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bluky999:
jiakechong 写道跟啊里爸爸那个差不多吧跟 dubb ...
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jiakechong:
跟啊里爸爸那个差不多吧
淘宝-HSF -
lookqlp:
您好!我写了个关键词关联度统计的mapreduce,排序非常慢 ...
排序--MapReduce -
liubang201010:
DB Audit更多参考资料:
http://www.inno ...
8--《DB Audit》 -
裴小星:
这些数据库专家,同样不了解程序员。SQL(或PL/SQL,TS ...
MapReduce:一个重大的倒退
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. > >= 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 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 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 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.
(本文转自:http://hi.baidu.com/edeed/blog/item/73c46538d2614d2796ddd864.html)
发表评论
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OCP认证考试指南全册10g
2012-01-06 18:49 957常用命令: 1,lsnrctl status;emctl s ... -
exits 和in 深度分析(转载,出处不明)
2011-08-22 21:03 830EXISTS的执行流程 select * from t1 wh ... -
索引有点靠后
2011-08-10 21:51 689记得在taobao时候需要统计一个数据,但是通过不了SQLWE ... -
《MySQL 性能调优与架构设计》
2011-08-10 21:47 751MySQL的基本架构:SQL Layer和Storage En ... -
《Oracle Database 10g 性能调整与优化》
2011-08-02 21:31 10008.2 当从表中访问数据 ... -
Oracle零数据丢失
2011-07-25 20:28 865Oracle在版本7的时候, ... -
硬盘存储和磁盘存储比较
2011-05-30 14:02 707如图 -
解析oracle的ROWNUM -- 作者: chen_liang
2011-05-17 11:32 757对于rownum来说它是oracle系统顺序分配为从查询 ... -
flashback 需要的条件
2011-04-13 19:00 735flashback table 需要下面几个条件 1. ... -
oracle表列名大小写
2011-03-23 22:06 3960不是我写的sql,比较烂,但是有一个可以关注的点 oracle ... -
MapReduce:一个重大的倒退
2011-03-22 23:09 1601转载http://www.cnblogs.com/c ... -
ORA-12154
2011-03-20 23:16 853看看监听: lsnrctl status 是不是正常 ... -
Oracle -other
2011-03-12 13:10 764JackJw 的杂货铺 http://jackjw.itpu ... -
数据库的最佳布局
2011-03-09 23:26 7581,并发访问对象的分离 2,数据和相应的索引的分开 3,回 ... -
优化器
2011-03-07 23:47 699访问路径:FULL TABLE SCAN 全表扫描;Index ... -
B树索引和位图索引的使用环境
2011-03-07 00:18 784B树索引:是联机事务处理应用的最好选择,键值列是最适合的 位 ... -
sqlplus autotrace
2011-03-05 14:14 695出自eygle http://www.eygle.com/fa ... -
Oracle 重要文件系统解释 转载
2011-02-25 23:01 771Oracle 中要了解的文件实在太多了,一不小心就被弄 ... -
聚族索引、非聚族索引、组合索引的含义和用途 转载
2011-02-25 21:50 1200http://blog.csdn.net/zhaohuabin ... -
Oracle analyze
2011-02-25 21:19 8421、三大功能 搜集和删除索引、表和簇的统计信息 验证表、索 ...
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