SQL Syntax Reference : PARTIAL
 
PARTIAL
Remarks
To allow indexing on CHAR and VARCHAR columns wider than 255 bytes, include the PARTIAL keyword in the CREATE INDEX statement. If the columns that make up the partial index, including overhead, contain less than 255 bytes, PARTIAL is ignored.
Note The DROP INDEX statement does not require PARTIAL to remove a partial index.
UNIQUE and PARTIAL are mutually exclusive and cannot be used in the same CREATE INDEX statement.
See Also
CREATE INDEX
DROP INDEX
UNIQUE