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实用心得：Oracle中监控索引的使用

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: 

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意，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 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：

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;

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. 

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;

注意在此为了方便，我们删除部门id为10的记录。如果你删除其他的部门，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： 

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

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。如果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： 

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;

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： 

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');

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。