https://dev.mysql.com/doc/refman/8.0/en/optimize-table.html OPTIMIZE TABLE works for InnoDB, MyISAM, and ARCHIVE tables. OPTIMIZE TABLE is also supported for dynamic columns of in-memory NDB tables. It does not work for fixed-width columns of in-memory tables , nor does it work for Disk Data tables.

https://dev.mysql.com/doc/refman/8.0/en/memory-storage-engine.html#memory-storage-engine-managing-memory-use To free up the memory used by deleted rows, use ALTER TABLE ENGINE=MEMORY to force a table rebuild.