Move JSON histograms code into its own files

sql/opt_histogram_json.h

@@ -0,0 +1,95 @@
+/*
+   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
+*/
+
+class Histogram_json_hb : 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;
+
+  // Array of histogram bucket endpoints in KeyTupleFormat.
+  std::vector<std::string> histogram_bounds;
+
+public:
+  static constexpr const char* JSON_NAME="histogram_hb_v1";
+
+  bool parse(MEM_ROOT *mem_root, Field *field, Histogram_type type_arg,
+             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
+      Legacy: this returns the size of the histogram on disk.
+
+    @detail
+      This is only called at collection time when json_text is non-empty.
+  */
+  uint get_size() override
+  {
+    return json_text.size();
+  }
+
+  void init_for_collection(MEM_ROOT *mem_root, Histogram_type htype_arg,
+                           ulonglong size) override;
+
+  bool is_available() override {return true; }
+
+  bool is_usable(THD *thd) override
+  {
+    return thd->variables.optimizer_use_condition_selectivity > 3 &&
+           is_available();
+  }
+
+  double point_selectivity(Field *field, key_range *endpoint,
+                           double avg_selection) override;
+  double range_selectivity(Field *field, key_range *min_endp,
+                           key_range *max_endp) override;
+
+  void set_json_text(ulonglong sz, uchar *json_text_arg)
+  {
+    size = (uint8) sz;
+    json_text.assign((const char*)json_text_arg,
+                     strlen((const char*)json_text_arg));
+  }
+
+private:
+  int find_bucket(Field *field, const uchar *lookup_val, bool equal_is_less);
+};
+