研究發現，Oracle數據庫使用的索引不會超過總數的25%，或者不易他們期望被使用的方式使用。通過監控數據庫索引的使用，釋放那些未被使用的索引，從而節省維護索引的開銷，優化性能。
1、在Oracle8i中，確定使用了那個索引的方法意味著要對存在語共享SQL區中的所有語句運行EXPLIAN PALN，然後查詢計劃表中的OperaTION列，從而識別有OBJECT_OWNER和OBJECT_NAME列所確定的那個索引上的索引訪問。
下面是一個監控索引使用的腳本，這個腳本僅僅是一個樣品，在某種條件下成立：
條件：
運行這個腳本的用戶擁有權限解釋所有的v$sqlarea中的sql，除了不是被SYS裝載的。
plan_table.remarks能夠別用來決定與特權習慣的錯誤。
對所有的共享池中SQL，參數OPTIMIZER_GOAL是一個常量，無視v$sqlarea.optimizer_mode。
兩次快照之間，統計資料被再次分析過。
沒有語句別截斷。
所有的對象都是局部的。
所有被引用的表或視圖或者是被運行腳本的用戶所擁有，或者完全有資格的名字或同義詞被使用。
自從上次快照以來，沒有不受"歡迎"的語句被沖洗出共享池(例如，在裝載)。
對於所有的語句， v$sqlarea.version_count = 1 (children)。
腳本:
CODE:
set echo off Rem Drop and recreate PLAN_TABLE for EXPLAIN PLAN drop table plan_table; create table PLAN_TABLE ( statement_id varchar2(30), timestamp date, remarks varchar2(80), Operation varchar2(30), options varchar2(255), object_node varchar2(128), object_owner varchar2(30), object_name varchar2(30), object_instance numeric, object_type varchar2(30), optimizer varchar2(255), search_columns number, id numeric, parent_id numeric, position numeric, cost numeric, cardinality numeric, bytes numeric, other_tag varchar2(255), partition_start varchar2(255), partition_stop varchar2(255), partition_id numeric, other long, distribution varchar2(30), cpu_cost numeric, io_cost numeric, temp_space numeric, Access_predicates varchar2(4000), filter_predicates varchar2(4000)); Rem Drop and recreate SQLTEMP for taking a snapshot of the SQLAREA drop table sqltemp; create table sqltemp ( ADDR VARCHAR2 (16), SQL_TEXT VARCHAR2 (2000), DISK_READS NUMBER, EXECUTIONS NUMBER, PARSE_CALLS NUMBER); set echo on Rem Create procedure to populate the plan_table by executing Rem explain plan...for 'sqltext' dynamically create or replace procedure do_explain ( addr IN varchar2, sqltext IN varchar2) as dummy varchar2 (1100); mycursor integer; ret integer; my_sqlerrm varchar2 (85); begin dummy:='EXPLAIN PLAN SET STATEMENT_ID=' ; dummy:=dummy||''''||addr||''''||' FOR '||sqltext; mycursor := dbms_sql.open_cursor; dbms_sql.parse(mycursor,dummy,dbms_sql.v7); ret := dbms_sql.execute(mycursor); dbms_sql.close_cursor(mycursor); commit; exception -- Insert errors into PLAN_TABLE... when others then my_sqlerrm := substr(sqlerrm,1,80); insert into plan_table(statement_id,remarks) values (addr,my_sqlerrm); -- close cursor if exception raised on EXPLAIN PLAN dbms_sql.close_cursor(mycursor); end; / Rem Start EXPLAINing all S/I/U/D statements in the shared pool declare -- exclude statements with v$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$sqlarea.parse_calls%type; begin open c1; fetch c1 into addr2,sqltext,dreads,execs,pcalls; while (c1%found) loop insert into sqltemp values(addr2,sqltext,dreads,execs,pcalls); commit; fetch c1 into addr2,sqltext,dreads,execs,pcalls; end