MYSQL MYISAM_SORT_BUFFER_SIZE Variable

MYSQL MYISAM_SORT_BUFFER_SIZE Variable

 This is the The size of the buffer that is allocated when sorting MyISAM indexes during a REPAIR TABLE or when creating indexes with CREATE INDEX or ALTER TABLE. The maximum size is 4 GB.

This variable should be set as large as the largest index in the table for index builds if there is sufficient RAM and it is not over 4 GB.

This variable can be set as global variable or session variable.
 
MYSQL>set session MYISAM_SORT_BUFFER_SIZE=1073741824;
or
MYSQL>set global MYISAM_SORT_BUFFER_SIZE=1073741824;

Or, to set it in my.cnf file:
MYISAM_SORT_BUFFER_SIZE=1073741824

References:

• MySQL Manual: MyISAM_Sort_Buffer_Size
• Other References On The Web