MySQL 8.0 Release Notes
MySQL 8.0 Source Code Documentation
Online DDL operations that create or rebuild secondary indexes
allocate temporary buffers during different phases of index
creation. The
innodb_ddl_buffer_size variable,
introduced in MySQL 8.0.27, defines the maximum buffer size for
online DDL operations. The default setting is 1048576 bytes (1
MB). The setting applies to buffers created by threads executing
online DDL operations. Defining an appropriate buffer size limit
avoids potential out of memory errors for online DDL operations
that create or rebuild secondary indexes. The maximum buffer size
per DDL thread is the maximum buffer size divided by the number of
DDL threads
(innodb_ddl_buffer_size/innodb_ddl_threads).
Prior to MySQL 8.0.27,
innodb_sort_buffer_size variable
defines the buffer size for online DDL operations that create or
rebuild secondary indexes.