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By default, the database itself automatically collects the necessary optimizer statistics. Every night, the database schedules a statistics collection job during the maintenance window of the Oracle Scheduler. The maintenance window, by default, extends from 10 p.m. to 6 a.m. on weekdays and all weekend as well. The job is named GATHER_STATS_JOB and runs by default in every Oracle Database 11g database. You have the ability to disable the GATHER_STATS_JOB if you wish. You can get details about this default GATHER_STATS_JOB by querying the DBA_SCHEDULER_JOBS view. The GATHER_STATS_JOB collects statistics for all tables that either don t have optimizer statistics or have stale (outdated) statistics. Oracle considers an object s statistics stale if more than 10 percent of its data has changed since the last time it collected statistics for that object. By default, Oracle monitors all DML changes such as inserts, updates, and deletes made to all database objects. You can also view the information about these changes in the DBA_TAB_MODIFICATIONS view. Based on this default object monitoring, Oracle decides whether to collect new statistics for an object. To check that the GATHER_STATS_JOB is indeed collecting statistics on a regular basis, use the following: SQL> SELECT last_analyzed, table_name, owner, num_rows, sample_size 2 FROM dba_tables 3* ORDER by last_analyzed;

TABLE_NAME ----------iR_LICENSE ROLL_AUDIT HISTORY_TAB YTDM_200505 REGS163X_200505 UNITS CAMPAIGN FET$ . . . SQL>

LAST_ANALYZED ----------------------22/JUN/2008 12:38:56 AM 06/JUN/2008 11:34:29 PM 04/JUN/2008 07:28:40 AM 04/JUN/2008 07:23:21 AM 04/JUN/2008 07:23:08 AM 07/JUN/2008 01:18:48 AM 16/JUN/2008 02:01:45 AM 30/JUN/2008 12:03:50 AM

Rendezvous -IsNotificationPending : Boolean -NotificationPayload : String FireEvent()

NUM_ROWS -------142 8179264 388757 113582 115631 33633262 29157889 5692

SAMPLE_SIZE ----------142 5444 88066 6142 5375 5144703 29157889 5692

Note the following points about the preceding output: The job collects statistics during the maintenance window of the database, which is, by default, scheduled between 10 pm and 6 am during weekdays and all weekend The statistics are collected by the nightly GATHER_STATS_JOB run by the Scheduler If a table is created that day, the job uses all the rows of the table the first time it collects statistics for the table The sampling percentage varies from less than 1 percent to 100 percent The size of the table and the percentage of the sample aren t correlated The job doesn t collect statistics for all the tables each day If a table s data doesn t change after it s created, the job never collects a second time.

Check IsNotificationPending() Check IsNotificationPending() Write Payload()

Oracle determines the sample size for each object based on its internal algorithms; there is no standard sample size for all objects Once you verify the collection of statistics, you can pretty much leave statistics collection to the database and focus your attention elsewhere This way, you can potentially run huge production databases for years on end, without ever having to run a manual statistics collection job using the DBMS_STATS package Of course, if you load data during the day, or after the GATHER_STATS_JOB starts running, you ll miss the boat and the object won t have any statistics collected for it Therefore, keep any eye on objects that might undergo huge changes during the day You might want to schedule a statistics collection job right after the data changes occur.

In addition, you can provide the necessary statistics to the optimizer with the DBMS_STATS package yourself (the automatic statistics collection process managed by the GATHER_STATS_JOB uses the same package internally to collect statistics), which you ll learn about later on in this chapter The necessary statistics are as follows: The number of rows in a table The number of rows per database block The average row length The total number of database blocks in a table The number of levels in each index The number of leaf blocks in each index The number of distinct values in each column of a table Data distribution histograms.

Set Notification Flag() Get Payload() Set IsNotificationPending() Blocks Until IsNotificationPending Is Set