More code cleanups

Remove Histogram_*::is_available(), it is not applicable anymore.
Fix compilation on Windows

sql/opt_histogram_json.cc

@@ -126,7 +126,7 @@ void Histogram_json_hb::init_for_collection(MEM_ROOT *mem_root,
                                             ulonglong size_arg)
 {
   DBUG_ASSERT(htype_arg == JSON_HB);
-  size= (uint8) size_arg;
+  size= (size_t)size_arg;
 }
 
 

sql/opt_histogram_json.h

@@ -69,14 +69,6 @@ class Histogram_json_hb : public Histogram_base
   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,
@@ -84,7 +76,7 @@ class Histogram_json_hb : public Histogram_base
 
   void set_json_text(ulonglong sz, uchar *json_text_arg)
   {
-    size = (uint8) sz;
+    size= (size_t) sz;
     json_text.assign((const char*)json_text_arg,
                      strlen((const char*)json_text_arg));
   }

sql/sql_statistics.cc

@@ -1271,7 +1271,7 @@ void Histogram_binary::init_for_collection(MEM_ROOT *mem_root,
                                            ulonglong size_arg)
 {
   type= htype_arg;
-  values = (uchar*)alloc_root(mem_root, size_arg);
+  values= (uchar*)alloc_root(mem_root, (size_t)size_arg);
   size= (uint8) size_arg;
 }
 

sql/sql_statistics.h

@@ -175,9 +175,15 @@ class Histogram_base : public Sql_alloc
   virtual Histogram_builder *create_builder(Field *col, uint col_len,
                                             ha_rows rows)=0;
 
-  virtual bool is_available()=0;
+  /*
+    This function checks that histograms should be usable only when
+      1) the level of optimizer_use_condition_selectivity > 3
+  */
+  bool is_usable(THD *thd)
+  {
+    return thd->variables.optimizer_use_condition_selectivity > 3;
+  }
 
-  virtual bool is_usable(THD *thd)=0;
 
   virtual double point_selectivity(Field *field, key_range *endpoint,
                                    double avg_selection)=0;
@@ -312,19 +318,6 @@ class Histogram_binary : public Histogram_base
   Histogram_builder *create_builder(Field *col, uint col_len,
                                     ha_rows rows) override;
 
-  bool is_available() override { return (values!=NULL); }
-
-  /*
-    This function checks that histograms should be usable only when
-      1) the level of optimizer_use_condition_selectivity > 3
-      2) histograms have been collected
-  */
-  bool is_usable(THD *thd) override
-  {
-    return thd->variables.optimizer_use_condition_selectivity > 3 &&
-           is_available();
-  }
-
   void set_value(uint i, double val)
   {
     switch (type) {