Add Primary Key column at the end of an Index
In InnoDB, every "secondary" index (that is, index other than "PRIMARY") automatically adds the columns of the PK.

These are 'identical':

PRIMARY KEY(id),  INDEX(from, id)
PRIMARY KEY(id),  INDEX(from)


You could even do

PRIMARY KEY(from, id),  INDEX(id)


This combo is beneficial when id is AUTO_INCREMENT, but you often do a range on from.  Now the rows are "clustered" together, thereby being more efficient to fetch.  Meanwhile, INDEX(id) keeps AUTO_INCREMENT happy -- since it needs to know where it left off.

I assume you are doing:

SELECT ...
    WHERE `from` = '...'
    ORDER BY id DESC
    LIMIT 20;