands Druid Segments
There is a newer version: 30.0.1
/*
 * 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;

import com.google.common.collect.Iterables;
import org.apache.druid.guice.annotations.PublicApi;
import org.apache.druid.segment.column.ColumnCapabilities;
import org.apache.druid.segment.column.RowSignature;
import org.apache.druid.segment.data.Indexed;
import org.joda.time.DateTime;
import org.joda.time.Interval;

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

/**
 */
@PublicApi
public interface StorageAdapter extends CursorFactory, ColumnInspector
{
  Interval getInterval();
  Indexed getAvailableDimensions();
  Iterable getAvailableMetrics();

  /**
   * Returns the row signature of the data available from this adapter. For mutable adapters, even though the signature
   * may evolve over time, any particular object returned by this method is an immutable snapshot.
   */
  default RowSignature getRowSignature()
  {
    final RowSignature.Builder builder = RowSignature.builder();
    builder.addTimeColumn();

    for (final String column : Iterables.concat(getAvailableDimensions(), getAvailableMetrics())) {
      builder.add(
          column,
          Optional.ofNullable(getColumnCapabilities(column)).map(ColumnCapabilities::toColumnType).orElse(null)
      );
    }

    return builder.build();
  }

  /**
   * Returns the number of distinct values for a column, or {@link DimensionDictionarySelector#CARDINALITY_UNKNOWN}
   * if unknown.
   *
   * If the column doesn't exist, returns 1, because a column that doesn't exist is treated as a column of default
   * (or null) values.
   */
  int getDimensionCardinality(String column);

  /**
   * Metadata-only operation that returns a lower bound on
   * {@link org.apache.druid.segment.column.ColumnHolder#TIME_COLUMN_NAME} values for this adapter. May be earlier than
   * the actual minimum data timestamp.
   *
   * For {@link QueryableIndexStorageAdapter} and {@link org.apache.druid.segment.incremental.IncrementalIndexStorageAdapter}
   * specifically, which back regular tables (i.e. {@link org.apache.druid.query.TableDataSource}), this method
   * contract is tighter: it does return the actual minimum data timestamp. This fact is leveraged by
   * {@link org.apache.druid.query.timeboundary.TimeBoundaryQuery} to return results using metadata only.
   */
  DateTime getMinTime();

  /**
   * Metadata-only operation that returns an upper bound on
   * {@link org.apache.druid.segment.column.ColumnHolder#TIME_COLUMN_NAME} values for this adapter. May be later than
   * the actual maximum data timestamp.
   *
   * For {@link QueryableIndexStorageAdapter} and {@link org.apache.druid.segment.incremental.IncrementalIndexStorageAdapter}
   * specifically, which back regular tables (i.e. {@link org.apache.druid.query.TableDataSource}), this method
   * contract is tighter: it does return the actual maximum data timestamp. This fact is leveraged by
   * {@link org.apache.druid.query.timeboundary.TimeBoundaryQuery} to return results using metadata only.
   */
  DateTime getMaxTime();

  /**
   * Returns the minimum value of the provided column, if known through an index, dictionary, or cache. Returns null
   * if not known. Does not scan the column to find the minimum value.
   */
  @Nullable
  Comparable getMinValue(String column);

  /**
   * Returns the minimum value of the provided column, if known through an index, dictionary, or cache. Returns null
   * if not known. Does not scan the column to find the maximum value.
   */
  @Nullable
  Comparable getMaxValue(String column);

  /**
   * Returns capabilities of a particular column, if known. May be null if the column doesn't exist, or if
   * the column does exist but the capabilities are unknown. The latter is possible with dynamically discovered
   * columns.
   *
   * Note that StorageAdapters are representations of "real" segments, so they are not aware of any virtual columns
   * that may be involved in a query. In general, query engines should instead use the method
   * {@link ColumnSelectorFactory#getColumnCapabilities(String)}, which returns capabilities for virtual columns as
   * well.
   *
   * @param column column name
   *
   * @return capabilities, or null
   */
  @Override
  @Nullable
  ColumnCapabilities getColumnCapabilities(String column);

  int getNumRows();
  DateTime getMaxIngestedEventTime();

  @Nullable
  Metadata getMetadata();

  /**
   * Returns true if this storage adapter can filter some rows out. The actual column cardinality can be lower than
   * what {@link #getDimensionCardinality} returns if this returns true. Dimension selectors for such storage adapter
   * can return non-contiguous dictionary IDs because the dictionary IDs in filtered rows will not be returned.
   * Note that the number of rows accessible via this storage adapter will not necessarily decrease because of
   * the built-in filters. For inner joins, for example, the number of joined rows can be larger than
   * the number of rows in the base adapter even though this method returns true.
   */
  default boolean hasBuiltInFilters()
  {
    return false;
  }

  /**
   * @return true if this index was created from a tombstone or false otherwise
   */
  default boolean isFromTombstone()
  {
    return false;
  }
}