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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 org.apache.druid.guice.annotations.PublicApi;
import org.apache.druid.query.dimension.DimensionSpec;
import org.apache.druid.segment.column.ColumnCapabilities;

import javax.annotation.Nullable;

/**
 * Factory class for MetricSelectors
 *
 * @see org.apache.druid.segment.vector.VectorColumnSelectorFactory, the vectorized version
 */
@PublicApi
public interface ColumnSelectorFactory extends ColumnInspector
{
  DimensionSelector makeDimensionSelector(DimensionSpec dimensionSpec);

  /**
   * Returns ColumnValueSelector corresponding to the given column name, or {@link NilColumnValueSelector} if the
   * column with such name is absent.
   */
  ColumnValueSelector makeColumnValueSelector(String columnName);

  /**
   * 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.
   *
   * @param column column name
   *
   * @return capabilities, or null
   */
  @Override
  @Nullable
  ColumnCapabilities getColumnCapabilities(String column);

  /**
   * Returns a {@link RowIdSupplier} that allows callers to detect whether the selectors returned by this
   * factory have moved or not. Useful for selectors that wrap other selectors, such as virtual columns,
   * as it allows them to cache their outputs.
   */
  @Nullable
  default RowIdSupplier getRowIdSupplier()
  {
    return null;
  }
}