MySQL 9.5 Release Notes
Online DDL operations that create or rebuild secondary indexes
allocate temporary buffers during different phases of index
creation. The
innodb_ddl_buffer_size variable
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).