histogram.h

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#ifndef HISTOGRAMS_HISTOGRAM_INCLUDED
#define HISTOGRAMS_HISTOGRAM_INCLUDED
 
/* Copyright (c) 2016, 2024, Oracle and/or its affiliates.
 
   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License, version 2.0,
   as published by the Free Software Foundation.
 
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   This program is distributed in the hope that it will be useful,
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   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License, version 2.0, for more details.
 
   You should have received a copy of the GNU General Public License
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   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA */
 
/**
  @file sql/histograms/histogram.h
  Histogram base class.
 
  This file defines the base class for all histogram types. We keep the base
  class itself non-templatized in order to more easily send a histogram as an
  argument, collect multiple histograms in a single collection etc.
 
  A histogram is stored as a JSON object. This gives the flexibility of storing
  virtually an unlimited number of buckets, data values in its full length and
  easily expanding with new histogram types in the future. They are stored
  persistently in the system table mysql.column_stats.
 
  We keep all histogram code in the namespace "histograms" in order to avoid
  name conflicts etc.
*/
 
#include <cstddef>  // size_t
#include <functional>
#include <map>  // std::map
#include <memory>
#include <set>      // std::set
#include <string>   // std::string
#include <utility>  // std::pair
 
#include "lex_string.h"  // LEX_CSTRING
#include "my_base.h"     // ha_rows
#include "sql/field.h"   // Field
#include "sql/histograms/value_map_type.h"
#include "sql/mem_root_allocator.h"   // Mem_root_allocator
#include "sql/stateless_allocator.h"  // Stateless_allocator
 
class Item;
class Json_dom;
class Json_object;
class THD;
struct TYPELIB;
class Field;
 
namespace dd {
class Table;
}  // namespace dd
namespace histograms {
struct Histogram_comparator;
template <class T>
class Value_map;
}  // namespace histograms
struct CHARSET_INFO;
struct MEM_ROOT;
class Table_ref;
class Json_dom;
 
namespace histograms {
 
/// The default (and invalid) value for "m_null_values_fraction".
static const double INVALID_NULL_VALUES_FRACTION = -1.0;
 
enum class Message {
  FIELD_NOT_FOUND,
  UNSUPPORTED_DATA_TYPE,
  TEMPORARY_TABLE,
  ENCRYPTED_TABLE,
  VIEW,
  HISTOGRAM_CREATED,
  MULTIPLE_TABLES_SPECIFIED,
  COVERED_BY_SINGLE_PART_UNIQUE_INDEX,
  NO_HISTOGRAM_FOUND,
  HISTOGRAM_DELETED,
  SERVER_READ_ONLY,
  MULTIPLE_COLUMNS_SPECIFIED,
  SYSTEM_SCHEMA_NOT_SUPPORTED,
 
  // JSON validation errors. See Error_context.
  JSON_FORMAT_ERROR,
  JSON_NOT_AN_OBJECT,
  JSON_MISSING_ATTRIBUTE,
  JSON_WRONG_ATTRIBUTE_TYPE,
  JSON_WRONG_BUCKET_TYPE_2,
  JSON_WRONG_BUCKET_TYPE_4,
  JSON_WRONG_DATA_TYPE,
  JSON_UNSUPPORTED_DATA_TYPE,
  JSON_UNSUPPORTED_HISTOGRAM_TYPE,
  JSON_UNSUPPORTED_CHARSET,
  JSON_INVALID_SAMPLING_RATE,
  JSON_INVALID_NUM_BUCKETS_SPECIFIED,
  JSON_INVALID_FREQUENCY,
  JSON_INVALID_NUM_DISTINCT,
  JSON_VALUE_FORMAT_ERROR,
  JSON_VALUE_OUT_OF_RANGE,
  JSON_VALUE_NOT_ASCENDING_1,
  JSON_VALUE_NOT_ASCENDING_2,
  JSON_VALUE_DESCENDING_IN_BUCKET,
  JSON_CUMULATIVE_FREQUENCY_NOT_ASCENDING,
  JSON_INVALID_NULL_VALUES_FRACTION,
  JSON_INVALID_TOTAL_FREQUENCY,
  JSON_NUM_BUCKETS_MORE_THAN_SPECIFIED,
  JSON_IMPOSSIBLE_EMPTY_EQUI_HEIGHT,
};
 
