ORACLE中查看执行计划
转载地址：http://hi.baidu.com/xyooo/blog/item/eba0dc1f40072967f624e49e.html
有三种方法：
explain plan for
select * from aa;
查看结果：
select * from table(dbms_xplan.display());
3. SQL_TRACE
ORACLE SQL_TRACE
“SQL TRACE”是Oracle提供的用于进行SQL跟踪的手段，是强有力的辅助诊断工具。在日常的数据库问题诊断和解决中，“SQL TRACE”是非常常用的方法。
一般，一次跟踪可以分为以下几步：
1、界定需要跟踪的目标范围，并使用适当的命令启用所需跟踪。
2、经过一段时间后，停止跟踪。此时应该产生了一个跟踪结果文件。
3、找到跟踪文件，并对其进行格式化，然后阅读或分析。
本文就“SQL TRACE”的这些使用作简单探讨，并通过具体案例对SQL_TRACE的使用进行说明。
一、“SQL TRACE”的启用。
(A)SQL_TRACE说明
SQL_TRACE可以作为初始化参数在全局启用，也可以通过命令行方式在具体session启用。
1． 在全局启用
在参数文件(pfile/spfile)中指定: SQL_TRACE = true
在全局启用SQL_TRACE会导致所有进程的活动被跟踪，包括后台进程及所有用户进程，这通常会导致比较严重的性能问题，所以在生产环境中要谨慎使用。
提示: 通过在全局启用SQL_TRACE，我们可以跟踪到所有后台进程的活动，很多在文档中的抽象说明，通过跟踪文件的实时变化，我们可以清晰的看到各个进程之间的紧密协调。
2． 在当前session级设置
大多数时候我们使用SQL_TRACE跟踪当前会话的进程。通过跟踪当前进程可以发现当前操作的后台数据库递归活动(这在研究数据库新特性时尤其有效)，研究SQL执行，发现后台错误等。
在session级启用和停止SQL_TRACE方式如下:
启用当前session的跟踪:
SQL> alter session set SQL_TRACE=true;
Session altered.
此时的SQL操作将被跟踪:
SQL> select count(*) from dba_users;
COUNT(*)
----------
34
结束跟踪:
SQL> alter session set SQL_TRACE=false;
Session altered.
3． 跟踪其它用户进程
在很多时候我们需要跟踪其他用户的进程，而不是当前用户，这可以通过Oracle提供的系统包DBMS_SYSTEM.SET_SQL_TRACE_IN_SESSION来完成
通过v$session我们可以获得sid、serial#等信息:
获得进程信息，选择需要跟踪的进程:
SQL> select sid,serial#,username from v$session where username =’***’
SID SERIAL# USERNAME
---------- ---------- ------------------------------
8 2041 SYS
9 437 EYGLE
设置跟踪:
SQL> exec dbms_system.set_SQL_TRACE_in_session(9,437,true)
PL/SQL procedure successfully completed.
….
可以等候片刻，跟踪session执行任务,捕获sql操作…
….
停止跟踪:
SQL> exec dbms_system.set_SQL_TRACE_in_session(9,437,false)
PL/SQL procedure successfully completed.
(B) 10046事件说明
10046事件是Oracle提供的内部事件，是对SQL_TRACE的增强.
10046事件可以设置以下四个级别:
1 - 启用标准的SQL_TRACE功能,等价于SQL_TRACE。
4 - Level 1 加上绑定值(bind values)
8 - Level 1 + 等待事件跟踪
12 - Level 1 + Level 4 + Level 8
类似SQL_TRACE方法，10046事件可以在全局设置，也可以在session级设置。
对于10046事件的设置，涉及到了oracle的“诊断事件”的概念。
可以参考以下链接了解详情。http://www.itpub.net/323537,1.html
1． 在全局设置
在参数文件中增加:
EVENT="10046 trace name context forever,level 12"
此设置对所有用户的所有进程生效、包括后台进程.
2． 对当前session设置
通过alter session的方式修改，需要alter session的系统权限:
SQL> alter session set events '10046 trace name context forever';
Session altered.
SQL> alter session set events '10046 trace name context forever, level 8';
Session altered.
SQL> alter session set events '10046 trace name context off';
Session altered.
3． 对其他用户session设置
通过DBMS_SYSTEM.SET_EV系统包来实现:
SQL> desc DBMS_SYSTEM.SET_EV;
Parameter Type Mode Default?
