Oracle的執行計畫詳解

一、什麼是執行計畫

An explain plan is a representation of the access path that is taken when a query is executed within Oracle.

二、如何訪問資料

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)   --全表掃描
Index Lookup (unique & non-unique)   --索引掃描（唯一和非唯一）
Rowid   --物理行id

三、執行計畫層次關係

When looking at a plan, the rightmost (ie most inndented) uppermost operation is the first thing that is executed. --採用最右最上最先執行的原則看層次關係，在同一級如果某個動作沒有子ID就最先執行

1.看一個簡單的例子：

Query Plan
-----------------------------------------
SELECT STATEMENT [CHOOSE] Cost=1234
**TABLE ACCESS FULL LARGE [:Q65001] [ANALYZED] --[:Q65001]表示是並行方式，[ANALYZED]表示該對象已經分析過了

最佳化模式是CHOOSE的情況下，看Cost參數是否有值來決定採用CBO還是RBO：
SELECT STATEMENT [CHOOSE] Cost=1234 --Cost有值，採用CBO
SELECT STATEMENT [CHOOSE] Cost= --Cost為空白，採用RBO

2.層次的父子關係，看比較複雜的例子：

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、例子解說

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)

左側的兩排資料，前面的是序號ID，後面的是對應的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、表訪問方式

1.Full Table Scan (FTS) 全表掃描

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.  --全表掃描模式下會讀資料到表的高水位線（HWM即表示表曾經擴充的最後一個資料區塊），讀取速度依賴於Oracle初始化參數db_block_multiblock_read_count

Query Plan
------------------------------------
SELECT STATEMENT [CHOOSE] Cost=1
**INDEX UNIQUE SCAN EMP_I1  --如果索引裡就找到了所要的資料，就不會再去訪問表了

2.Index Lookup 索引掃描

There are 5 methods of index lookup:

index unique scan  --索引唯一掃描
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  --索引局部掃描
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  --索引全域掃描
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  --索引快速全域掃描，不帶order by情況下常發生
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  --索引跳躍掃描，where條件列是非索引的前置列情況下常發生
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掃描

This is the quickest access method available.Oracle retrieves the specified block and extracts the rows it is interested in. --Rowid掃描是最快的訪問資料方式

六、表串連方式

有三種串連方式：

1.Sort Merge Join (SMJ)   --由於sort是非常耗資源的，所以這種串連方式要避免

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)   --比較高效的一種串連方式

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   --最為高效的一種串連方式

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   --卡迪爾積，不算真正的串連方式，sql肯定寫的有問題

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、運算子

1.sort   --排序，很消耗資源

There are a number of different operations that promote sorts:
order by clauses
group by
sort merge join

2.filter   --過濾，如not in、min函數等容易產生

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   --視圖，大都由內聯視圖產生

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     --分區視圖

 

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.

（本文轉自：http://hi.baidu.com/edeed/blog/item/73c46538d2614d2796ddd864.html）