struct Histogram_psi_key_alloc {
  void *operator()(size_t s) const;
};
 
template <class T>
using Histogram_key_allocator = Stateless_allocator<T, Histogram_psi_key_alloc>;
 
template <class T>
using value_map_allocator = Mem_root_allocator<std::pair<const T, ha_rows>>;
 
template <typename T>
using value_map_type =
    std::map<T, ha_rows, Histogram_comparator, value_map_allocator<T>>;
 
using columns_set = std::set<std::string, std::less<std::string>,
                             Histogram_key_allocator<std::string>>;
 
// Used as an array, so duplicate values are not checked.
// TODO((tlchrist): Convert this std::map to an array.
using results_map =
    std::map<std::string, Message, std::less<std::string>,
             Histogram_key_allocator<std::pair<const std::string, Message>>>;
 
/**
  The different operators we can ask histogram statistics for selectivity
  estimations.
*/
enum class enum_operator {
  EQUALS_TO,
  GREATER_THAN,
  LESS_THAN,
  IS_NULL,
  IS_NOT_NULL,
  LESS_THAN_OR_EQUAL,
  GREATER_THAN_OR_EQUAL,
  NOT_EQUALS_TO,
  BETWEEN,
  NOT_BETWEEN,
  IN_LIST,
  NOT_IN_LIST
};
 
/**
  Error context to validate given JSON object which represents a histogram.
 
  A validation error consists of two pieces of information:
 
    1) error code  - what kind of error it is
    2) JSON path   - where the error occurs
 
  Errors are classified into a few conceptual categories, namely
 
    1) absence of required attributes
    2) unexpected JSON type of attributes
    3) value encoding corruption
    4) value out of domain
    5) breaking bucket sequence semantics
    6) breaking certain constraint between pieces of information
 
  @see histograms::Message for the list of JSON validation errors.
 
  Use of the Error_context class
  ------------------------------
 
  An Error_context object is passed along with other parameters to the
  json_to_histogram() function that is used to create a histogram object (e.g.
  Equi_height<longlong>) from a JSON string.
 
  The json_to_histogram() function has two different use cases, with different
  requirements for validation:
 
  1) Deserializing a histogram that was retrieved from the dictionary. In this
     case the histogram has already been validated, and the user is not
     expecting validation feedback, so we pass along a default-constructed
     "empty shell" Error_context object with no-op operations.
 
  2) When validating the user-supplied JSON string to the UPDATE HISTOGRAM ...
     USING DATA commmand. In this case we pass along an active Error_context
     object that uses a Field object to validate bucket values, and stores
     results in a results_map.
 
  The binary() method is used to distinguish between these two contexts/cases.
*/
class Error_context {
 public:
  /// Default constructor. Used when deserializing binary JSON that has already
  /// been validated, e.g. when retrieving a histogram from the dictionary, and
  /// the Error_context object is not actively used for validation.
  Error_context()
      : m_thd{nullptr}, m_field{nullptr}, m_results{nullptr}, m_binary{true} {}
 
  /**
    Constructor. Used in the context of deserializing the user-supplied JSON
    string to the UPDATE HISTOGRAM ... USING DATA command.
 
    @param thd      Thread context
    @param field    The field for values on which the histogram is built
    @param results  Where reported errors are stored
    */
  Error_context(THD *thd, Field *field, results_map *results)
      : m_thd(thd), m_field(field), m_results(results), m_binary(false) {}
 
  /**
    Report a global error to this context.
 
    @param err_code  The global error code
  */
  void report_global(Message err_code);
 
  /**
    Report to this context that a required attribute is missing.
 
    @param name  Name of the missing attribute
   */
  void report_missing_attribute(const std::string &name);
 
  /**
    Report to this context that an error occurs on the given dom node.
 
    @param dom       The given dom node
    @param err_code  The error code
   */
  void report_node(const Json_dom *dom, Message err_code);
 
  /**
    Check if the value is in the field definition domain.
 
    @param v Pointer to the value.
 
