If MySQL exits in the middle of an in-place ALTER TABLE operation (ALGORITHM=INPLACE), you may be left with an orphan intermediate table that takes up space on your system. This section describes how to identify and remove orphan intermediate tables.
Intermediate table names begin with an #sql-ib prefix (e.g., #sql-ib87-856498050). The accompanying .frm file has an #sql-* prefix and is named differently (e.g., #sql-36ab_2.frm).
To identify orphan intermediate tables on your system, you can view Table Monitor output or query INFORMATION_SCHEMA.INNODB_SYS_TABLES. Look for table names that begin with #sql. If the original table resides in a file-per-tabletablespace, the tablespace file (the #sql-*.ibd file) for the orphan intermediate table should be visible in the database directory.