Storage Indexes

-Storage Indexes provide the third level of optimization for Smart Scans. 

-Storage Indexes are in-memory structures on the storage cells that maintain a maximum and minimum value for each 1MB disk storage unit, for up to eight columns of a table. -Storage Indexes are a little different than most Smart Scan optimizations. 

-The goal of Storage Indexes is not to reduce the amount of data being transferred back to the database tier. In fact, whether they are used on a given query or not, the amount of data returned to the database tier remains constant. 

-On the contrary, Storage Indexes are designed to eliminate time spent reading data from disk on the storage servers themselves. Think of this feature as a  pre-filter.

-Since Smart Scans pass the query predicates to the storage servers, and Storage Indexes contain a map of values in each 1MB storage region, any region that can’t possibly contain a matching row can be eliminated without ever being read. 

-You can also think of Storage Indexes as an alternate partitioning mechanism. Disk I/O is eliminated in analogous fashion to partition elimination. If a partition can’t contain any records of interest, the partition’s blocks will not be read. 

-Similarly, if a storage region cannot contain any records of interest, that storage region need not be read. Storage Indexes cannot be used in all cases, and there is little that can be done to affect when or how they are used. But in the right situations, the results from this optimization technique can be astounding.