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

Performance Considerations 4 Sorting DBMS Data 37Repeatedly Accessing DataCAUTION:If you need to access the most current DBMS data, access it directly from the databaseevery time. Do not follow the extraction suggestions in this section. 4It is sometimes more efficient to extract (copy) DBMS data to a SAS data file than torepeatedly read the data by using a SAS view. SAS data files are organized to provideoptimal performance with PROC and DATA steps. Programs that use SAS data files areoften more efficient than SAS programs that read DBMS data directly.Consider extracting data when you work with a large DBMS table and plan to use thesame DBMS data in several procedures or DATA steps during the same SAS session.You can extract DBMS data to a SAS data file by using the OUT= option, a DATAstep, or ACCESS procedures.Sorting DBMS DataSorting DBMS data can be resource-intensive—whether you use the SORTprocedure, a BY statement, or an ORDER BY clause on a DBMS data source or in theSQL procedure SELECT statement. Sort data only when it is needed for your program.Here are guidelines for sorting data.3 If you specify a BY statement in a DATA or PROC step that references a DBMSdata source, it is recommended for performance reasons that you associate the BYvariable (in a DATA or PROC step) with an indexed DBMS column. If youreference DBMS data in a SAS program and the program includes a BY statementfor a variable that corresponds to a column in the DBMS table, the SAS/ACCESSLIBNAME engine automatically generates an ORDER BY clause for that variable.The ORDER BY clause causes the DBMS to sort the data before the DATA orPROC step uses the data in a SAS program. If the DBMS table is very large, thissorting can adversely affect your performance. Use a BY variable that is based onan indexed DBMS column in order to reduce this negative impact.3 The outermost BY or ORDER BY clause overrides any embedded BY or ORDERBY clauses, including those specified by the DBCONDITION= option, thosespecified in a WHERE clause, and those in the selection criteria in a viewdescriptor. In the following example, the EXEC_EMPLOYEES data set includes aBY statement that sorts the data by the variable SENIORITY. However, when thatdata set is used in the following PROC SQL query, the data is ordered by theSALARY column and not by SENIORITY.libname mydblib oracle user=testuser password=testpass;data exec_employees;set mydblib.staff (keep=lname fname idnum);by seniority;where salary >= 150000;run;proc sql;select * from exec_employeesorder by salary;3 Do not use PROC SORT to sort data from SAS back into the DBMS because thisimpedes performance and has no effect on the order of the data.