Mysql Alter table with Instant algorithm
The manual goes into a lot of gory details; you can mostly ignore them.  In general, if you do not provide an &quot;algorithm&quot;, the ALTER will pick the fastest way.  If you specify INSTANT, but that algorithm is not possible for what you want to change, it will spit at you.  (No harm done.)
At one extreme (the COPY end):  Changing the PRIMARY KEY (on InnoDB) requires rebuilding the entire table, reordering the data, and rebuilding all the indexes (because the PK is part of every secondary index).
At the other extreme (the INSTANT end):  Adding an option to the end of an ENUM list does not require touching the data at all.