--------- -------------- ---- --------
SI BINARY_INTEGER IN
SE BINARY_INTEGER IN
EV BINARY_INTEGER IN
LE BINARY_INTEGER IN
NM VARCHAR2 IN
其中的参数SI、SE来自v$session视图:
查询获得需要跟踪的session信息:
SQL> select sid,serial#,username from v$session where username is not null;
SID SERIAL# USERNAME
---------- ---------- ------------------------------
8 2041 SYS
9 437 EYGLE
执行跟踪:
SQL> exec dbms_system.set_ev(9,437,10046,8,'eygle');
PL/SQL procedure successfully completed.
结束跟踪:
SQL> exec dbms_system.set_ev(9,437,10046,0,'eygle');
PL/SQL procedure successfully completed.
(C)对启用方法的一些总结。
因为trace的目标范围不同，导致必须使用不同的方法。
?nbsp; 作用于数据库全局的，就改初始化参数。
?nbsp; 只作用于本session的，就用alter session 命令。
?nbsp; 作用于其它session的，就用DBMS_SYSTEM包。
再加上10046诊断事件，是SQL_TRACE的增强，又多了一套方法。
二、获取跟踪文件
以上生成的跟踪文件位于“user_dump_dest”参数所指定的目录中，位置及文件名可以通过以下SQL查询获得:
1.如果是查询当前session的跟踪文件，使用如下查询：
SELECT d.value||'/'||lower(rtrim(i.instance, chr(0)))||'_ora_'||p.spid||'.trc' trace_file_name
from
( select p.spid from v$mystat m,v$session s, v$process p
where m.statistic# = 1 and s.sid = m.sid and p.addr = s.paddr) p,
( select t.instance from v$thread t,v$parameter v
where v.name = 'thread' and (v.value = 0 or t.thread# = to_number(v.value))) i,
( select value from v$parameter where name = 'user_dump_dest') d
TRACE_FILE_NAME
-------------------------------------------------------------------------------
D:/ORACLE/PRODUCT/10.2.0/ADMIN/MYORACLE/UDUMP/hsjf_ora_1026.trc
2.如果是查询其他用户session的跟踪文件，则根据用户的sid和#serial使用如下查询：
SELECT d.value||'/'||lower(rtrim(i.instance, chr(0)))||'_ora_'||p.spid||'.trc' trace_file_name
from
( select p.spid from v$session s, v$process p
where s.sid=’’ and s. SERIAL#='' and p.addr = s.paddr) p,
( select t.instance from v$thread t,v$parameter v
where v.name = 'thread' and (v.value = 0 or t.thread# = to_number(v.value))) i,
( select value from v$parameter where name = 'user_dump_dest') d
TRACE_FILE_NAME
-------------------------------------------------------------------------------
D:/ORACLE/PRODUCT/10.2.0/ADMIN/MYORACLE/UDUMP/hsjf_ora_1026.trc
三、格式化跟踪文件。
原始的跟踪文件是很难读懂的。需要使用oracle自带的tkprof命令行工具格式化一下。
SQL>$tkprof D:/ORACLE/PRODUCT/10.2.0/ADMIN/MYORACLE/UDUMP/hsjf_ora_1026.trc D:/ORACLE/PRODUCT/10.2.0/ADMIN/MYORACLE/UDUMP/hsjf_ora_1026.txt
这个就可以方便的阅读了。可以在hsjf_ora_1026.txt文件中看到所有的sql语句执行次数,CPU使用时间等数据。
备注：可以通过以下方法读取当前已经设置的参数
对于全局的SQL_TRACE参数的设置，可以通过show parameter命令获得。
当我们通过alter session的方式设置了SQL_TRACE,这个设置是不能通过show parameter的方式得到的,我们需要通过dbms_system.read_ev来获取：
SQL> set feedback off
SQL> set serveroutput on
SQL> declare
2 event_level number;
3 begin
4 for event_number in 10000..10999 loop
5 sys.dbms_system.read_ev(event_number, event_level);
6 if (event_level > 0) then
7 sys.dbms_output.put_line(
8 'Event ' ||
9 to_char(event_number) ||
10 ' is set at level ' ||
11 to_char(event_level)
12 );
13 end if;
14 end loop;
15 end;
16 /
Event 10046 is set at level 1
PS：如何看懂ORACLE的执行计划
一、什么是执行计划
An explain plan is a representation of the access path that is taken when a query is executed within Oracle.
二、如何访问数据
At the physical level Oracle reads blocks of data. The smallest amount of data read is a single Oracle block, the largest is constrained by operating system limits (and multiblock i/o). Logically Oracle finds the data to read by using the following methods:
Full Table Scan (FTS) --全表扫描
Index Lookup (unique & non-unique) --索引扫描（唯一和非唯一）
Rowid --物理行id
三、执行计划层次关系
When looking at a plan, the rightmost (ie most inndented) uppermost operation is the first thing that is executed. --采用最右最上最先执行的原则看层次关系，在同一级如果某个动作没有子ID就最先执行
1.看一个简单的例子：
Query Plan
-----------------------------------------
SELECT STATEMENT [CHOOSE] Cost=1234
**TABLE ACCESS FULL LARGE [:Q65001] [ANALYZED] --[:Q65001]表示是并行方式，[ANALYZED]表示该对象已经分析过了
优化模式是CHOOSE的情况下，看Cost参数是否有值来决定采用CBO还是RBO：
SELECT STATEMENT [CHOOSE] Cost=1234 --Cost有值，采用CBO
SELECT STATEMENT [CHOOSE] Cost= --Cost为空，采用RBO
2.层次的父子关系，看比较复杂的例子：
PARENT1
**FIRST CHILD
****FIRST GRANDCHILD
**SECOND CHILD
Here the same principles apply, the FIRST GRANDCHILD is the initial operation then the FIRST CHILD followed by the SECOND CHILD and finally the PARENT collates the output.
四、例子解说
Execution Plan
----------------------------------------------------------
0 **SELECT STATEMENT Optimizer=CHOOSE (Cost=3 Card=8 Bytes=248)
1 0 **HASH JOIN (Cost=3 Card=8 Bytes=248)
2 1 ****TABLE ACCESS (FULL) OF 'DEPT' (Cost=1 Card=3 Bytes=36)
3 1 ****TABLE ACCESS (FULL) OF 'EMP' (Cost=1 Card=16 Bytes=304)
左侧的两排数据，前面的是序列号ID，后面的是对应的PID（父ID）。
A shortened summary of this is:
Execution starts with ID=0: SELECT STATEMENT but this is dependand on it's child objects
So it executes its first child step: ID=1 PID=0 HASH JOIN but this is dependand on it's child objects
So it executes its first child step: ID=2 PID=1 TABLE ACCESS (FULL) OF 'DEPT'
Then the second child step: ID=3 PID=2 TABLE ACCESS (FULL) OF 'EMP'
Rows are returned to the parent step(s) until finished
五、表访问方式
1.Full Table Scan (FTS) 全表扫描
In a FTS operation, the whole table is read up to the high water mark (HWM). The HWM marks the last block in the table that has ever had data written to it. If you have deleted all the rows then you will still read up to the HWM. Truncate resets the HWM back to the start of the table. FTS uses multiblock i/o to read the blocks from disk. --全表扫描模式下会读数据到表的高水位线（HWM即表示表曾经扩展的最后一个数据块），读取速度依赖于Oracle初始化参数db_block_multiblock_read_count
Query Plan
------------------------------------
SELECT STATEMENT [CHOOSE] Cost=1
**INDEX UNIQUE SCAN EMP_I1 --如果索引里就找到了所要的数据，就不会再去访问表了
2.Index Lookup 索引扫描
There are 5 methods of index lookup:
index unique scan --索引唯一扫描
Method for looking up a single key value via a unique index. always returns a single value, You must supply AT LEAST the leading column of the index to access data via the index.
eg:
SQL> explain plan for select empno,ename from emp where empno=10;
index range scan --索引局部扫描
Index range scan is a method for accessing a range values of a particular column. AT LEAST the leading column of the index must be supplied to access data via the index. Can be used for range operations (e.g. > < <> >= <= between) .
eg:
SQL> explain plan for select mgr from emp where mgr = 5;
index full scan --索引全局扫描
Full index scans are only available in the CBO as otherwise we are unable to determine whether a full scan would be a good idea or not. We choose an index Full Scan when we have statistics that indicate that it is going to be more efficient than a Full table scan and a sort. For example we may do a Full index scan when we do an unbounded scan of an index and want the data to be ordered in the index order.
eg:
SQL> explain plan for select empno,ename from big_emp order by empno,ename;
index fast full scan --索引快速全局扫描，不带order by情况下常发生
Scans all the block in the index, Rows are not returned in sorted order, Introduced in 7.3 and requires V733_PLANS_ENABLED=TRUE and CBO, may be hinted using INDEX_FFS hint, uses multiblock i/o, can be executed in parallel, can be used to access second column of concatenated indexes. This is because we are selecting all of the index.
eg:
SQL> explain plan for select empno,ename from big_emp;
index skip scan --索引跳跃扫描，where条件列是非索引的前导列情况下常发生
Index skip scan finds rows even if the column is not the leading column of a concatenated index. It skips the first column(s) during the search.
eg:
SQL> create index i_emp on emp(empno, ename);
SQL> select /*+ index_ss(emp i_emp)*/ job from emp where ename='SMITH';
3.Rowid 物理ID扫描
This is the quickest access method available.Oracle retrieves the specified block and extracts the rows it is interested in. --Rowid扫描是最快的访问数据方式
六、表连接方式
有三种连接方式：
1.Sort Merge Join (SMJ) --由于sort是非常耗资源的，所以这种连接方式要避免
Rows are produced by Row Source 1 and are then sorted Rows from Row Source 2 are then produced and sorted by the same sort key as Row Source 1. Row Source 1 and 2 are NOT accessed concurrently.
