Expert Oracle Exadata - Parent Directory

C H A P T E R 6Exadata Parallel OperationsExadata doesn’t have a special way of executing parallel operations that is not available on otherplatforms running 11gR2. However, parallel processing is a key component of Exadata because efficienthandling of Data Warehouse workloads was a primary design goal for Exadata. In addition, becauseOffloading/Smart Scan depends on direct path reads, which are used by parallel query slaves, paralleloperations take on a whole new importance. Traditionally, the use of parallel query has required carefulcontrol of concurrency in order to maximize the use of available resources without overwhelming thesystem. Oracle’s previous attempts at throttling parallel operations to allow them to be used in multiuserenvironments have not been entirely successful. 11gR2 provides some new capabilities for controllingparallel operations. In particular, a queuing mechanism has been introduced that allows the number ofconcurrent parallel processes to be managed more effectively. This approach appears to be much bettersuited to allowing a high degree of parallelism without overwhelming the available resources thanprevious attempts. 11gR2 also introduced the ability for Oracle to automatically calculate a degree ofparallelism on a statement-by-statement basis.This chapter is about parallel query. Other forms of parallelism provided by the database, such asrecovery parallelism, will not be covered here. We will briefly cover parallel query mechanics anddemonstrate specifically how the queuing mechanism and automatic degree of parallelism work withExadata. This chapter will also briefly cover the new 11gR2 In-Memory Parallel Execution feature anddiscuss how Exadata storage parallelizes I/O operations. It will not cover all the details of parallel queryor parallel DML and assumes that the reader is familiar with basic Oracle parallel concepts. Note alsothat the discussion and examples will refer to behavior in Oracle Database 11g Release 2 (11.2.0.2). Inmost cases, the comments apply equally to version 11.2.0.1 as well. Exceptions are explicitly called out.ParametersBefore describing the various new features and how they apply to Exadata, you should review theparameters that affect how parallel queries are handled by the database. Table 6-1 shows the parametersalong with a brief description of each one’s purpose.143