Add ability to compress existing tables

MySQL 5.7 is able to compress existing tables by altering the table and then issuing an OPTIMIZE TABLE query:

ALTER TABLE t1 COMPRESSION="zlib";
OPTIMZE TABLE t1;

The alter table command sets the value for new writes, and the optimize table command compresses existing pages.

source: https://dev.mysql.com/doc/refman/5.7/en/innodb-page-compression.html

This would be a very nice feature to have for organizations attempting to transition their tables to use compression.