Physical I/O Saved With Exadata Hybrid Columnar Compression

In my previous post I highlighted the space savings of Exadata HCC. This post is a simple example of the physical I/O savings of Exadata HCC:

alter system flush buffer_cache;
alter system flush shared_pool;
select /* not_compressed */ count(*) from all_objs where object_type = 'TABLE';
select /* compressed     */ count(*) from all_objs_compressed_query where object_type = 'TABLE';

select substr(sql_text,1,27)||'...' sql_text,disk_reads,
	   (physical_read_bytes /1024 / 1024) physical_io_mb 
  from v$sql 
 where sql_text like '%compressed%' 
   and sql_text not like '%v$sql%';

SQL_TEXT                       DISK_READS  PHYSICAL_IO_MB
------------------------------ ---------- ---------------
select /* compressed     */...        124             .97
select /* not_compressed */...      20520          160.31

Obviously your results will vary based on the nature of the data, but reducing physical I/O from 160 MB to 1 MB for the same query could have a dramatic impact on performance.