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

SAS/ACCESS Interface to Teradata 4 Using the PreFetch Facility 801How PreFetch WorksWhen reading DBMS tables, SAS/ACCESS submits SQL statements on your behalfto Teradata. Each SQL statement that is submitted has an execution cost: the amountof time Teradata spends processing the statement before it returns the requested datato SAS/ACCESS.When PreFetch is enabled the first time you run your SAS job, SAS/ACCESSidentifies and selects statements with a high execution cost. SAS/ACCESS then stores(caches) the selected SQL statements to one or more Teradata macros that it creates.On subsequent runs of the job, when PreFetch is enabled, SAS/ACCESS extractsstatements from the cache and submits them to Teradata in advance. The rows thatthese SQL statements select are immediately available to SAS/ACCESS becauseTeradata prefetches them. Your SAS job runs faster because PreFetch reduces the waitfor SQL statements with a high execution cost. However, PreFetch improves elapsedtime only on subsequent runs of a SAS job. During the first run, SAS/ACCESS onlycreates the SQL cache and stores selected SQL statements; no prefetching is performed.The PreFetch Option Argumentsunique_storenameAs mentioned, when PreFetch is enabled, SAS/ACCESS creates one or moreTeradata macros to store the selected SQL statements that PreFetch caches. Youcan easily distinguish a PreFetch macro from other Teradata macros. ThePreFetch Teradata macro contains a comment that is prefaced with this text:"SAS/ACCESS PreFetch Cache"The name that the PreFetch facility assigns for the macro is the value that youenter for the unique_storename argument. The unique_storename must be unique.Do not specify a name that exists in the Teradata DBMS already for a DBMStable, view, or macro. Also, do not enter a name that exists already in anotherSAS job that uses the Prefetch facility.#sessionsThis argument specifies how many cached SQL statements SAS/ACCESS submitsin parallel to Teradata. In general, your SAS job completes faster if you increasethe number of statements that Teradata works on in advance. However, a largenumber (too many sessions) can strain client and server resources. A valid valueis 1 through 9. If you do not specify a value for this argument, the default is 3.In addition to the specified number of sessions, SAS/ACCESS adds anadditional session for submitting SQL statements that are not stored in thePreFetch cache. Thus, if the default is 3, SAS/ACCESS actually opens up to foursessions on the Teradata server.algorithmThis argument is present to handle future enhancements. Currently PreFetch onlysupports one algorithm, SEQUENTIAL.When and Why Use PreFetchIf you have a read-only SAS job that runs frequently, this is an ideal candidate forPreFetch; for example, a daily job that extracts data from Teradata tables. To help youdecide when to use PreFetch, consider these daily jobs:3 Job 1Reads and collects data from the Teradata DBMS.