The database stores a set of rows in an internal structure called a compression unit in case of Exadata Hybrid Columnar Compression.
a. Compression unit can span across multiple data blocks
b. The value of particular column can span across multiple blocks
c. If a column in a compression unit is updated, the database will take a lock only on the updated rows
d. If a column in a compression unit is updated, the database will take a lock on the entire compression unit