MDEV-25259 JSON_TABLE: Illegal mix of collations upon executing query with...

MDEV-25259 JSON_TABLE: Illegal mix of collations upon executing query with combination of charsets via view.

now the ::print printed too much. Limit it for fields with no CHARSET
possible.

sql/field.cc

@@ -2331,6 +2331,17 @@ bool Field_str::can_be_substituted_to_equal_item(const Context &ctx,
 }
 
 
+void Field_str::change_charset(const DTCollation &new_cs)
+{
+  if (!has_charset())
+    return;
+
+  field_length= (field_length * new_cs.collation->mbmaxlen) /
+    m_collation.collation->mbmaxlen;
+  m_collation= new_cs;
+}
+
+
 void Field_num::make_send_field(Send_field *field)
 {
   Field::make_send_field(field);

sql/field.h

@@ -2111,12 +2111,7 @@ class Field_str :public Field {
   {
     return m_collation;
   }
-  void change_charset(const DTCollation &new_cs) override
-  {
-    field_length= (field_length * new_cs.collation->mbmaxlen) /
-      m_collation.collation->mbmaxlen;
-    m_collation= new_cs;
-  }
+  void change_charset(const DTCollation &new_cs) override;
   bool binary() const override { return field_charset() == &my_charset_bin; }
   uint32 max_display_length() const override { return field_length; }
   uint32 character_octet_length() const override { return field_length; }

sql/json_table.cc

@@ -926,7 +926,7 @@ bool Create_json_table::add_json_table_fields(THD *thd, TABLE *table,
        executing a prepared statement for the second time.
     */
     sql_f->length= sql_f->char_length;
-    if (!(jc->m_explicit_cs= sql_f->charset))
+    if (!sql_f->charset)
       sql_f->charset= &my_charset_utf8mb4_general_ci;
 
     if (sql_f->prepare_stage1(thd, thd->mem_root, table->file,
@@ -1098,7 +1098,7 @@ int Json_table_column::print(THD *thd, Field **f, String *str)
     (*f)->sql_type(column_type);
 
     if (str->append(column_type) ||
-        (m_explicit_cs &&
+        ((*f)->has_charset() && m_explicit_cs &&
            (str->append(" CHARSET ") || str->append(m_explicit_cs->csname))) ||
         str->append(m_column_type == PATH ? " PATH " : " EXISTS PATH ") ||
         print_path(str, &m_path))

sql/json_table.h

@@ -161,7 +161,7 @@ class Json_table_column : public Sql_alloc
   }
   int set(THD *thd, enum_type ctype, const LEX_CSTRING &path);
   Json_table_column(Create_field *f, Json_table_nested_path *nest) :
-    m_field(f), m_nest(nest)
+    m_field(f), m_nest(nest), m_explicit_cs(NULL)
   {
     m_on_error.m_response= RESPONSE_NOT_SPECIFIED;
     m_on_empty.m_response= RESPONSE_NOT_SPECIFIED;

sql/sql_yacc.yy

@@ -11634,10 +11634,12 @@ json_table_field_type:
         | field_type_string opt_collate
           {
             set_collation($2);
+            Lex->json_table->m_cur_json_table_column->m_explicit_cs= Lex->charset;
           }
         | field_type_lob opt_collate
           {
             set_collation($2);
+            Lex->json_table->m_cur_json_table_column->m_explicit_cs= Lex->charset;
           }
         ;