    @return true on error, false otherwise
 
    @note Uses Field::store() on the field for which the user-defined histogram
    is to be constructed in order to check the validity of the supplied value.
    This will have the side effect of writing to the record buffer so this
    should only be used with an active Error_context (with a non-nullptr field)
    when we do not otherwise expect to use the record buffer. Currently the only
    use case is to validate the JSON input to the command UPDATE HISTOGRAM ...
    USING DATA where it should be OK to use the field for this purpose.
   */
  template <typename T>
  bool check_value(T *v);
 
  /**
    Tell whether the input json is an internal persisted copy or
    a user-defined input. If the input is an internal copy, there
    should never be type/format errors. If it is a user-defined input,
    errors may occur and should be handled, and some type casting may
    be needed.
 
    @return true for JSON, false otherwise
   */
  bool binary() const { return m_binary; }
 
  /**
    Return data-type of field in context if present. Used to enforce
    that histogram datatype matches column datatype for user-defined
    histograms.
 
    @return datatype string if present, nullptr if not
   */
  Field *field() const { return m_field; }
 
 private:
  /// Thread context for error handlers
  THD *m_thd;
  /// The field for checking endpoint values
  Field *m_field;
  /// Where reported errors are stored
  results_map *m_results;
  /// Whether or not the JSON object to process is in binary format
  bool m_binary;
};
 
/**
  Histogram base class.
 
  This is an abstract class containing the interface and shared code for
  concrete histogram subclasses.
 
  Histogram subclasses (Singleton, Equi_height) are constructed through factory
  methods in order to catch memory allocation errors during construction.
 
  The histogram subclasses have no public copy or move constructors. In order to
  copy a histogram onto a given MEM_ROOT, use the public clone method. The clone
  method ensures that members of the histogram, such String type buckets,
  are also allocated on the given MEM_ROOT. Modifications to these methods need
  to be careful that histogram buckets are cloned/copied correctly.
*/
class Histogram {
 public:
  /// All supported histogram types in MySQL.
  enum class enum_histogram_type { EQUI_HEIGHT, SINGLETON };
 
  /// String representation of the JSON field "histogram-type".
  static constexpr const char *histogram_type_str() { return "histogram-type"; }
 
  /// String representation of the JSON field "data-type".
  static constexpr const char *data_type_str() { return "data-type"; }
 
  /// String representation of the JSON field "collation-id".
  static constexpr const char *collation_id_str() { return "collation-id"; }
 
  /// String representation of the histogram type SINGLETON.
  static constexpr const char *singleton_str() { return "singleton"; }
 
  /// String representation of the histogram type EQUI-HEIGHT.
  static constexpr const char *equi_height_str() { return "equi-height"; }
 
 protected:
  double m_sampling_rate;
 
  /// The fraction of NULL values in the histogram (between 0.0 and 1.0).
  double m_null_values_fraction;
 
  /// The character set for the data stored
  const CHARSET_INFO *m_charset;
 
  /// The number of buckets originally specified
  size_t m_num_buckets_specified;
 
  /// String representation of the JSON field "buckets".
  static constexpr const char *buckets_str() { return "buckets"; }
 
  /// String representation of the JSON field "last-updated".
  static constexpr const char *last_updated_str() { return "last-updated"; }
 
  /// String representation of the JSON field "null-values".
  static constexpr const char *null_values_str() { return "null-values"; }
 
  static constexpr const char *sampling_rate_str() { return "sampling-rate"; }
 
  /// String representation of the JSON field "number-of-buckets-specified".
  static constexpr const char *numer_of_buckets_specified_str() {
    return "number-of-buckets-specified";
  }
 
  /// String representation of the JSON field "auto-update".
  static constexpr const char *auto_update_str() { return "auto-update"; }
 
  /**
    Constructor.
 
    @param mem_root  the mem_root where the histogram contents will be allocated
    @param db_name   name of the database this histogram represents
    @param tbl_name  name of the table this histogram represents
    @param col_name  name of the column this histogram represents
    @param type      the histogram type (equi-height, singleton)
    @param data_type the type of data that this histogram contains
    @param[out] error is set to true if an error occurs
  */
  Histogram(MEM_ROOT *mem_root, const std::string &db_name,
            const std::string &tbl_name, const std::string &col_name,
            enum_histogram_type type, Value_map_type data_type, bool *error);
 
  /**
    Copy constructor
 
    This will make a copy of the provided histogram onto the provided MEM_ROOT.
 
