Unexpected slow ALTER TABLE in MySQL 5.7 (opens in new tab)

Usually one would expect that ALTER TABLE with ALGORITHM=COPY will be slower than the default ALGORITHM=INPLACE. In this blog post we describe the case when this is not so. One of the reasons for such behavior is the lesser known limitation of ALTER TABLE (with default ALGORITHM=INPLACE) that avoids REDO operations. As a result, all dirty pages of the altered table/tablespace have to be flushed before the ALTER TABLE completion.