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

SAS/ACCESS Interface to Teradata 4 Overview 833Table 28.8Read Isolation Levels for TeradataIsolation LevelACCESSREADWRITEDefinitionObtains an ACCESS lock by ignoring other users’ ACCESS,READ, and WRITE locks. Permits other users to obtain a lock onthe table or view.Can return inconsistent or unusual results.Obtains a READ lock if no other user holds a WRITE orEXCLUSIVE lock. Does not prevent other users from reading theobject.Specify this isolation level whenever possible, it is usuallyadequate for most SAS/ACCESS processing.Obtains a WRITE lock on the table or view if no other user has aREAD, WRITE, or EXCLUSIVE lock on the resource. You cannotexplicitly release a WRITE lock. It is released only when the tableis closed. Prevents other users from acquiring any lock butACCESS.This is unnecessarily restrictive, because it locks the entire tableuntil the read operation is finished.UPDATE_ISOLATION_LEVEL= ACCESS | READ | WRITEThe valid values for this option, ACCESS, READ, and WRITE, are defined in thefollowing table.Table 28.9Update Isolation Levels for TeradataIsolation LevelACCESSREADWRITEDefinitionObtains an ACCESS lock by ignoring other users’ ACCESS,READ, and WRITE locks. Avoids a potential deadlock but cancause data corruption if another user is updating the same data.Obtains a READ lock if no other user holds a WRITE orEXCLUSIVE lock. Prevents other users from being granted aWRITE or EXCLUSIVE lock.Locks the entire table or view, allowing other users to acquireREAD locks. Can lead to deadlock situations.Obtains a WRITE lock on the table or view if no other user has aREAD, WRITE, or EXCLUSIVE lock on the resource. You cannotexplicitly release a WRITE lock. It is released only when the tableis closed. Prevents other users from acquiring any lock butACCESS.Prevents all users, except those with ACCESS locks, fromaccessing the table. Prevents the possibility of a deadlock, butlimits concurrent use of the table.These locking options cause the LIBNAME engine to transmit a locking request tothe DBMS; Teradata performs all data-locking. If you correctly specify a set ofSAS/ACCESS read or update locking options, SAS/ACCESS generates locking modifiersthat override the Teradata standard locking.If you specify an incomplete set of locking options, SAS/ACCESS returns an errormessage. If you do not use SAS/ACCESS locking options, Teradata lock defaults are in