innodb DEPENDENT SUBQUERY very slow
Some notes:


Why is there no PRIMARY KEY in the table?
Why date is of type int and not date or datetime or timestamp?
Why is it called date when it stores date and time?


Regarding the actual question, the efficiency of the query, an index on (tindex, tn, col, date) would help in my opinion much more than the other suggestions. And since you have no primary key and the (tindex, tn, col) is unique (you have added another unique index), my suggestion is:


(optionally) drop that unique index)
define the primary key as (tindex, tn, col):

ALTER TABLE mytable
  DROP INDEX __the_name_of_the_unique_index,  -- this is optional
  ADD PRIMARY KEY (tindex, tn, col) ;



This will take some time (that's why I suggest you do both operations in one pass.)

Then you can measure your query and all the suggested rewritings of the query (and check if the primary index is used).


Why is this a better index?


Because the tindex column is the first column in the index, all the rows with tindex='anticorrosive' will be in consecutive pages in the index (and all the rows with tindex='corrosive' will be in another part of index but still in consecutive pages.) So, reading (scanning) these 2 parts of the index will be much faster than scanning the whole table or scanning the whole index twice (which is what mysql does essentially with the (tn, col, tindex) index.)

Another benefit of making this index the primary key is that you get rid of the (hidden) 6-byte column that InnoDB added and has been using as the clustered index of the table (since you did not provide any primary or unique constraint/index) so the table is now less wide. The (tindex, tn, col) will be the clustered index of the table from now on. That also means that the date values will be available for the query - after the 2 parts of the index have been scanned.