Which three are true concerning Hybrid Columnar Compression (HCC) deployed on Exadata storage?
A.
HCC can be used only when the Exadata Smart Flash Cache is configured in Write-Through mode.
B.
HCC data is never cached in the Exadata Smart Flash Cache.
C.
By default, decompression is performed by Exadata Storage Servers.
D.
Rows residing in HCC compressed segments, are always self- contained in a single database block.
E.
Rows residing in HCC compressed segments, are always self- contained in a single compression unit.
F.
Row-level locks are supported on HCC compressed data.
Explanation:
C: The decompression process typically takes place on the Oracle Exadata Storage Server in order to maximize performance and offload processing from the database server.
E: A logical construct called the compression unit is used to store a set of hybrid columnar compressed rows.
When data is loaded, column values for a set of rows are grouped together and compressed. After the column data for a set of rows has been compressed, it is stored in a compression unit.
F: What happens when I update a row on compressed tables? What about locks?
BASIC and OLTP:
the updated row stays in the compressed block ‘usual’ Oracle’s row-level locks
Hybrid columnar:
Updated row is moved, as in a delete + insert
How to see that? With dbms_rowid package
New row is OLTP compressed if possible
Lock affects the entire CU that contains the row
Note: Oracle’s Hybrid Columnar Compression technology is a new method for organizing data within a database block. As the name implies, this technology utilizes a combination of both row and columnar methods for storing data. This hybrid approach achieves the compression benefits of columnar storage, while avoiding the performance shortfalls of a pure columnar format.
Incorrect Answers:
B: Data remains compressed not only on disk, but also remains compressed in the Exadata Smart Flash
Cache, on Infiniband, in the database server buffer cache, as well as when doing back-ups or log shipping to
Data Guard.
References: http://www.oracle.com/technetwork/database/exadata/ehcc-twp-131254.pdf