Bug #49838: DROP INDEX and ADD UNIQUE INDEX for same index may corrupt

definition at engine

If a single ALTER TABLE contains both DROP INDEX and ADD INDEX using 
the same index name (a.k.a. index modification) we need to disable 
in-place alter table because we can't ask the storage engine to have 
two copies of the index with the same name even temporarily (if we 
first do the ADD INDEX and then DROP INDEX) and we can't modify 
indexes that are needed by e.g. foreign keys if we first do 
DROP INDEX and then ADD INDEX.
Fixed the problem by disabling in-place ALTER TABLE for these cases.

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