processing/src/main/java/org/apache/druid/segment/virtual/SingleStringInputDeferredEvaluationExpressionDimensionVectorSelector.java

/*
 * 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.virtual;

import org.apache.druid.java.util.common.ISE;
import org.apache.druid.math.expr.Expr;
import org.apache.druid.math.expr.ExprType;
import org.apache.druid.math.expr.vector.ExprVectorProcessor;
import org.apache.druid.segment.DimensionDictionarySelector;
import org.apache.druid.segment.IdLookup;
import org.apache.druid.segment.vector.SingleValueDimensionVectorSelector;

import javax.annotation.Nullable;

/**
 * A {@link SingleValueDimensionVectorSelector} decorator that directly exposes the underlying dictionary ids in
 * {@link #getRowVector}, saving expression computation until {@link #lookupName} is called. This allows for
 * performing operations like grouping on the native dictionary ids, and deferring expression evaluation until
 * after, which can dramatically reduce the total number of evaluations.
 *
 * @see ExpressionVectorSelectors for details on how expression vector selectors are constructed.
 *
 * @see SingleStringInputDeferredEvaluationExpressionDimensionSelector for the non-vectorized version of this selector.
 */
public class SingleStringInputDeferredEvaluationExpressionDimensionVectorSelector
    implements SingleValueDimensionVectorSelector
{
  private final SingleValueDimensionVectorSelector selector;
  private final ExprVectorProcessor<String[]> stringProcessor;
  private final StringLookupVectorInputBindings inputBinding;

  public SingleStringInputDeferredEvaluationExpressionDimensionVectorSelector(
      SingleValueDimensionVectorSelector selector,
      Expr expression
  )
  {
    // Verify selector has a working dictionary.
    if (selector.getValueCardinality() == DimensionDictionarySelector.CARDINALITY_UNKNOWN
        || !selector.nameLookupPossibleInAdvance()) {
      throw new ISE(
          "Selector of class[%s] does not have a dictionary, cannot use it.",
          selector.getClass().getName()
      );
    }
    this.selector = selector;
    this.inputBinding = new StringLookupVectorInputBindings();
    this.stringProcessor = expression.buildVectorized(inputBinding);
  }

  @Override
  public int getValueCardinality()
  {
    return CARDINALITY_UNKNOWN;
  }

  @Nullable
  @Override
  public String lookupName(int id)
  {
    inputBinding.currentValue[0] = selector.lookupName(id);
    return stringProcessor.evalVector(inputBinding).values()[0];
  }

  @Override
  public boolean nameLookupPossibleInAdvance()
  {
    return true;
  }

  @Nullable
  @Override
  public IdLookup idLookup()
  {
    return null;
  }

  @Override
  public int[] getRowVector()
  {
    return selector.getRowVector();
  }

  @Override
  public int getMaxVectorSize()
  {
    return selector.getMaxVectorSize();
  }

  @Override
  public int getCurrentVectorSize()
  {
    return selector.getCurrentVectorSize();
  }

  /**
   * Special single element vector input bindings used for processing the string value for {@link #lookupName(int)}
   *
   * Vector size is fixed to 1 because {@link #lookupName} operates on a single dictionary value at a time. If a
   * bulk lookup method is ever added, these vector bindings should be modified to process the results with actual
   * vectors.
   */
  private static final class StringLookupVectorInputBindings implements Expr.VectorInputBinding
  {
    private final String[] currentValue = new String[1];

    @Nullable
    @Override
    public ExprType getType(String name)
    {
      return ExprType.STRING;
    }

    @Override
    public int getMaxVectorSize()
    {
      return 1;
    }

    @Override
    public int getCurrentVectorSize()
    {
      return 1;
    }

    @Override
    public int getCurrentVectorId()
    {
      return -1;
    }

    @Override
    public <T> T[] getObjectVector(String name)
    {
      return (T[]) currentValue;
    }

    @Override
    public long[] getLongVector(String name)
    {
      throw new UnsupportedOperationException("attempt to get long[] from string[] only scalar binding");
    }

    @Override
    public double[] getDoubleVector(String name)
    {
      throw new UnsupportedOperationException("attempt to get double[] from string[] only scalar binding");
    }

    @Nullable
    @Override
    public boolean[] getNullVector(String name)
    {
      throw new UnsupportedOperationException("attempt to get boolean[] null vector from string[] only scalar binding");
    }
  }
}