diff --git a/mysql-test/suite/innodb/r/instant_alter_index_rename.result b/mysql-test/suite/innodb/r/instant_alter_index_rename.result
index 52051eff0bd..69f7cc8df4c 100644
--- a/mysql-test/suite/innodb/r/instant_alter_index_rename.result
+++ b/mysql-test/suite/innodb/r/instant_alter_index_rename.result
@@ -74,7 +74,6 @@ select @a_key_returns_id = get_index_id(@table_id, 'a_key_returns'),
 @a_key_returns_id = get_index_id(@table_id, 'a_key_returns')	@b_key_id = get_index_id(@table_id, 'b_key')	@c_key_id = get_index_id(@table_id, 'c_key2')	@primary_id = get_index_id(@table_id, 'primary')
 1	1	1	1
 drop table t;
-drop function get_index_id;
 create table errors (
 a int,
 unique key a_key (a),
@@ -183,3 +182,19 @@ CREATE TABLE t1 (f1 INT, f2 INT, f3 INT);
 ALTER TABLE t1 ADD FOREIGN KEY f (f2) REFERENCES xx(f2);
 ALTER TABLE t1 ADD FOREIGN KEY (f2) REFERENCES t1(f2), ADD KEY (f3), ADD KEY (f1);
 DROP TABLE t1;
+#
+# MDEV-21669 InnoDB: Table ... contains <n> indexes inside InnoDB, which is different from the number of indexes <n> defined in the MariaDB
+#
+CREATE TABLE t1 (col_int INTEGER, col_char CHAR(20), col_varchar VARCHAR(500)) ENGINE=InnoDB;
+SET @table_id = (SELECT table_id FROM information_schema.innodb_sys_tables WHERE name='test/t1');
+ALTER TABLE t1 ADD KEY idx3 (col_varchar(9)), ADD KEY idX2 (col_char(9));
+SET @idx3_key_id = get_index_id(@table_id, 'iDx3');
+ALTER TABLE t1 DROP KEY iDx3, ADD KEY Idx3 (col_varchar(9));
+SELECT @idx3_key_id = get_index_id(@table_id, 'Idx3');
+@idx3_key_id = get_index_id(@table_id, 'Idx3')
+1
+CHECK TABLE t1 EXTENDED ;
+Table	Op	Msg_type	Msg_text
+test.t1	check	status	OK
+DROP TABLE t1;
+DROP FUNCTION get_index_id;
diff --git a/mysql-test/suite/innodb/t/instant_alter_index_rename.test b/mysql-test/suite/innodb/t/instant_alter_index_rename.test
index 3a608a00837..dd6cee7c860 100644
--- a/mysql-test/suite/innodb/t/instant_alter_index_rename.test
+++ b/mysql-test/suite/innodb/t/instant_alter_index_rename.test
@@ -73,7 +73,6 @@ select @a_key_returns_id = get_index_id(@table_id, 'a_key_returns'),
   @primary_id = get_index_id(@table_id, 'primary');
 
 drop table t;
-drop function get_index_id;
 
 create table errors (
   a int,
@@ -194,3 +193,17 @@ CREATE TABLE t1 (f1 INT, f2 INT, f3 INT);
 ALTER TABLE t1 ADD FOREIGN KEY f (f2) REFERENCES xx(f2);
 ALTER TABLE t1 ADD FOREIGN KEY (f2) REFERENCES t1(f2), ADD KEY (f3), ADD KEY (f1);
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-21669 InnoDB: Table ... contains <n> indexes inside InnoDB, which is different from the number of indexes <n> defined in the MariaDB
+--echo #
+CREATE TABLE t1 (col_int INTEGER, col_char CHAR(20), col_varchar VARCHAR(500)) ENGINE=InnoDB;
+SET @table_id = (SELECT table_id FROM information_schema.innodb_sys_tables WHERE name='test/t1');
+ALTER TABLE t1 ADD KEY idx3 (col_varchar(9)), ADD KEY idX2 (col_char(9));
+SET @idx3_key_id = get_index_id(@table_id, 'iDx3');
+ALTER TABLE t1 DROP KEY iDx3, ADD KEY Idx3 (col_varchar(9));
+SELECT @idx3_key_id = get_index_id(@table_id, 'Idx3');
+CHECK TABLE t1 EXTENDED ;
+DROP TABLE t1;
+
+DROP FUNCTION get_index_id;
diff --git a/sql/field.h b/sql/field.h
index f07eab3a197..5fe3a9ed106 100644
--- a/sql/field.h
+++ b/sql/field.h
@@ -694,7 +694,7 @@ public:
   enum imagetype { itRAW, itMBR};
 
   utype		unireg_check;
-  uint32	field_length;		// Length of field
+  const uint32	field_length;		// Length of field
   uint32	flags;
   uint16        field_index;            // field number in fields array
   uchar		null_bit;		// Bit used to test null bit
diff --git a/sql/sql_table.cc b/sql/sql_table.cc
index 8c695988853..851a63bf05a 100644
--- a/sql/sql_table.cc
+++ b/sql/sql_table.cc
@@ -6634,7 +6634,7 @@ Compare_keys compare_keys_but_name(const KEY *table_key, const KEY *new_key,
     */
     const Field *old_field= table->field[key_part->fieldnr - 1];
 
-    bool is_equal= key_part->field->is_equal(*new_field);
+    bool is_equal= old_field->is_equal(*new_field);
     /* TODO: below is an InnoDB specific code which should be moved to InnoDB */
     if (!is_equal)
     {
diff --git a/sql/table.cc b/sql/table.cc
index 6426cb57524..4962a002cb9 100644
--- a/sql/table.cc
+++ b/sql/table.cc
@@ -3759,7 +3759,7 @@ enum open_frm_error open_table_from_share(THD *thd, TABLE_SHARE *share,
           */
           field= key_part->field=field->make_new_field(&outparam->mem_root,
                                                        outparam, 0);
-          field->field_length= key_part->length;
+          const_cast<uint32_t&>(field->field_length)= key_part->length;
         }
       }
       if (!share->use_ext_keys)