SQL> explain plan for
select /*+ ordered */ e.deptno,d.deptno
from emp e,dept d
where e.deptno = d.deptno
order by e.deptno,d.deptno;
Query Plan
-------------------------------------
SELECT STATEMENT [CHOOSE] Cost=17
**MERGE JOIN
****SORT JOIN
******TABLE ACCESS FULL EMP [ANALYZED]
****SORT JOIN
******TABLE ACCESS FULL DEPT [ANALYZED]
Sorting is an expensive operation, especially with large tables. Because of this, SMJ is often not a particularly efficient join method.
2.Nested Loops (NL) --比较高效的一种连接方式
Fetches the first batch of rows from row source 1, Then we probe row source 2 once for each row returned from row source 1.
For nested loops to be efficient it is important that the first row source returns as few rows as possible as this directly controls the number of probes of the second row source. Also it helps if the access method for row source 2 is efficient as this operation is being repeated once for every row returned by row source 1.
SQL> explain plan for
select a.dname,b.sql
from dept a,emp b
where a.deptno = b.deptno;
Query Plan
-------------------------
SELECT STATEMENT [CHOOSE] Cost=5
**NESTED LOOPS
****TABLE ACCESS FULL DEPT [ANALYZED]
****TABLE ACCESS FULL EMP [ANALYZED]
3.Hash Join --最为高效的一种连接方式
New join type introduced in 7.3, More efficient in theory than NL & SMJ, Only accessible via the CBO. Smallest row source is chosen and used to build a hash table and a bitmap The second row source is hashed and checked against the hash table looking for joins. The bitmap is used as a quick lookup to check if rows are in the hash table and are especially useful when the hash table is too large to fit in memory.
SQL> explain plan for
select /*+ use_hash(emp) */ empno
from emp,dept
where emp.deptno = dept.deptno;
Query Plan
----------------------------
SELECT STATEMENT [CHOOSE] Cost=3
**HASH JOIN
****TABLE ACCESS FULL DEPT
****TABLE ACCESS FULL EMP
Hash joins are enabled by the parameter HASH_JOIN_ENABLED=TRUE in the init.ora or session. TRUE is the default in 7.3.
3.Cartesian Product --卡迪尔积，不算真正的连接方式，sql肯定写的有问题
A Cartesian Product is done where they are no join conditions between 2 row sources and there is no alternative method of accessing the data. Not really a join as such as there is no join! Typically this is caused by a coding mistake where a join has been left out.
It can be useful in some circumstances - Star joins uses cartesian products.Notice that there is no join between the 2 tables:
SQL> explain plan for
select emp.deptno,dept,deptno
from emp,dept
Query Plan
------------------------------
SLECT STATEMENT [CHOOSE] Cost=5
**MERGE JOIN CARTESIAN
****TABLE ACCESS FULL DEPT
****SORT JOIN
******TABLE ACCESS FULL EMP
The CARTESIAN keyword indicate that we are doing a cartesian product.
七、运算符
1.sort --排序，很消耗资源
There are a number of different operations that promote sorts:
order by clauses
group by
sort merge join
2.filter --过滤，如not in、min函数等容易产生
Has a number of different meanings, used to indicate partition elimination, may also indicate an actual filter step where one row source is filtering, another, functions such as min may introduce filter steps into query plans.
3.view --视图，大都由内联视图产生
When a view cannot be merged into the main query you will often see a projection view operation. This indicates that the 'view' will be selected from directly as opposed to being broken down into joins on the base tables. A number of constructs make a view non mergeable. Inline views are also non mergeable.
eg:
SQL> explain plan for
select ename,tot
from emp,(select empno,sum(empno) tot from big_emp group by empno) tmp
where emp.empno = tmp.empno;
Query Plan
------------------------
SELECT STATEMENT [CHOOSE]
**HASH JOIN
**TABLE ACCESS FULL EMP [ANALYZED]
**VIEW
****SORT GROUP BY
******INDEX FULL SCAN BE_IX
4.partition view --分区视图
Partition views are a legacy technology that were superceded by the partitioning option. This section of the article is provided as reference for such legacy systems.