Address review input

mysql-test/main/statistics_json.result

@@ -7397,3 +7397,51 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	r_rows	filtered	r_f
 set histogram_type=@save_histogram_type;
 set histogram_size=@save_histogram_size;
 DROP SCHEMA world;
+use test;
+create table t10 (
+a varchar(10)
+);
+#
+# Histograms are not collected for empty tables:
+#
+analyze table t10 persistent for all;
+Table	Op	Msg_type	Msg_text
+test.t10	analyze	status	Engine-independent statistics collected
+test.t10	analyze	status	Table is already up to date
+select histogram
+from mysql.column_stats where table_name='t10' and db_name=database();
+histogram
+NULL
+#
+# Try with n_buckets > n_rows
+#
+insert into t10 values ('Berlin'),('Paris'),('Rome');
+set histogram_size=10, histogram_type='json_hb';
+analyze table t10 persistent for all;
+Table	Op	Msg_type	Msg_text
+test.t10	analyze	status	Engine-independent statistics collected
+test.t10	analyze	status	OK
+select histogram
+from mysql.column_stats where table_name='t10' and db_name=database();
+histogram
+{
+  "histogram_hb_v2": [
+    {
+      "start": "Berlin",
+      "size": 0.333333333,
+      "ndv": 1
+    },
+    {
+      "start": "Paris",
+      "size": 0.333333333,
+      "ndv": 1
+    },
+    {
+      "start": "Rome",
+      "end": "Rome",
+      "size": 0.333333333,
+      "ndv": 1
+    }
+  ]
+}
+drop table t10;

mysql-test/main/statistics_json.test

@@ -149,3 +149,26 @@ set histogram_type=@save_histogram_type;
 set histogram_size=@save_histogram_size;
 
 DROP SCHEMA world;
+use test;
+
+create table t10 (
+  a varchar(10)
+);
+
+--echo #
+--echo # Histograms are not collected for empty tables:
+--echo #
+analyze table t10 persistent for all;
+select histogram
+from mysql.column_stats where table_name='t10' and db_name=database();
+
+--echo #
+--echo # Try with n_buckets > n_rows
+--echo #
+insert into t10 values ('Berlin'),('Paris'),('Rome');
+set histogram_size=10, histogram_type='json_hb';
+analyze table t10 persistent for all;
+select histogram
+from mysql.column_stats where table_name='t10' and db_name=database();
+
+drop table t10;

sql/opt_histogram_json.cc

@@ -59,6 +59,8 @@ class Histogram_json_builder : public Histogram_builder
     : Histogram_builder(col, col_len, rows), histogram(hist)
   {
     bucket_capacity= records / histogram->get_width();
+    if (bucket_capacity == 0)
+      bucket_capacity= 1;
     hist_width= histogram->get_width();
     n_buckets_collected= 0;
     bucket.ndv= 0;
@@ -227,7 +229,8 @@ class Histogram_json_builder : public Histogram_builder
     writer.end_object();
     Binary_string *json_string= (Binary_string *) writer.output.get_string();
     histogram->set_json_text(n_buckets_collected,
-                             (uchar *) json_string->c_ptr());
+                             json_string->c_ptr(),
+                             (size_t)json_string->length());
   }
 };
 

sql/opt_histogram_json.h

@@ -113,11 +113,11 @@ class Histogram_json_hb : public Histogram_base
   double range_selectivity(Field *field, key_range *min_endp,
                            key_range *max_endp) override;
 
-  void set_json_text(ulonglong sz, uchar *json_text_arg)
+  void set_json_text(ulonglong sz, const char *json_text_arg,
+                     size_t json_text_len)
   {
     size= (size_t) sz;
-    json_text.assign((const char*)json_text_arg,
-                     strlen((const char*)json_text_arg));
+    json_text.assign(json_text_arg, json_text_len);
   }
 
 private: