MDEV-8253 EXPLAIN SELECT prints unexpected characters

Item_string::clone_item() creates a new Item_string that
points exactly to the same buffer that the original one does.
Later, Item_string::print() uses c_ptr() for the original Item_string,
which reallocs the original buffer, and the clone remain with
the old freed buffer.
Refactoring the code not to use c_ptr() in Item_string::print().

mysql-test/r/ctype_latin1.result

@@ -8168,5 +8168,18 @@ SELECT * FROM t1 WHERE a=_latin1'a' AND a=_latin1'A';
 a
 DROP TABLE t1;
 #
+# MDEV-8253 EXPLAIN SELECT prints unexpected characters
+#
+SET NAMES latin1;
+CREATE TABLE t1 (a DECIMAL(10,1),b DECIMAL(10,1),c VARCHAR(10),d VARCHAR(10));
+INSERT INTO t1 VALUES (1.5,1.5,'1','1'),(3.5,3.5,'3','3');
+EXPLAIN EXTENDED
+SELECT * FROM t1 WHERE COALESCE(c,0)='3 ' AND COALESCE(d,0)=COALESCE(c,0);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b`,`test`.`t1`.`c` AS `c`,`test`.`t1`.`d` AS `d` from `test`.`t1` where ((coalesce(`test`.`t1`.`c`,0) = '3 ') and (coalesce(`test`.`t1`.`d`,0) = '3 '))
+DROP TABLE t1;
+#
 # End of 10.1 tests
 #

mysql-test/r/ctype_ucs.result

@@ -5627,5 +5627,18 @@ a
 1
 DROP TABLE t1;
 #
+# MDEV-8253 EXPLAIN SELECT prints unexpected characters
+#
+SET NAMES latin1, character_set_connection=ucs2;
+CREATE TABLE t1 (a DECIMAL(10,1),b DECIMAL(10,1),c VARCHAR(10),d VARCHAR(10));
+INSERT INTO t1 VALUES (1.5,1.5,'1','1'),(3.5,3.5,'3','3');
+EXPLAIN EXTENDED
+SELECT * FROM t1 WHERE COALESCE(c,0)='3 ' AND COALESCE(d,0)=COALESCE(c,0);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b`,`test`.`t1`.`c` AS `c`,`test`.`t1`.`d` AS `d` from `test`.`t1` where ((coalesce(`test`.`t1`.`c`,0) = '3 ') and (coalesce(`test`.`t1`.`d`,0) = '3 '))
+DROP TABLE t1;
+#
 # End of 10.1 tests
 #

mysql-test/t/ctype_latin1.test

@@ -363,6 +363,16 @@ SELECT * FROM t1 WHERE a=_latin1'A' AND a=_latin1'a';
 SELECT * FROM t1 WHERE a=_latin1'a' AND a=_latin1'A';
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-8253 EXPLAIN SELECT prints unexpected characters
+--echo #
+SET NAMES latin1;
+CREATE TABLE t1 (a DECIMAL(10,1),b DECIMAL(10,1),c VARCHAR(10),d VARCHAR(10));
+INSERT INTO t1 VALUES (1.5,1.5,'1','1'),(3.5,3.5,'3','3');
+EXPLAIN EXTENDED
+SELECT * FROM t1 WHERE COALESCE(c,0)='3 ' AND COALESCE(d,0)=COALESCE(c,0);
+DROP TABLE t1;
+
 --echo #
 --echo # End of 10.1 tests
 --echo #

mysql-test/t/ctype_ucs.test

@@ -934,6 +934,18 @@ INSERT INTO t1 VALUES ('1');
 SELECT * FROM t1 WHERE a LIKE 1;
 DROP TABLE t1;
 
+
+--echo #
+--echo # MDEV-8253 EXPLAIN SELECT prints unexpected characters
+--echo #
+SET NAMES latin1, character_set_connection=ucs2;
+CREATE TABLE t1 (a DECIMAL(10,1),b DECIMAL(10,1),c VARCHAR(10),d VARCHAR(10));
+INSERT INTO t1 VALUES (1.5,1.5,'1','1'),(3.5,3.5,'3','3');
+EXPLAIN EXTENDED
+SELECT * FROM t1 WHERE COALESCE(c,0)='3 ' AND COALESCE(d,0)=COALESCE(c,0);
+DROP TABLE t1;
+
+
 --echo #
 --echo # End of 10.1 tests
 --echo #

sql/item.cc

@@ -2939,25 +2939,7 @@ void Item_string::print(String *str, enum_query_type query_type)
     }
     else
     {
-      if (my_charset_same(str_value.charset(), system_charset_info))
-        str_value.print(str); // already in system_charset_info
-      else // need to convert
-      {
-        THD *thd= current_thd;
-        LEX_STRING utf8_lex_str;
-
-        thd->convert_string(&utf8_lex_str,
-                            system_charset_info,
-                            str_value.c_ptr_safe(),
-                            str_value.length(),
-                            str_value.charset());
-
-        String utf8_str(utf8_lex_str.str,
-                        utf8_lex_str.length,
-                        system_charset_info);
-
-        utf8_str.print(str);
-      }
+      str_value.print(str, system_charset_info);
     }
   }
   else

sql/sql_string.cc

@@ -954,6 +954,16 @@ void String::print(String *str) const
   str->append_for_single_quote(Ptr, str_length);
 }
 
+
+void String::print_with_conversion(String *print, CHARSET_INFO *cs) const
+{
+  StringBuffer<256> tmp(cs);
+  uint errors= 0;
+  tmp.copy(this, cs, &errors);
+  tmp.print(print);
+}
+
+
 /*
   Exchange state of this object and argument.
 

sql/sql_string.h

@@ -568,7 +568,15 @@ class String
     str_length+= arg_length;
     return FALSE;
   }
-  void print(String *print) const;
+  void print(String *to) const;
+  void print_with_conversion(String *to, CHARSET_INFO *cs) const;
+  void print(String *to, CHARSET_INFO *cs) const
+  {
+    if (my_charset_same(charset(), cs))
+      print(to);
+    else
+      print_with_conversion(to, cs);
+  }
 
   bool append_for_single_quote(const char *st, uint len);
   bool append_for_single_quote(const String *s)