如何监控oracle索引的使用详细说明

2/9/2008来源:Oracle教程人气:5890

如何监控索引的使用?
  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$SQL_PLAN使用ADDRESS和HASH_VALUE列 来识别语句, 而计划表使用用户提供得STATEMENT_ID来识别语句。下面的SQL显示了在一个oracle9i数据库中,由出现在共享SQL区中语句使用的所有索引
  [/code]
  select object_owner, object_name, options, count(*)
  from  v$sql_plan
  where operation='INDEX'
  and  object_owner!='SYS'
  group by object_owner, object_name, operation, options
  order by count(*) desc;
  [/code]
  
  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$object_usage: the monitoring has not started yet
  column index_name format a12
  column monitoring format a10
  column used format a4
  column start_monitoring format a19
  column end_monitoring format a19
  select index_name,monitoring,used,start_monitoring,end_monitoring
  from v$object_usage;
  
  no rows selected
  
  (d) Start monitoring of the index usage
  alter index products_pk monitoring usage;
  Index altered.
  
  (e) Query v$object_usage to see the monitoring in progress
  select index_name,monitoring,used,start_monitoring,end_monitoring
  from v$object_usage;
  INDEX_NAME  MONITORING USED START_MONITORING  END_MONITORING
  ---------------------------------------------------------------
  PRODUCTS_PK YES    NO  04/25/2001 15:43:13
  Note: Column MONITORING='YES', START_MONITORING gives the timestamp.
  
  (f) Issue the SELECT statement which uses the index First, make sure that index will
  be used for this statement. Create plan_table in your schema, as required by Oracle
  Autotrace utility:
  @$ORACLE_HOME/rdbms/admin/utlxplan
  Table created.
  Use Oracle Autotrace utility to oBTain the execution plan:
  set autotrace on explain
  select * from products where prod_id = 2;
  
  Execution Plan
  ------------------------------------------------------
  0   SELECT STATEMENT Optimizer=CHOOSE
  1 0 TABLE ACCESS (BY INDEX ROWID) OF 'PRODUCTS'
  2 1  INDEX (UNIQUE SCAN) OF 'PRODUCTS_PK' (UNIQUE)
  set autotrace off Now, since you know the index will be used for this query,
  issue the actual SELECT statement:
  
  select * from products where prod_id = 2;
  PROD_ID  PROD_
  ---------- -----
  2     bbbbb
  
  (g) Query v$object_usage again to see that the index has been used
  select index_name,monitoring,used,start_monitoring,end_monitoring
  from v$object_usage;
  
  INDEX_NAME  MONITORING USED START_MONITORING  END_MONITORING
  ------------ ---------- ---- ------------------- ---- ------------
  PRODUCTS_PK YES    YES 04/25/2001 15:43:13
  Note: Column USED='YES'.
  
  (h) Stop monitoring of the index usage
  alter index products_pk nomonitoring usage;
  Index altered.
  
  (i) Query v$object_usage to see that the monitoring stopped
  select index_name,monitoring,used,start_monitoring,end_monitoring
  from v$object_usage;
  INDEX_NAME  MONITORING USED START_MONITORING  END_MONITORING
  ------------ ---------- ---- ------------------- -------------------
  PRODUCTS_PK NO     YES 04/25/2001 15:43:13 04/25/2001 15:48:44
  Note: Column MONITORING='NO', END_MONITORING gives the timestamp.
  
  下面的PL/SQL块对数据库中的所有索引(SYS和SYSTEM拥有的索引除外)启用监控:
  [/code]
  declare
  l_sql  varchar2(128);
  begin
  for rec in
  (select 'alter index 'owner.'.'index_name' monitoring usage' mon
  from  dba_indexes
  where owner not in ('SYS', 'SYSTEM')
  and  index_type='NORMAL') loop
  l_sql:=rec.mon;
  execute immediate l_sql;
  end loop;
  end;
  /
  [/code]
  
  下面我们来看一下Oracle 9i 这个新特性能不能识别在进行DML操作时外键列上索引的使用情况:
  以9i中HR模式为例:
  标准的dept和emp表是一个与外键关联的父子表的例子。这个例子主要想看一下,在父表上删除一个记录,会不会调用子表上外键上的索引。 首先监控HR模式下所有索引的使用,为了便于在主表上删除一条记录,不违反引用完整性约束。我们首先丢弃原有的约束,重新创建支持级联删除的约束.
  [/code]
  alter table employees drop constraint emp_dept_fk;
  alter table employees add constraint emp_dept_fk foreign key (department_id) references departments on delete cascade;
  alter table job_history drop constraint jhist_emp_fk;
  alter table job_history add constraint jhist_emp_fk foreign key(employee_id) references employees on delete cascade;
  delete from departments where department_id=10;
  [/code]
  注重在此为了方便,我们删除部门id为10的记录。假如你删除其他的部门,可能你还要更改表job_history中相关的约束。
  现在我们看看索引使用的情况:
  [/code]
  select index_name, table_name, monitoring, used
  from  v$object_usage
  where  used='YES'
  
  INDEX_NAME           TABLE_NAME      MON USE
  ------------------------------ -------------------- --- ---
  DEPT_ID_PK           DEPARTMENTS     YES YES
  EMP_EMP_ID_PK         EMPLOYEES      YES YES
  EMP_DEPT_FK          EMPLOYEES       YES YES
  [/code]
  很明显删除父表上的记录,也利用了子表中相关的索引。
  
  v$object_usage 视图的一个异常之处是, 它只能显示属于连接用户的索引的信息。Oracle可能在将来会解决这个问题。假如您的数据库只显示连接用户的对象使用信息,下面的视图(必须被创建为SYS)可用于提供来自任何帐户的所有被监控的索引的信息:
  [/code]
  create or replace view
  V$ALL_OBJECT_USAGE(INDEX_NAME, TABLE_NAME, MONITORING, USED,
  START_MONITORING, END_MONITORING) as
  select io.name, t.name, decode(bitand(i.flags, 65536),0,'NO','YES'),
  decode(bitand(ou.flags,1),0,'NO','YES'), ou.start_monitoring,
  ou.end_monitoring
  from sys.obj$ io, sys.obj$ t, sys.ind$ i, sys.object_usage ou
  where i.obj#=ou.obj#
  and  io.obj#=ou.obj#
  and  t.obj#=i.bo#;
  
  grant select on v$all_object_usage to public;
  
  create public synonym v$all_object_usage for v$all_object_usage;
  [/code]
  
  3、最后我们简单的说一下,如何监控最近被使用的索引
  下列查询将列出最近被访问的索引:
  [/code]
  column owner format a20 trunc
  column segment_name format a30 trunc
  select distinct b.owner, b.segment_name
  from x$bh a, dba_extents b
  where b.file_id=a.dbafil
  and     a.dbablk between b.block_id and b.block_id+blocks-1
  and     segment_type='INDEX'
  and     b.owner not in ('SYS','SYSTEM');
  [/code]
  这个过程可能要耗费一定时间,建议在数据库不太繁忙的时候运行。
  
  to be continued... 2004-05-28
  假如大家觉得浏览不方便, 我可以上传文本文件。
  
  [ Last edited by amaome on 2004-5-28 at 02:59 PM ]