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diff --git a/sql/opt_histogram_json.h b/sql/opt_histogram_json.h
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+/*
+   Copyright (c) 2021, MariaDB Corporation.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; version 2 of the License.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1335 USA */
+
+#include "sql_statistics.h"
+
+/*
+  An equi-height histogram which stores real values for bucket bounds.
+
+  Handles @@histogram_type=JSON_HB
+
+  Histogram format in JSON:
+
+  {
+    // The next three are saved but not currently analyzed:
+    "target_histogram_size": nnn,
+    "collected_at": "(date and time)",
+    "collected_by": "(server version)",
+
+    "histogram_hb": [
+      { "start": "value", "size":nnn.nn, "ndv": nnn },
+      ...
+
+      // Optionally, start and/or end can be replaced with _hex variant
+      { "start_hex: "value", "size":nnn.nn, "ndv":nnn},
+
+      ...
+      { "start": "value", "size":nnn.nn, "ndv": nnn, "end": "value"},
+    ]
+  }
+
+  Histogram is a JSON object. It has some global properties and "histogram_hb"
+  member whose value is a JSON array of histogram buckets.
+
+  Each bucket is an object with these members:
+    "start" - the first value in the bucket.
+    "size"  - fraction of table rows that is contained in the bucket.
+    "ndv"   - Number of Distinct Values in the bucket.
+    "end"   - Optionally, the last value in the bucket.
+
+  A bucket is a single-point bucket if it has ndv=1.
+
+  Most buckets have no "end" member: the bucket is assumed to contain all
+  values up to the "start" of the next bucket.
+
+  The exception is single-point buckets where last value is the same as the
+  first value.
+
+  start/end can be replaced with start_hex/end_hex. In _hex variant, the
+  constant is encoded in hex. This encoding is used to handle so called 
+  "unassigned characters": some non-UTF8 charsets have byte combinations that
+  are not mapped to any UTF8 character.
+*/
+
+class Histogram_json_hb final : public Histogram_base
+{
+  size_t size; /* Number of elements in the histogram */
+
+  /* Collection-time only: collected histogram in the JSON form. */
+  std::string json_text;
+
+  struct Bucket
+  {
+    // The left endpoint in KeyTupleFormat. The endpoint is inclusive, this
+    // value is in this bucket.
+    std::string start_value;
+
+    // Cumulative fraction: The fraction of table rows that fall into this
+    //  and preceding buckets.
+    double cum_fract;
+
+    // Number of distinct values in the bucket.
+    longlong ndv;
+  };
+
+  std::vector<Bucket> buckets;
+
+  std::string last_bucket_end_endp;
+
+public:
+  static constexpr const char* JSON_NAME="histogram_hb";
+
+  bool parse(MEM_ROOT *mem_root, const char *db_name, const char *table_name,
+             Field *field, const char *hist_data,
+             size_t hist_data_len) override;
+
+  void serialize(Field *field) override;
+
+  Histogram_builder *create_builder(Field *col, uint col_len,
+                                    ha_rows rows) override;
+
+  // returns number of buckets in the histogram
+  uint get_width() override
+  {
+    return (uint)size;
+  }
+
+  Histogram_type get_type() override
+  {
+    return JSON_HB;
+  }
+
+  /*
+    @brief
+      This used to be the size of the histogram on disk, which was redundant
+      (one can check the size directly). Return the number of buckets instead.
+  */
+  uint get_size() override
+  {
+    return (uint)size;
+  }
+  void init_for_collection(MEM_ROOT *mem_root, Histogram_type htype_arg,
+                           ulonglong size) override;
+
+  double point_selectivity(Field *field, key_range *endpoint,
+                           double avg_sel) override;
+  double range_selectivity(Field *field, key_range *min_endp,
+                           key_range *max_endp, double avg_sel) override;
+
+  void set_json_text(ulonglong sz, const char *json_text_arg,
+                     size_t json_text_len)
+  {
+    size= (size_t) sz;
+    json_text.assign(json_text_arg, json_text_len);
+  }
+
+private:
+  int parse_bucket(json_engine_t *je, Field *field, double *cumulative_size,
+                   bool *assigned_last_end, const char **err);
+
+  double get_left_fract(int idx);
+  std::string& get_end_value(int idx);
+  int find_bucket(const Field *field, const uchar *lookup_val, int *cmp);
+};
+