A. Because the PGA_AGGREGATE_TARGET parameter is not set<??http://www.2cto.com/kf/ware/vc/" target="_blank" class="keylink">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"brush:sql;"> statistics_level 参数是oracle9.2开始引入的一个控制系统统计参数收集的一个开关.一共有三个值:basic,typical,all.支持alter session,alter system 动态修改.如果要用statspack或者AWR收集系统性能统计数据.那么这个参数的值必须为typical或all.通常all是一个全面收集,包括 OS以及sql执行路径方面的一些统计信息,除非遇见非常严重的性能问题或在一些特殊的性能挣断方面才会用到statistics_level=all, 平常statistics_level=typeical已经足够诊断99%的性能问题了. example: seting statistics_level alter system set statistics_level=basic; alter system set statistics_level=typical; alter system set statistics_level=all; or alter session set statistics_level=basic; alter session set statistics_level=typical; alter session set statistics_level=all; statistics_level=basic的情况下,oracle关闭了所有性能数据的收集,也就是如果要关闭AWR或statspack收集,只要设置alter system set statistics_level=basic;就行了; statistics_level=typical的时 候,除了plan_executetion_statistics和OS Statistics不能收集外,其他的都可以收集,如要要收集这个两项,必须设置statistics_level=all; 如果当statistics_level=ALL,系统收集所有的统计信息. 设置 STATISTICS_LEVEL = BASIC 会禁用一下oracle 的功能特性 Automatic Workload Repository (AWR) Snapshots Automatic Database Diagnostic Monitor (ADDM) All server-generated alerts Automatic SGA Memory Management （ASMM）和自动内存管理 Automatic optimizer statistics collection Object level statistics End to End Application Tracing (V$CLIENT_STATS) Database time distribution statistics (V$SESS_TIME_MODEL and V$SYS_TIME_MODEL) Service level statistics Buffer cache advisory MTTR advisory Shared pool sizing advisory Segment level statistics PGA Target advisory Timed statistics Monitoring of statistics