半小时看懂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 ptimizer=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: