Expert Oracle Exadata - Parent Directory

CHAPTER 2 OFFLOADING / SMART SCAN3. The third component that the DIY proposals generally address is the databaseservers themselves. The Exadata hardware specifications are readily available, soit is a simple matter to buy exactly the same Sun models. Unfortunately, you’llneed to plan for more CPU’s since you won’t be able to offload any processing tothe CPUs on the Exadata storage servers. This in turn will drive up the number ofOracle database licenses.4. Assuming we could match the Exadata hardware performance in every area, wewould still not expect to be able come close to the performance provided byExadata. That’s because it is the software that provides the lion’s share of theperformance benefit of Exadata. This is easily demonstrated by disablingOffloading on Exadata and running comparisons. This allows us to see theperformance of the hardware without the software enhancements. A big part ofwhat Exadata software does is eliminate totally unnecessary work, such astransferring columns and rows that will eventually be discarded, back to thedatabase servers.As our friend Cary Millsap likes to say, “The fastest way to do anything is to not do it!”What Offloading IncludesThere are many optimizations that can be lumped under the Offloading banner. This chapter focuses onSQL statement optimizations that are implemented via Smart Scans. The big three Smart Scanoptimizations are Column Projection, Predicate Filtering, and Storage Indexes. The primary goal of mostof the Smart Scan optimizations is to reduce the amount of data that needs to be transmitted back to thedatabase servers during scan execution. However, some of the optimizations also attempt to offloadCPU-intensive operations, decompression for example. We won’t have much to say about optimizationsthat are not related to SQL statement processing in this chapter, such as Smart File Creation and RMANrelatedoptimizations. Those topics will be covered in more detail elsewhere in the book.■ Kevin Says: This aspect of Offload Processing seems quite complicated. The authors are correct in stating thatthe primary benefit of Smart Scan is payload reduction between storage and the database grid. And it’s true thatsome CPU-offload benefit is enjoyed by decompressing Exadata Hybrid Columnar Compression units in the storagecells. However, therein lies one case where Offload Processing actually aims to increase the payload between thecells and the database grid. The trade-off is important, however. It makes sense to decompress EHCC data in thecells (after filtration) in spite of the fact that more data is sent to the database grid due to the decompression. Alltechnology solutions have trade-offs.Column ProjectionThe term Column Projection refers to Exadata’s ability to limit the volume of data transferred betweenthe storage tier and the database tier by only returning columns of interest (that is, those in the select list27