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

58 Data Ordering in SAS/ACCESS 4 Chapter 6andSELECT CHR1, CHR2 FROM DBTAB WHERE (MOD(INTCOL,2)=0)SELECT CHR1, CHR2 FROM DBTAB WHERE (MOD(INTCOL,2)=1)Rows with an even value for IntCol are retrieved by the first thread. Rows with an oddvalue for IntCol are retrieved by the second thread. Distribution of rows across the twothreads is optimal if IntCol has a 50/50 distribution of even and odd values.SAS modifies the SQL for columns containing negative integers, for nullablecolumns, and to combine SAS WHERE clauses with the partitioning WHERE clauses.SAS can also run more than two threads. You use the second parameter of theDBSLICEPARM= option to increase the number of threads.The success of this technique depends on the distribution of the values in the chosenintegral column. Without knowledge of the distribution, your SAS/ACCESS productattempts to pick the best possible column. For example, indexed columns are givenpreference for some DBMSs. However, column selection is more or less a guess, and theSAS guess might cause poor distribution of the result set across the threads. If nosuitable numeric column is found, MOD cannot be used at all, and threaded reads willnot occur if your SAS/ACCESS product has no other partitioning technique. For thesereasons, you should explore autopartitioning particulars for your DBMS and judiciouslyuse DBSLICE= to augment autopartitioning. See the information for your DBMS forspecific autopartitioning details.3 Aster nCluster3 DB2 Under UNIX and PC Hosts3 DB2 Under z/OS3 Greenplum3 HP Neoview3 Informix3 ODBC3 Oracle (not supported under z/OS)3 Sybase3 Sybase IQ3 Teradata (supports only FastExport threaded reads on z/OS and UNIX; seeTeradata documentation for details)Data Ordering in SAS/ACCESSThe order in which table rows are delivered to SAS varies each time a step is rerunwith threaded reads. Most DBMS editions, especially increasingly popular paralleleditions, do not guarantee consistent ordering.Two-Pass Processing for SAS Threaded ApplicationsTwo-pass processing occurs when a SAS Teradata requests that data be madeavailable for multiple pass reading (that is, more than one pass through the data set).In the context of DBMS engines, this requires that as the data is read from thedatabase, temporary spool files are written containing the read data. There is onetemporary spool file per thread, and each spool file will contain all data read on that