    @param mem_root  the mem_root where the histogram contents will be allocated
    @param other     the histogram to copy
    @param[out] error is set to true if an error occurs
  */
  Histogram(MEM_ROOT *mem_root, const Histogram &other, bool *error);
 
  /**
    Write the data type of this histogram into a JSON object.
 
    @param json_object the JSON object where we will write the histogram
                       data type
 
    @return true on error, false otherwise
  */
  bool histogram_data_type_to_json(Json_object *json_object) const;
 
  /**
    Return the value that is contained in the JSON DOM object.
 
    For most types, this function simply returns the contained value. For String
    values, the value is allocated on this histograms MEM_ROOT before it is
    returned. This allows the String value to survive the entire lifetime of the
    histogram object.
 
    @param json_dom the JSON DOM object to extract the value from
    @param out      the value from the JSON DOM object
    @param context  error context for validation
 
    @return true on error, false otherwise
  */
  template <class T>
  bool extract_json_dom_value(const Json_dom *json_dom, T *out,
                              Error_context *context);
 
  /**
    Populate the histogram with data from the provided JSON object. The base
    class also provides an implementation that subclasses must call in order
    to populate fields that are shared among all histogram types (character set,
    null values fraction).
 
    @param json_object  the JSON object to read the histogram data from
    @param context      error context for validation
 
    @return true on error, false otherwise
  */
  virtual bool json_to_histogram(const Json_object &json_object,
                                 Error_context *context) = 0;
 
 private:
  /// The MEM_ROOT where the histogram contents will be allocated.
  MEM_ROOT *m_mem_root;
 
  /// The type of this histogram.
  const enum_histogram_type m_hist_type;
 
  /// The type of the data this histogram contains.
  const Value_map_type m_data_type;
 
  /// Name of the database this histogram represents.
  LEX_CSTRING m_database_name;
 
  /// Name of the table this histogram represents.
  LEX_CSTRING m_table_name;
 
  /// Name of the column this histogram represents.
  LEX_CSTRING m_column_name;
 
  /// True if the histogram was created with the AUTO UPDATE option, false if
  /// MANUAL UPDATE.
  bool m_auto_update;
 
  /**
    An internal function for getting a selectivity estimate prior to adustment.
    @see get_selectivity() for details.
   */
  bool get_raw_selectivity(Item **items, size_t item_count, enum_operator op,
                           double *selectivity) const;
 
  /**
    An internal function for getting the selecitvity estimation.
 
    This function will read/evaluate the value from the given Item, and pass
    this value on to the correct selectivity estimation function based on the
    data type of the histogram. For instance, if the data type of the histogram
    is INT, we will call "val_int" on the Item to evaluate the value as an
    integer and pass this value on to the next function.
 
    @param item The Item to read/evaluate the value from.
    @param op The operator we are estimating the selectivity for.
    @param typelib In the case of ENUM or SET data type, this parameter holds
                   the type information. This is needed in order to map a
                   string representation of an ENUM/SET value into its correct
                   integer representation (ENUM/SET values are stored as
                   integer values in the histogram).
    @param[out] selectivity The estimated selectivity, between 0.0 and 1.0
                inclusive.
 
    @return true on error (i.e the provided item was NULL), false on success.
  */
  bool get_selectivity_dispatcher(Item *item, const enum_operator op,
                                  const TYPELIB *typelib,
                                  double *selectivity) const;
 
  /**
    An internal function for getting the selecitvity estimation.
 
    This function will cast the histogram to the correct class (using down_cast)
    and pass the given value on to the correct selectivity estimation function
    for that class.
 
    @param value The value to estimate the selectivity for.
 
    @return The estimated selectivity, between 0.0 and 1.0 inclusive.
  */
  template <class T>
  double get_less_than_selectivity_dispatcher(const T &value) const;
 
  /// @see get_less_than_selectivity_dispatcher
  template <class T>
  double get_greater_than_selectivity_dispatcher(const T &value) const;
 
  /// @see get_less_than_selectivity_dispatcher
  template <class T>
  double get_equal_to_selectivity_dispatcher(const T &value) const;
 
  /**
    An internal function for applying the correct function for the given
    operator.
 
