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.