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Tham khảo tài liệu 'oca: oracle database 11g administrator certified associate- p20', công nghệ thông tin, cơ sở dữ liệu phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Proactive Database Maintenance 781 EXTSTAT SYS_STUZVS6GX30A0GN_5YRYSD2LPM SQL SQL exec dbms_stats.gather_table_stats null customers method_opt for all columns size skewonly PL SQL procedure successfully completed. SQL select column_name num_distinct histogram 2 from user_tab_col_statistics 3 where table_name CUSTOMERS SQL COLUMN_NAME NUM_DISTINCT HISTOGRAM CUST_NAME 47692 HEIGHT BALANCED CUST_STATE 6 FREQUENCY CUST_COUNTRY 3 FREQUENCY SYS_STUZVS6GX30A0GN_5YRYSD2LPM 8 FREQUENCY SQL select from customers where cust_country India and cust_state TN I Id I Operation Name Rows Bytes Cost CPU Time I 0 SELECT STATEMENT I 86 2580 137 1 00 00 o2 I 1 1 TABLE ACCESS FULL CUSTOMERS 86 2580 137 1 00 00 02 Predicate Information identified by operation id 1 - filter CUST_STATE TN AND CUST_COUNTRY India As you can see in the example before extended statistics were collected the estimated number of rows was 1447 whereas after the extended statistics collection the number of rows optimizer estimated to return is 86. 782 Chapter 14 Maintaining the Database and Managing Performance To drop the extend statistics use the DROP_EXTENDED_STATISTICS procedure SQL exec dbms_stats.drop_extended_stats null CUSTOMERS CUST_COUNTRY CUST_STATE PL SQL procedure successfully completed. SQL To define the extension and collect statistics in one step you can do the following SQL exec dbms_stats.gather_table_stats null customers method_opt for all columns size skewonly for columns cust_country cust_state PL SQL procedure successfully completed. SQL select extension_name extension from user_stat_extensions 2 where table_name CUSTOMERS EXTENSION_NAME EXTENSION SYS_STUZVS6GX30A0GN_5YRYSD2LPM CUST_COUNTRY CUST_STATE In the next section you ll learn to enable and disable the automatic statistics collection as well as perform other AutoTask jobs. Configuring Automated Maintenance Tasks Using EM The following are three default automated maintenance tasks Û Gathering optimizer statistics Running the Segment .
