SAS/ACCESS 9.2 for Relational Databases: Reference, Fourth Edition

Threaded Reads 4 Autopartitioning Techniques in SAS/ACCESS 57Performance Impact of Threaded ReadsThreaded reads only increase performance when the DBMS result set is large.Performance is optimal when the partitions are similar in size. Using threaded readsshould reduce the elapsed time of your SAS step, but unusual cases can slow the SASstep. They generally increase the workload on your DBMS.For example, threaded reads for DB2 under z/OS involve a tradeoff, generallyreducing job elapsed time but increasing DB2 workload and CPU usage. See the autopartitioning documentation for DB2 under z/OS for details.SAS automatically tries to autopartition table references for SAS in threadedapplications. To determine whether autopartitioning is occurring and to assess itsperformance, complete these tasks:3 Turn on SAS tracing to see whether SAS is autopartitioning and to view the SQLassociated with each thread.3 Know your DBMS algorithm for autopartitioning.3 Turn threaded reads on and off, and compare the elapsed times.Follow these guidelines to ensure optimal tuning of threaded reads.3 Use it only when pulling large result sets into SAS from the DBMS.3 Use DBSLICE= to partition if SAS autopartitioning does not occur.3 Override autopartitioning with DBSLICE= if you can manually providesubstantially better partitioning. The best partitioning equally distributes theresult set across the threads.3 See the DBMS-specific reference section in this document for information and tipsfor your DBMS.Threaded reads are most effective on new, faster computer hardware running SAS,and with a powerful parallel edition of the DBMS. For example, if SAS runs on a fastuniprocessor or on a multiprocessor machine and your DBMS runs on a high-end SMPserver, you can experience substantial performance gains. However, you can experienceminimal gains or even performance degradation when running SAS on an old desktopmodel with a nonparallel DBMS edition running on old hardware.Autopartitioning Techniques in SAS/ACCESSSAS/ACCESS products share an autopartitioning scheme based on the MODfunction. Some products support additional techniques. For example, if your Oracletables are physically partitioned in the DBMS, SAS/ACCESS Interface to Oracleautomatically partitions in accordance with Oracle physical partitions rather thanusing MOD. SAS/ACCESS Interface to Teradata uses FastExport, if available, whichlets the FastExport Utility direct partitioning.MOD is a mathematical function that produces the remainder of a division operation.Your DBMS table must contain a column to which SAS can apply the MOD function —a numeric column constrained to integral values. DBMS integer and small integercolumns suit this purpose. Integral decimal (numeric) type columns can work as well.On each thread, SAS appends a WHERE clause to your SQL that uses the MODfunction with the numeric column to create a subset of the result set. Combined, thesesubsets add up to exactly the result set for your original single SQL statement.For example, assume that your original SQL that SAS produced is SELECT CHR1,CHR2 FROM DBTAB and that table Dbtab contains integer column IntCol. SAS createstwo threads and issues: