equi_height_bucket.h

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#ifndef HISTOGRAMS_EQUI_HEIGHT_BUCKET_INCLUDED
#define HISTOGRAMS_EQUI_HEIGHT_BUCKET_INCLUDED
 
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/**
  @file sql/histograms/equi_height_bucket.h
  Equi-height bucket.
 
  This file defines the class representing an equi-height bucket. A bucket holds
  four different values:
    - Lower inclusive value.
    - Upper inclusive value.
    - The cumulative frequency (between 0.0 and 1.0).
    - Number of distinct values in this bucket.
*/
 
#include "my_base.h"  // ha_rows
#include "my_inttypes.h"
#include "mysql_time.h"
#include "sql/my_decimal.h"
#include "sql_string.h"
 
class Json_array;
 
namespace histograms {
namespace equi_height {
 
/**
  Equi-height bucket.
*/
template <class T>
class Bucket {
 private:
  /// Lower inclusive value contained in this bucket.
  const T m_lower_inclusive;
 
  /// Upper inclusive value contained in this bucket.
  const T m_upper_inclusive;
 
  /// The cumulative frequency. 0.0 <= m_cumulative_frequency <= 1.0.
  const double m_cumulative_frequency;
 
  /// Number of distinct values in this bucket.
  const ha_rows m_num_distinct;
 
  /**
    Add values to a JSON bucket.
 
    This function adds the lower and upper inclusive value to the supplied
    JSON array. The lower value is added first.
 
    @param      lower_value The lower inclusive value to add.
    @param      upper_value The upper inclusive value to add.
    @param[out] json_array  A JSON array where the bucket data is stored.
 
    @return     True on error, false otherwise.
  */
  static bool add_values_json_bucket(const T &lower_value, const T &upper_value,
                                     Json_array *json_array);
 
 public:
  /**
    Equi-height bucket constructor.
 
    Does nothing more than setting the member variables.
 
    @param lower         Lower inclusive value.
    @param upper         Upper inclusive value.
    @param freq          The cumulative frequency.
    @param num_distinct  Number of distinct values in this bucket.
  */
  Bucket(T lower, T upper, double freq, ha_rows num_distinct);
 
  /**
    @return Lower inclusive value.
  */
  const T &get_lower_inclusive() const { return m_lower_inclusive; }
 
  /**
    @return Upper inclusive value.
  */
  const T &get_upper_inclusive() const { return m_upper_inclusive; }
 
  /**
    @return Cumulative frequency.
  */
  double get_cumulative_frequency() const { return m_cumulative_frequency; }
 
  /**
    @return Number of distinct values.
  */
  ha_rows get_num_distinct() const { return m_num_distinct; }
 
  /**
    Convert this equi-height bucket to a JSON array.
 
    This function will take the contents of the current equi-height bucket
    and put it in the output parameter "json_array". The result is an array
    with the following contents:
      Index 0: Lower inclusive value.
      Index 1: Upper inclusive value.
      Index 2: Cumulative frequency.
      Index 3: Number of distinct values.
 
    @param[out] json_array Output where the bucket content is to be stored.
 
    @return     True on error, false otherwise.
  */
  bool bucket_to_json(Json_array *json_array) const;
 
  /**
    Finds the relative location of "value" between bucket endpoints.
    This is used to determine the fraction of a bucket to include in selectivity
    estimates in the case where the query value lies inside a bucket.
 
    get_distance_from_lower returns the fraction of all elements between bucket
    endpoints [a, b] that lie in the interval [a, value), i.e., strictly less
    than value. For some histogram types the return value is only an estimate.
 
    @param value The value to calculate the distance for.
 
    @return The distance between "value" and lower inclusive value.
  */
  double get_distance_from_lower(const T &value) const;
 
  /**
    Returns the fraction of all elements between bucket endpoints [a, b] that
    are strictly greater than "value". For some histogram types the return value
    is only an estimate.
 
    @return The distance between "value" and upper inclusive value.
  */
  double get_distance_from_upper(const T &value) const;
};
 
}  // namespace equi_height
}  // namespace histograms
 
#endif