    @param op    The operator to apply
    @param value The value to find the selectivity for.
 
    @return The estimated selectivity, between 0.0 and 1.0 inclusive.
  */
  template <class T>
  double apply_operator(const enum_operator op, const T &value) const;
 
 public:
  Histogram() = delete;
  Histogram(const Histogram &other) = delete;
 
  /// Destructor.
  virtual ~Histogram() = default;
 
  /// @return the MEM_ROOT that this histogram uses for allocations
  MEM_ROOT *get_mem_root() const { return m_mem_root; }
 
  /**
    @return name of the database this histogram represents
  */
  const LEX_CSTRING get_database_name() const { return m_database_name; }
 
  /**
    @return name of the table this histogram represents
  */
  const LEX_CSTRING get_table_name() const { return m_table_name; }
 
  /**
    @return name of the column this histogram represents
  */
  const LEX_CSTRING get_column_name() const { return m_column_name; }
 
  /**
    @return type of this histogram
  */
  enum_histogram_type get_histogram_type() const { return m_hist_type; }
 
  /**
    @return the fraction of NULL values, in the range [0.0, 1.0]
  */
  double get_null_values_fraction() const;
 
  /// @return the character set for the data this histogram contains
  const CHARSET_INFO *get_character_set() const { return m_charset; }
 
  /// @return the sampling rate used to generate this histogram
  double get_sampling_rate() const { return m_sampling_rate; }
 
  /**
    Returns the histogram type as a readable string.
 
    @return a readable string representation of the histogram type
  */
  virtual std::string histogram_type_to_str() const = 0;
 
  /**
    @return number of buckets in this histogram
  */
  virtual size_t get_num_buckets() const = 0;
 
  /**
    Get the estimated number of distinct non-NULL values.
    @return number of distinct non-NULL values
  */
  virtual size_t get_num_distinct_values() const = 0;
 
  /**
    @return the data type that this histogram contains
  */
  Value_map_type get_data_type() const { return m_data_type; }
 
  /**
    @return number of buckets originally specified by the user. This may be
            higher than the actual number of buckets in the histogram.
  */
  size_t get_num_buckets_specified() const { return m_num_buckets_specified; }
 
  /**
    @return True if automatic updates are enabled for the histogram, false
    otherwise.
  */
  bool get_auto_update() const { return m_auto_update; }
 
  /**
    Sets the auto update property for the histogram.
  */
  void set_auto_update(bool auto_update) { m_auto_update = auto_update; }
 
  /**
    Converts the histogram to a JSON object.
 
    @param[in,out] json_object output where the histogram is to be stored. The
                   caller is responsible for allocating/deallocating the JSON
                   object
 
    @return     true on error, false otherwise
  */
  virtual bool histogram_to_json(Json_object *json_object) const = 0;
 
  /**
    Converts JSON object to a histogram.
 
    @param  mem_root    MEM_ROOT where the histogram will be allocated
    @param  schema_name the schema name
    @param  table_name  the table name
    @param  column_name the column name
    @param  json_object output where the histogram is stored
    @param  context     error context for validation
 
    @return nullptr on error. Otherwise a histogram allocated on the provided
            MEM_ROOT.
  */
  static Histogram *json_to_histogram(MEM_ROOT *mem_root,
                                      const std::string &schema_name,
                                      const std::string &table_name,
                                      const std::string &column_name,
                                      const Json_object &json_object,
                                      Error_context *context);
 
  /**
    Make a clone of the current histogram
 
    @param mem_root the MEM_ROOT on which the new histogram will be allocated.
 
    @return a histogram allocated on the provided MEM_ROOT. Returns nullptr
            on error.
  */
  virtual Histogram *clone(MEM_ROOT *mem_root) const = 0;
 
  /**
    Store this histogram to persistent storage (data dictionary). The MEM_ROOT
    that the histogram is allocated on is transferred to the dictionary.
 
    @param thd Thread handler.
 
    @return false on success, true on error.
  */
  bool store_histogram(THD *thd) const;
 
  /**
    Get selectivity estimation.
 
    This function will try and get the selectivity estimation for a predicate
    on the form "COLUMN OPERATOR CONSTANT", for instance "SELECT * FROM t1
    WHERE col1 > 23;".
 
