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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 | |
