研究发现，Oracle数据库使用的索引不会超过总数的25%，或者不易他们期望被使用的方式使用。通过 监控数据库索引的使用，释放那些未被使用的索引，从而节省维护索引的开销，优化性能。 1、在oracle8i中，确定使用了那个索引的方法意味着要对存在语共享SQL区中的所有语句运行EXPLIAN PALN，然后查询计划表中的OperaTION列，从而识别有OBJECT_OWNER和OBJECT_NAME列所确定的那个索引上的索引访问。 下面是一个监控索引使用的脚本，这个脚本仅仅是一个样品，在某种条件下成立： 条件： 运行这个脚本的用户拥有权限解释所有的v$sqlarea中的sql，除了不是被SYS装载的。plan_table.remarks能够别用来决定与特权习惯的错误。对所有的共享池中SQL，参数OPTIMIZER_GOAL是一个常量，无视v$sqlarea.optimizer_mode。两次快照之间，统计资料被再次分析过。没有语句别截断。所有的对象都是局部的。所有被引用的表或视图或者是被运行脚本的用户所拥有，或者完全有资格的名字或同义词被使用。自从上次快照以来，没有不受"欢迎"的语句被冲洗出共享池(例如，在装载)。对于所有的语句，v$sqlarea.version_count = 1 (children)。 脚本: Code: [Copy to clipboard] set echo off Rem Drop and recreate PLAN_TABLE for EXPLAIN PLAN drop table plan_table; create table PLAN_TABLE ( statement_id varchar2(30), timestamp date, remarks varchar2(80), operation varchar2(30), options varchar2(255), object_node varchar2(128), object_owner varchar2(30), object_name varchar2(30), object_instance numeric, object_type varchar2(30), optimizer varchar2(255), search_columns number, id numeric, parent_id numeric, position numeric, cost numeric, cardinality numeric, bytes numeric, other_tag varchar2(255), partition_start varchar2(255), partition_stop varchar2(255), partition_id numeric, other long, distribution varchar2(30), cpu_cost numeric, io_cost numeric, temp_space numeric, access_PRedicates varchar2(4000), filter_predicates varchar2(4000)); Rem Drop and recreate SQLTEMP for taking a snapshot of the SQLAREA drop table sqltemp; create table sqltemp ( ADDR VARCHAR2 (16), SQL_TEXT VARCHAR2 (2000), DISK_READS NUMBER, EXECUTIONS NUMBER, PARSE_CALLS NUMBER); set echo on Rem Create procedure to populate the plan_table by executing Rem explain plan...for 'sqltext' dynamically create or replace procedure do_explain ( addr IN varchar2, sqltext IN varchar2) as dummy varchar2 (1100); mycursor integer; ret integer; my_sqlerrm varchar2 (85); begin dummy:='EXPLAIN PLAN SET STATEMENT_ID=' ; dummy:=dummy''''addr'''' ' FOR 'sqltext; mycursor := dbms_sql.open_cursor; dbms_sql.parse(mycursor,dummy,dbms_sql.v7); ret := dbms_sql.execute(mycursor); dbms_sql.close_cursor(mycursor); commit; exception -- Insert errors into PLAN_TABLE... when others then my_sqlerrm := substr(sqlerrm,1,80); insert into plan_table(statement_id,
remarks) values (addr,my_sqlerrm); -- close cursor if exception raised on EXPLAIN PLAN dbms_sql.close_cursor(mycursor); end; / Rem Start EXPLAINing all S/I/U/D statements in the shared pool declare -- exclude statements with v$sqlarea.parsing_schema_id = 0 (SYS) cursor c1 is select address, sql_text, DISK_READS, EXECUTIONS, PARSE_CALLS from v$sqlarea where command_type in (2,3,6,7) and parsing_schema_id != 0; cursor c2 is select addr, sql_text from sqltemp; addr2 varchar(16); sqltext v$sqlarea.sql_text%type; dreads v$sqlarea.disk_reads%type; execs v$sqlarea.executions%type; pcalls v$sqlarea.parse_calls%type; begin open c1; fetch c1 into addr2,sqltext, dreads,execs,pcalls; while (c1%found) loop insert into sqltemp values (addr2,sqltext,dreads,execs,pcalls); commit; fetch c1 into addr2, sqltext,dreads,execs,pcalls; end loop; close c1; open c2; fetch c2 into addr2, sqltext; while (c2%found) loop do_explain(addr2,sqltext); fetch c2 into addr2, sqltext; end loop; close c2; end; / Rem Generate a report of index usage based on the number of times Rem a SQL statement using that index was executed select p.owner, p.name, sum(s.executions) totexec from sqltemp s, (select distinct statement_id stid, object_owner owner, object_name name from plan_table where operation = 'INDEX') p where s.addr = p.stid group by p.owner, p.name order by 2 desc; Rem Perform cleanup on exit (optional) delete from plan_table where statement_id in ( select addr from sqltemp ); drop table sqltemp;
关于这个脚本，有几个重要的地方需要注重，即它可能一起明显的开销，因此，应该在仔细地进行 权衡后才把它应用到繁忙的生产应用系统中区。 2、oracle9i中如何确定索引的使用情况: 在oracle9i中，情况会简单得多，因为有一个新得字典视图V$SQL_PLAN存储了实际计划，这些计划用于执行共享SQL区中得语句。V$SQL_PLAN视图很类似与计划表，但V$SQL_PLAN使用ADDRESS和HASH_VALUE列 来识别语句， 而计划表使用用户提供得STATEMENT_ID来识别语句。下面的SQL显示了在一个oracle9i数据库中，由出现在共享SQL区中语句使用的所有索引。 select object_owner, object_name, options, count(*) from v$sql_plan where operation='INDEX' and object_owner!='SYS' group by object_owner, object_name, operation, options order by count(*) desc;
所有基于共享SQL区中的信心来识别索引使用情况的方法， 都可能会收集到不完整的信息。共享SQL区是一 个动态结构，除非能对它进行足够频繁的采样， 否则在有关索引使用的情况的信息被收集之前，SQL语句可 能就已经(因为老化)被移出缓存了。oracle9i提供了解决这个问题的方案，即它为alter index提供了一个monitoring usage子句。当启用monitoring usage 时，oralce记录简单的yes或no值，以指出在监控间隔 期间某个索引是否被使用。 为了演示这个新特性，你可以使用下面的例子: (a) Create and populate a small test table (b) Create Primary Key index on that table (c) Query v$object_usage: the monitoring has not started yet (d) Start monitoring of the index usage (e) Query v$object_usage to see the monitoring in progress
(f) Issue the SELECT statement which uses the index (g) Query v$object_usage again to see that the index has been used (h) Stop monitoring of the index usage (i) Query v$object_usage to see that the monitoring sDetailed steps (a) Create and populate a small test table create table prodUCts ( prod_id number(3), prod_name_code varchar2(5)); insert into products values(1,'aaaaa'); insert into products values(2,'bbbbb'); insert into products values(3,'ccccc'); insert into products values(4,'ddddd'); commit; (b) Create Primary Key index on that table alter table products add (constraint products_pk primary key (prod_id)); (c) Query v$object_usage: the monitoring has not started yet column