    This function will take care of several of things, for instance checking
    that the value we are estimating the selectivity for is a constant value.
 
    The order of the Items provided does not matter. For instance, of the
    operator argument given is "EQUALS_TO", it does not matter if the constant
    value is provided as the first or the second argument; this function will
    take care of this.
 
    @param items            an array of items that contains both the field we
                            are estimating the selectivity for, as well as the
                            user-provided constant values.
    @param item_count       the number of Items in the Item array.
    @param op               the predicate operator
    @param[out] selectivity the calculated selectivity if a usable histogram was
                            found
 
    @retval true if an error occurred (the Item provided was not a constant
    value or similar).
    @return false if success
  */
  bool get_selectivity(Item **items, size_t item_count, enum_operator op,
                       double *selectivity) const;
 
  /**
    @return the fraction of non-null values in the histogram.
  */
  double get_non_null_values_fraction() const {
    return 1.0 - get_null_values_fraction();
  }
};
 
/** Return true if 'histogram' was built on an empty table.*/
inline bool empty(const Histogram &histogram) {
  return histogram.get_num_distinct_values() == 0 &&
         histogram.get_null_values_fraction() == 0.0;
}
 
/**
  Create a histogram from a value map.
 
  This function will build a histogram from a value map. The histogram type
  depends on both the size of the input data, as well as the number of buckets
  specified. If the number of distinct values is less than or equal to the
  number of buckets, a Singleton histogram will be created. Otherwise, an
  equi-height histogram will be created.
 
  The histogram will be allocated on the supplied mem_root, and it is the
  callers responsibility to properly clean up when the histogram isn't needed
  anymore.
 
  @param   mem_root        the MEM_ROOT where the histogram contents will be
                           allocated
  @param   value_map       a value map containing [value, frequency]
  @param   num_buckets     the maximum number of buckets to create
  @param   db_name         name of the database this histogram represents
  @param   tbl_name        name of the table this histogram represents
  @param   col_name        name of the column this histogram represents
 
  @return  a histogram, using at most "num_buckets" buckets. The histogram
           type depends on the size of the input data, and the number of
           buckets
*/
template <class T>
Histogram *build_histogram(MEM_ROOT *mem_root, const Value_map<T> &value_map,
                           size_t num_buckets, const std::string &db_name,
                           const std::string &tbl_name,
                           const std::string &col_name);
 
/**
  A simple struct containing the settings for a histogram to be built.
*/
struct HistogramSetting {
  /// A null-terminated C-style string with the name of the column to build the
  /// histogram for.
  const char *column_name;
 
  /// The target number of buckets for the histogram.
  size_t num_buckets = 100;
 
  /// Holds the JSON specification of the histogram for the UPDATE HISTOGRAM ...
  /// USING DATA command, otherwise empty.
  LEX_STRING data = {nullptr, 0};
 
  /// True if AUTO UPDATE, false for MANUAL UPDATE.
  bool auto_update = false;
 
  /// A pointer to the field, used internally by update_histograms().
  Field *field = nullptr;
};
 
/**
  Create or update histograms for a set of columns of a given table.
 
  This function will try to create a histogram for each HistogramSetting object
  passed to it. It operates in two stages:
 
  In the first stage it will attempt to resolve every HistogramSetting in
  settings, verifying that the specified column exists and supports histograms.
  If a setting cannot be resolved an error message will be generated (see note
  below for details on error reporting), but the function will continue
  executing. The collection of settings is modified in-place so that only the
  resolved settings remain when the function returns.
 
  In the second stage, after the settings have been resolved, the function
  attempts to build a histogram for each resolved column. If an error is
  encountered during this stage, the function will immediately abort and return
  true. In other words, if the function returns true, it will have made an
  attempt to update the histograms as specified in the output collection of
  settings, but it could have failed halfway.
 
  If no error occurs during the second stage the function will return false, and
  the histograms specified in the output collection of settings will succesfully
  have been updated.
 
  @param thd Thread handler.
  @param table The table where we should look for the columns/data.
  @param[in,out] settings The settings for the histograms to be built.
  @param[in,out] results A map where the result of each operation is stored.
 
