Expert Oracle Exadata - Parent Directory

CHAPTER 6 MAKING PCBS584 bgcmmcyyyvpg9 0 1404581711 4 204.91 select /*+ parallel (a 16) */1157 bgcmmcyyyvpg9 0 1404581711 4 204.91 select /*+ parallel (a 16) */35 rows selected.This listing shows that statements using the NO_STATEMENT_QUEUING hint completely bypass thequeuing mechanism. The steps we executed were to enable queuing, fire off 10 copies of a selectstatement that was hinted to use a DOP of 16. These statements were queued as shown by thequeued_sql.sql script. We then fired off 2 copies of a statement using the NO_STATEMENT_QUEUING hint.The queued_sql.sql script showed that the new statements were not at the bottom of the queue. In fact,they were not queued at all. They were run immediately. The as.sql script shows active sessions, and asyou can see, a side effect of bypassing the queuing is that the number of active slaves can end up beingconsiderably higher than the setting of the PARALLEL_SERVERS_TARGET parameter.Controlling Queuing with Resource ManagerOracle’s Database Resource Manager (DBRM) provides additional capability to control ParallelStatement Queuing. While a thorough discussion of DBRM is beyond the scope of this chapter, we willcover some specific features related to Parallel Query. Chapter 7 covers DBRM in more detail.Without DBRM, the parallel statement queue behaves strictly as a first-in, first-out (FIFO) queue.DBRM provides several directive attributes that can be used to provide additional control on a consumergroup basis. Many of these controls were introduced in version 11.2.0.2. Table 6-3 contains a list ofadditional capabilities provided by DBRM.Table 6-3. DBRM Parallel Statement Queuing ControlsControlSpecify a TimeoutSpecify Maximum DOPManage Order of DequeuingDescriptionThe PARALLEL_QUEUE_TIMEOUT directive attribute can be used to set amaximum queue time for a consumer group. The time limit is set inseconds and once it has expired, the statement will terminate withan error (ORA-07454). Note that this directive did not becomeavailable until version 11.2.0.2 of the database.The PARALLEL_DEGREE_LIMIT_P1 directive attribute sets a maximumnumber of parallel slaves that may be assigned to an individualstatement. This is equivalent to the PARALLEL_DEGREE_LIMIT databaseparameter but is used to set limits for different sets of users based onconsumer groups.The MGMT_P1, MGMT_P2, … MGMT_P8 directive attributes can be used toalter the normal FIFO processing. This attribute allows prioritizationof dequeuing. Each of the eight attributes essentially provides adistinct dequeuing priority level. All statements with an MGMT_P1attribute will be dequeued prior to any statement with MGMT_P2. Inaddition to the dequeuing priority, a probability number can beassigned to regulate dequeuing of statements within the same level.162