mysql index on derived table
What is your requirement? we cannot index "data" as it is not a column in table, It can defined with multiple columns within one table . 

Few things to be considered while using Fulltext.
Its only supported for MyISAM engine,
You can also set few variables "ft_min_word_len" and "ft_max_word_len" to search,
Some stopwords are ignored we can set change values at "ft_stopword_file", and 
Its not case sensitive search.