db file scattered read 等待事件
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参考自盖国强《深入解析oracle》
这个事件表明用户进程正在读数据到 Buffer Cache 中,等待直到物理 I/O调用返回。DB File Scattered Read 发出离散读,将存储上连续的数据块离散的读入到多个不连续的内存位置。Scattered Read 通常是多块读,在 Full Table Scan 或 Fast Full Scan 等访问方式下使用。每次多块读读取的数据块数量受初始化参数 DB_FILE_MULTIBLOCK_READ_COUNT 限制。根据经验,通常大量的 db file scattered read 等待可能意味着应用问题或者索引缺失。在实际环境的诊断过程中,可以通过 v$session_wait 视图发现 Session 的等待,再结合其他视图找到存在问题的 SQL 等根本原因,从而从根本上解决问题。获得全表扫描的对象SQL> select distinct object_name,object_owner from v$sql_plan p 2 where p.operation='TABLE ACCESS' and p.options='FULL' 3 and object_owner = 'TEST';OBJECT_NAME OBJECT_OWNER------------------------------ ------------------------------WORKER TEST获得全索引扫描对象SQL> select distinct object_name,object_owner from v$sql_plan p 2 where p.operation='INDEX' and p.options='FULL SCAN' ;OBJECT_NAME OBJECT_OWNER------------------------------ ------------------------------SCHEDULER$_WINGRP_MEMBER_UQ SYSWWV_FLOW_MAIL_QUEUE_PK APEX_030200I_UNDO1 SYSAQ$_SUBSCRIBER_TABLE_PRIMARY SYSI_USER# SYSBSLN_METRIC_DEFAULTS_PK DBSNMPBSLN_TIMEGROUPS_PK DBSNMP查找全表扫描的 SQL 语句可以参考如下语句:SELECT sql_text FROM v$sqltext t, v$sql_plan pWHERE t.hash_value = p.hash_value AND p.operation = 'TABLE ACCESS' AND p.options = 'FULL'ORDER BY p.hash_value, t.piece;查找 Fast Full Index 扫描的 SQL 语句可以参考如下语句:SELECT sql_text FROM v$sqltext t, v$sql_plan pWHERE t.hash_value = p.hash_value AND p.operation = 'INDEX' AND p.options = 'FULL SCAN'ORDER BY p.hash_value, t.piece;
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