diff --git a/mysql-test/r/innodb_mysql.result b/mysql-test/r/innodb_mysql.result
index 586cd5477a7..0e691611f02 100644
--- a/mysql-test/r/innodb_mysql.result
+++ b/mysql-test/r/innodb_mysql.result
@@ -2317,4 +2317,37 @@ ref	NULL
 rows	10
 Extra	Using index
 DROP TABLE t1;
+#
+# Bug #49838: DROP INDEX and ADD UNIQUE INDEX for same index may
+#   corrupt definition at engine
+#
+CREATE TABLE t1 (a INT NOT NULL, b INT NOT NULL, KEY k (a,b)) 
+ENGINE=InnoDB;
+ALTER TABLE t1 DROP INDEX k, ADD UNIQUE INDEX k (a,b);
+SHOW INDEXES FROM t1;;
+Table	t1
+Non_unique	0
+Key_name	k
+Seq_in_index	1
+Column_name	a
+Collation	A
+Cardinality	0
+Sub_part	NULL
+Packed	NULL
+Null	
+Index_type	BTREE
+Comment	
+Table	t1
+Non_unique	0
+Key_name	k
+Seq_in_index	2
+Column_name	b
+Collation	A
+Cardinality	0
+Sub_part	NULL
+Packed	NULL
+Null	
+Index_type	BTREE
+Comment	
+DROP TABLE t1;
 End of 5.1 tests
diff --git a/mysql-test/t/innodb_mysql.test b/mysql-test/t/innodb_mysql.test
index 75fff9656e2..88ca8d42e3d 100644
--- a/mysql-test/t/innodb_mysql.test
+++ b/mysql-test/t/innodb_mysql.test
@@ -575,5 +575,18 @@ INSERT INTO t1 VALUES (1,1,1,1,1,1), (2,2,2,2,2,2), (3,3,3,3,3,3),
 
 DROP TABLE t1;
 
+--echo #
+--echo # Bug #49838: DROP INDEX and ADD UNIQUE INDEX for same index may
+--echo #   corrupt definition at engine
+--echo #
+
+CREATE TABLE t1 (a INT NOT NULL, b INT NOT NULL, KEY k (a,b)) 
+  ENGINE=InnoDB;
+
+ALTER TABLE t1 DROP INDEX k, ADD UNIQUE INDEX k (a,b);
+
+--query_vertical SHOW INDEXES FROM t1;
+
+DROP TABLE t1;
 
 --echo End of 5.1 tests
diff --git a/sql/sql_table.cc b/sql/sql_table.cc
index eb88b1e70a5..ad72cab664e 100644
--- a/sql/sql_table.cc
+++ b/sql/sql_table.cc
@@ -5566,6 +5566,45 @@ err:
   DBUG_RETURN(-1);
 }
 
+/**
+  @brief Check if both DROP and CREATE are present for an index in ALTER TABLE
+ 
+  @details Checks if any index is being modified (present as both DROP INDEX 
+    and ADD INDEX) in the current ALTER TABLE statement. Needed for disabling 
+    online ALTER TABLE.
+  
+  @param table       The table being altered
+  @param alter_info  The ALTER TABLE structure
+  @return presence of index being altered  
+  @retval FALSE  No such index
+  @retval TRUE   Have at least 1 index modified
+*/
+
+static bool
+is_index_maintenance_unique (TABLE *table, Alter_info *alter_info)
+{
+  List_iterator<Key> key_it(alter_info->key_list);
+  List_iterator<Alter_drop> drop_it(alter_info->drop_list);
+  Key *key;
+
+  while ((key= key_it++))
+  {
+    if (key->name)
+    {
+      Alter_drop *drop;
+
+      drop_it.rewind();
+      while ((drop= drop_it++))
+      {
+        if (drop->type == Alter_drop::KEY &&
+            !my_strcasecmp(system_charset_info, key->name, drop->name))
+          return TRUE;
+      }
+    }
+  }
+  return FALSE;
+}
+
 
 /*
   SYNOPSIS
@@ -5654,6 +5693,7 @@ compare_tables(TABLE *table,
   */
   Alter_info tmp_alter_info(*alter_info, thd->mem_root);
   uint db_options= 0; /* not used */
+
   /* Create the prepared information. */
   if (mysql_prepare_create_table(thd, create_info,
                                  &tmp_alter_info,
@@ -6851,10 +6891,14 @@ view_err:
   */
   new_db_type= create_info->db_type;
 
+  if (is_index_maintenance_unique (table, alter_info))
+    need_copy_table= ALTER_TABLE_DATA_CHANGED;
+
   if (mysql_prepare_alter_table(thd, table, create_info, alter_info))
     goto err;
   
-  need_copy_table= alter_info->change_level;
+  if (need_copy_table == ALTER_TABLE_METADATA_ONLY)
+    need_copy_table= alter_info->change_level;
 
   set_table_default_charset(thd, create_info, db);