loop; close c1; open c2; fetch c2 into addr2, sqltext; while (c2%found) loop do_explain(addr2,sqltext); fetch c2 into addr2, sqltext; end loop; close c2; end; / Rem Generate a report of index usage based on the number of times Rem a SQL statement using that index was executed select p.owner, p.name, sum(s.executions) totexec from sqltemp s, (select distinct statement_id stid, object_owner owner, object_name name from plan_table where Operation = 'INDEX') p where s.addr = p.stid group by p.owner, p.name order by 2 desc; Rem Perform cleanup on exit (optional) delete from plan_table where statement_id in ( select addr from sqltemp ); drop table sqltemp; 關於這個腳本，有幾個重要的地方需要注意，即它可能一起明顯的開銷，因此，應該在仔細地進行 權衡後才把它應用到繁忙的生產應用系統中區。
2、 Oracle9i中如何確定索引的使用情況
在oracle9i中，情況會簡單得多，因為有一個新得字典視圖V$SQL_PLAN存儲了實際計劃，這些計劃用於執行共享SQL區中得語句。V$SQL_PLAN視圖很類似與計劃表，但V$SQL_PLAN使用ADDRESS和HASH_VALUE列 來識別語句， 而計劃表使用用戶提供得STATEMENT_ID來識別語句。下面的SQL顯示了在一個Oracle9i數據庫中，由出現在共享SQL區中語句使用的所有索引。
select object_owner, object_name, options, count(*)
from v$sql_plan
where Operation='INDEX'
and object_owner!='SYS'
group by object_owner, object_name, Operation, options
order by count(*) desc;
所有基於共享SQL區中的信心來識別索引使用情況的方法， 都可能會收集到不完整的信息。共享SQL區是一 個動態結構，除非能對它進行足夠頻繁的采樣， 否則在有關索引使用的情況的信息被收集之前，SQL語句可 能就已經(因為老化)被移出緩存了。Oracle9i提供了解決這個問題的方案，即它為alter index提供了一個monitoring usage子句。當啟用monitoring usage 時，oralce記錄簡單的yes或no值，以指出在監控間隔 期間某個索引是否被使用。
為了演示這個新特性，你可以使用下面的例子:
(a) Create and populate a small test table
(b) Create Primary Key index on that table
(c) Query v$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塊對數據庫中的所有索引(SYS和SYSTEM擁有的索引除外)啟用監控：
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;
下面我們來看一下Oracle 9i 這個新特性能不能識別在進行DML操作時外鍵列上索引的使用情況：
以9i中HR模式為例：
標准的dept和emp表是一個與外鍵關聯的父子表的例子。這個例子主要想看一下，在父表上刪除一個記錄，會不會調用子表上外鍵上的索引。 首先監控HR模式下所有索引的使用，為了便於在主表上刪除一條記錄，不違反引用完整性約束。我們首先丟棄原有的約束，重新創建支持級聯刪除的約束.
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; 注意在此為了方便，我們刪除部門id為10的記錄。如果你刪除其他的部門，可能你還要更改表job_history中相關的約束。
現在我們看看索引使用的情況：
select index_name, table_name, monitoring, used from v$object_usage where used='YES' INDEX_NAME TABLE_NAME MON USE ------------------------------ -------------------- --- --- DEPT_ID_PK DEPARTMENTS YES YES EMP_EMP_ID_PK EMPLOYEES YES YES EMP_DEPT_FK EMPLOYEES YES YES 很明顯刪除父表上的記錄，也利用了子表中相關的索引。
v$object_usage 視圖的一個異常之處是， 它只能顯示屬於連接用戶的索引的信息。Oracle可能在將來會解決這個問題。如果您的數據庫只顯示連接用戶的對象使用信息，下面的視圖(必須被創建為SYS)可用於提供來自任何帳戶的所有被監控的索引的信息：
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$all_object_usage; 3、最後我們簡單的說一下，如何監控最近被使用的索引
下列查詢將列出最近被訪問的索引：
column owner format a20 trunc column segment_name format a30 trunc select distinct b.owner, b.segment_name from x$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'); 這個過程可能要耗費一定時間，建議在數據庫不太繁忙的時候運行。
