Syntax:
CREATE [ONLINE|OFFLINE] [UNIQUE|FULLTEXT|SPATIAL] INDEX index_name
    [index_type]
    ON tbl_name (index_col_name,...)
    [index_option] ...

index_col_name:
    col_name [(length)] [ASC | DESC]

index_type:
    USING {BTREE | HASH}

index_option:
    KEY_BLOCK_SIZE [=] value
  | index_type
  | WITH PARSER parser_name
  | COMMENT 'string'

CREATE INDEX is mapped to an ALTER TABLE statement to create indexes.
See [HELP ALTER TABLE]. CREATE INDEX cannot be used to create a PRIMARY
KEY; use ALTER TABLE instead. For more information about indexes, see
https://mariadb.com/kb/en/optimization-and-indexes/.

URL: https://mariadb.com/kb/en/create-index/

