Syntax:
LOAD INDEX INTO CACHE
  tbl_index_list [, tbl_index_list] ...

tbl_index_list:
  tbl_name
    [PARTITION (partition_list)]
    [{INDEX|KEY} (index_name[, index_name] ...)]
    [IGNORE LEAVES]

partition_list: {
    partition_name[, partition_name] ...
  | ALL
}

The LOAD INDEX INTO CACHE statement preloads a table index into the key
cache to which it has been assigned by an explicit CACHE INDEX
statement, or into the default key cache otherwise.

LOAD INDEX INTO CACHE applies only to MyISAM tables, including
partitioned MyISAM tables. In addition, indexes on partitioned tables
can be preloaded for one, several, or all partitions.

The IGNORE LEAVES modifier causes only blocks for the nonleaf nodes of
the index to be preloaded.

IGNORE LEAVES is also supported for partitioned MyISAM tables.

URL: https://dev.mysql.com/doc/refman/8.0/en/load-index.html