  @return False on success, true if an error was encountered.
*/
bool update_histograms(THD *thd, Table_ref *table,
                       Mem_root_array<HistogramSetting> *settings,
                       results_map &results);
 
/**
  Updates existing histograms on a table that were specified with the AUTO
  UPDATE option. If any histograms were updated a new snapshot of the current
  collection of histograms for the table is inserted on the TABLE_SHARE.
 
  @note The caller must manually ensure that the table share is flushed or that
  tables are evicted from the table cache to guarantee that new queries will use
  the updated histograms. This can be done by calling tdc_remove_table() and
  passing the TDC_RT_REMOVE_UNUSED or TDC_RT_MARK_FOR_REOPEN option,
  respectively.
 
  @param thd Thread handle.
  @param table Table_ref for the table to update histograms on. The table should
  already be opened.
 
  @return False if all automatically updated histograms on the table
  (potentially none) were updated without encountering an error. True otherwise.
*/
bool auto_update_table_histograms(THD *thd, Table_ref *table);
 
/**
  Retrieve an updated snapshot of the histograms on a table directly from the
  dictionary (in an inefficient manner, querying all columns) and inserts this
  snapshot in the Table_histograms_collection on the TABLE_SHARE.
 
  @param thd The current thread.
  @param table The table to retrieve updated histograms for.
 
  @note This function assumes that the table is opened and generally depends on
  the surrounding context. It also locks/unlocks LOCK_OPEN.
 
  @return False on success. Returns true if an error occurred in which case the
  TABLE_SHARE will not have been updated.
*/
bool update_share_histograms(THD *thd, Table_ref *table);
 
/**
  Updates existing histograms on a table that were specified with the AUTO
  UPDATE option. Updated histograms are made available to the optimizer.
 
  This function wraps auto_update_table_histograms()) in an appropriate
  transaction-context for the background thread.
 
  @note This function temporarily disables the binary log as we are not
  interested in replicating or recovering updates to histograms that take place
  in the background.
 
  @note This function supresses some errors in order to avoid spamming the error
  log, but unexpected errors are written to the error log, following the same
  pattern as the event scheduler.
 
  @param thd Background thread handle.
  @param db_name Name of the database holding the table.
  @param table_name Name of the table to update histograms for.
 
  @return False on success, true on error.
*/
bool auto_update_table_histograms_from_background_thread(
    THD *thd, const std::string &db_name, const std::string &table_name);
 
/**
  Drop histograms for all columns in a given table.
 
  @param thd Thread handler.
  @param table The table where we should look for the columns.
  @param original_table_def Original table definition.
  @param results A map where the result of each operation is stored.
 
  @note Assumes that caller owns exclusive metadata lock on the table,
        so there is no need to lock individual statistics.
 
  @return false on success, true on error.
*/
bool drop_all_histograms(THD *thd, Table_ref &table,
                         const dd::Table &original_table_def,
                         results_map &results);
 
/**
  Drop histograms for a set of columns in a given table.
 
  This function will try to drop the histogram statistics for all specified
  columns. If one of the columns fail, it will continue to the next one and try.
 
  @param thd Thread handler.
  @param table The table where we should look for the columns.
  @param columns Columns specified by the user.
  @param results A map where the result of each operation is stored.
 
  @note Assumes that the caller has the appropriate metadata locks on both the
  table and column statistics. That can either be an exclusive metadata lock on
  the table itself, or a shared metadata lock on the table combined with
  exclusive locks on individual column statistics.
 
  @return false on success, true on error.
*/
bool drop_histograms(THD *thd, Table_ref &table, const columns_set &columns,
                     results_map &results);
 
/**
  Rename histograms for all columns in a given table.
 
  @param thd             Thread handler.
  @param old_schema_name The old schema name
  @param old_table_name  The old table name
  @param new_schema_name The new schema name
  @param new_table_name  The new table name
  @param results         A map where the result of each operation is stored.
 
  @return false on success, true on error.
*/
bool rename_histograms(THD *thd, const char *old_schema_name,
                       const char *old_table_name, const char *new_schema_name,
                       const char *new_table_name, results_map &results);
 
bool find_histogram(THD *thd, const std::string &schema_name,
                    const std::string &table_name,
                    const std::string &column_name,
                    const Histogram **histogram);
}  // namespace histograms
 
#endif