MDEV-26750: Estimation for filtered rows is far off with JSON_HB histogram

Fix a bug in position_in_interval(). Do not overwrite one interval endpoint
with another.

mysql-test/main/statistics_json.result

@@ -4243,12 +4243,12 @@ test	t1_json	a	a-0	a-9	0.0000	3.0000	1.0000	10	JSON_HB	{
 }
 explain extended select * from t1_json where a between 'a-3a' and 'zzzzzzzzz';
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
-1	SIMPLE	t1_json	ALL	NULL	NULL	NULL	NULL	10	50.00	Using where
+1	SIMPLE	t1_json	ALL	NULL	NULL	NULL	NULL	10	58.71	Using where
 Warnings:
 Note	1003	select `test`.`t1_json`.`a` AS `a` from `test`.`t1_json` where `test`.`t1_json`.`a` between 'a-3a' and 'zzzzzzzzz'
 analyze select * from t1_json where a between 'a-3a' and 'zzzzzzzzz';
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	r_rows	filtered	r_filtered	Extra
-1	SIMPLE	t1_json	ALL	NULL	NULL	NULL	NULL	10	10.00	50.00	60.00	Using where
+1	SIMPLE	t1_json	ALL	NULL	NULL	NULL	NULL	10	10.00	58.71	60.00	Using where
 explain extended select * from t1_json where a < 'b-1a';
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1_json	ALL	NULL	NULL	NULL	NULL	10	90.00	Using where
@@ -7280,7 +7280,7 @@ Percentage	0.0	99.9	47	JSON_HB	{
 }
 analyze select * from Country use index () where Code between 'BBC' and 'GGG';
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	r_rows	filtered	r_filtered	Extra
-1	SIMPLE	Country	ALL	NULL	NULL	NULL	NULL	239	239.00	25.10	25.52	Using where
+1	SIMPLE	Country	ALL	NULL	NULL	NULL	NULL	239	239.00	24.58	25.52	Using where
 analyze select * from Country use index () where Code < 'BBC';
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	r_rows	filtered	r_filtered	Extra
 1	SIMPLE	Country	ALL	NULL	NULL	NULL	NULL	239	239.00	8.37	7.11	Using where
@@ -7510,3 +7510,23 @@ histogram
   ]
 }
 drop table t1;
+#
+# MDEV-26750: Estimation for filtered rows is far off with JSON_HB histogram
+#
+create table t1 (c char(8));
+insert into t1 values ('1x');
+insert into t1 values ('1x');
+insert into t1 values ('1xx');
+insert into t1 values ('0xx');
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+set histogram_type= JSON_HB;
+analyze table t1 persistent for all;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	Engine-independent statistics collected
+test.t1	analyze	status	OK
+analyze
+select c from t1 where c > '1';
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	r_rows	filtered	r_filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	16	16.00	75.00	75.00	Using where
+drop table t1;

mysql-test/main/statistics_json.test

@@ -276,3 +276,22 @@ select histogram from mysql.column_stats where table_name = 't1';
 
 drop table t1;
 
+--echo #
+--echo # MDEV-26750: Estimation for filtered rows is far off with JSON_HB histogram
+--echo #
+create table t1 (c char(8));
+
+insert into t1 values ('1x');
+insert into t1 values ('1x');
+insert into t1 values ('1xx');
+insert into t1 values ('0xx');
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+
+set histogram_type= JSON_HB;
+analyze table t1 persistent for all;
+analyze
+select c from t1 where c > '1';
+
+drop table t1;
+

sql/opt_histogram_json.cc

@@ -569,21 +569,34 @@ double position_in_interval(Field *field, const  uchar *key, uint key_len,
   if (field->pos_through_val_str())
   {
     StringBuffer<64> buf1, buf2, buf3;
-    String empty_buf1, empty_buf2, empty_buf3;
 
     store_key_image_to_rec_no_null(field, left.data(), left.size());
-    String *min_str= field->val_str(&buf1, &empty_buf1);
+    String *min_str= field->val_str(&buf1);
+    /*
+      Make sure we've saved a copy of the data, not a pointer into the
+      field->ptr. We will overwrite the contents of field->ptr with the next
+      store_key_image_to_rec_no_null call
+    */
+    if (&buf1 != min_str)
+      buf1.copy(*min_str);
+    else
+      buf1.copy();
 
     store_key_image_to_rec_no_null(field, right.data(), right.size());
-    String *max_str= field->val_str(&buf2, &empty_buf2);
+    String *max_str= field->val_str(&buf2);
+    /* Same as above */
+    if (&buf2 != max_str)
+      buf2.copy(*max_str);
+    else
+      buf2.copy();
 
     store_key_image_to_rec_no_null(field, (const char*)key, key_len);
-    String *midp_str= field->val_str(&buf3, &empty_buf3);
+    String *midp_str= field->val_str(&buf3);
 
     res= pos_in_interval_for_string(field->charset(),
            (const uchar*)midp_str->ptr(), midp_str->length(),
-           (const uchar*)min_str->ptr(), min_str->length(),
-           (const uchar*)max_str->ptr(), max_str->length());
+           (const uchar*)buf1.ptr(), buf1.length(),
+           (const uchar*)buf2.ptr(), buf2.length());
   }
   else
   {