New getter to read Engine Independent JSON histogram buckets directly

	This would allow Columnstore to leverage EI data in its cost-based and rewrite optimizer parts

Author: drrtuy

sql/opt_histogram_json.h

@@ -16,6 +16,20 @@
 
 #include "sql_statistics.h"
 
+struct Histogram_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;
+};
+
 /*
   An equi-height histogram which stores real values for bucket bounds.
 
@@ -71,21 +85,7 @@ class Histogram_json_hb final : public Histogram_base
   /* 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::vector<Histogram_bucket> buckets;
 
   std::string last_bucket_end_endp;
 
@@ -129,6 +129,16 @@ class Histogram_json_hb final : public Histogram_base
   double range_selectivity(Field *field, key_range *min_endp,
                            key_range *max_endp, double avg_sel) override;
 
+  const std::vector<Histogram_bucket>& get_json_histogram() const
+  {
+    return buckets;
+  }
+
+  const std::string& get_last_bucket_end_endp() const
+  {
+    return last_bucket_end_endp;
+  }
+
   void set_json_text(ulonglong sz, const char *json_text_arg,
                      size_t json_text_len)
   {