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

51CHAPTER6Threaded ReadsOverview of Threaded Reads in SAS/ACCESS 51Underlying Technology of Threaded Reads 51SAS/ACCESS Interfaces and Threaded Reads 52Scope of Threaded Reads 52Options That Affect Threaded Reads 53Generating Trace Information for Threaded Reads 54Performance Impact of Threaded Reads 57Autopartitioning Techniques in SAS/ACCESS 57Data Ordering in SAS/ACCESS 58Two-Pass Processing for SAS Threaded Applications 58When Threaded Reads Do Not Occur 59Summary of Threaded Reads 59Overview of Threaded Reads in SAS/ACCESSIn SAS 8 and earlier, SAS opened a single connection to the DBMS to read a table.SAS statements requesting data were converted to an SQL statement and passed to theDBMS. The DBMS processed the SQL statement, produced a result set consisting oftable rows and columns, and transferred the result set back to SAS on the singleconnection.With a threaded read, you can reduce the table read time by retrieving the result seton multiple connections between SAS and the DBMS. SAS can create multiple threads,and a read connection is established between the DBMS and each SAS thread. Theresult set is partitioned across the connections, and rows are passed to SASsimultaneously (in parallel) across the connections, which improves performance.Underlying Technology of Threaded ReadsTo perform a threaded read, SAS first creates threads within the SAS session.Threads are standard operating system tasks that SAS controls. SAS then establishes aDBMS connection on each thread, causes the DBMS to partition the result set, andreads one partition per thread. To cause the partitioning, SAS appends a WHEREclause to the SQL so that a single SQL statement becomes multiple SQL statements,one for each thread. Here is an example.