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 --笛卡尔积，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.
