【書評:Oracle查詢最佳化改寫】第二章

BLOG文件結構圖

在上一篇中http://blog.itpub.net/26736162/viewspace-1652985/，我們主要分析了一些單表查詢的時候需要注意的內容，今天第二章也很簡單，主要是關於排序方面的內容，以下貼出第二章的內容：

第 2 章給查詢結果排序

2.1 以指定的次序返回查詢結果

2.2 按多個欄位排序

2.3 按子串排序

2.4 TRANSLATE

2.5 按數字和字母混合字串中的字母排序

2.6 處理排序空值

2.7 根據條件取不同列中的值來排序

排序基本上沒有什麼可以講的，不過書中著重介紹了下translate的用法。

1 translate用法

語法：TRANSLATE(char, from, to)

用法：

1. 返回將出現在from中的每個字元替換為to中的相應字元以後的字串。

2. 若from比to字串長，那麼在from中比to中多出的字元將會被刪除，或者認為from中多出的字元在to中與空對應

3. 三個引數中有一個是空，返回值也將是空值。

09:43:50 SQL> select translate('abcdefga','abc','wo') from dual;

TRANSLA

-------

wodefgw

Elapsed: 00:00:00.14

09:43:57 SQL> select translate('abcdefga','abc','') from dual;

T

-

Elapsed: 00:00:00.00

SELECT translate('ab 你好 bcadefg','abcdefg','1234567'),translate('ab 你好 bcadefg','1abcdefg','1') FROM dual;

2 按數字和字母混合字串中的字母排序，採用translate函式來實現

09:52:01 SQL> create or replace view v as select empno || ' '||ename as data from scott.emp;

View created.

Elapsed: 00:00:00.54

09:52:07 SQL> select * from V

09:52:15 2 ;

DATA

---------------------------------------------------

9000 lastwiner

9001 lastwiner

7369 SMITH

7499 ALLEN

7521 WARD

7566 JONES

7654 MARTIN

7698 BLAKE

7782 CLARK

7788 SCOTT

7839 KING

7844 TURNER

7876 ADAMS

7900 JAMES

7902 FORD

7934 MILLER

16 rows selected.

Elapsed: 00:00:00.20

09:55:07 SQL> select data,translate(data,'- 0123456789','-') from V order by 2;

DATA TRANSLATE(DATA,'-0123456789','-')

--------------------------------------------------- ------------------------------------------------------------------------------------------------------

7876 ADAMS ADAMS

7499 ALLEN ALLEN

7698 BLAKE BLAKE

7782 CLARK CLARK

7902 FORD FORD

7900 JAMES JAMES

7566 JONES JONES

7839 KING KING

7654 MARTIN MARTIN

7934 MILLER MILLER

7788 SCOTT SCOTT

7369 SMITH SMITH

7844 TURNER TURNER

7521 WARD WARD

9001 lastwiner lastwiner

9000 lastwiner lastwiner

16 rows selected.

Elapsed: 00:00:00.10

09:55:33 SQL>

3 關於order by 排序的最佳化

關於SQL最佳化中有一個原則叫：避免使用耗費資源的操作(DISTINCT、UNION、MINUS、INTERSECT、ORDER BY、group by、SMJ、created index)

帶有DISTINCT,UNION,MINUS,INTERSECT,ORDER BY的SQL語句會啟動SQL引擎執行耗費資源的排序(SORT)功能. DISTINCT需要一次排序操作, 而其他的至少需要執行兩次排序.

例如,一個UNION查詢,其中每個查詢都帶有GROUP BY子句, GROUP BY會觸發嵌入排序(NESTED SORT) ; 這樣, 每個查詢需要執行一次排序, 然後在執行UNION時, 又一個唯一排序(SORT UNIQUE)操作被執行而且它只能在前面的嵌入排序結束後才能開始執行. 嵌入的排序的深度會大大影響查詢的效率.

通常, 帶有UNION, MINUS , INTERSECT的SQL語句都可以用其他方式重寫.

ORDER BY語句決定了Oracle如何將返回的查詢結果排序。Order by語句對要排序的列沒有什麼特別的限制，也可以將函式加入列中（象聯接或者附加等）。任何在Order by語句的非索引項或者有計算表示式都將降低查詢速度。

仔細檢查order by語句以找出非索引項或者表示式，它們會降低效能。解決這個問題的辦法就是重寫order by語句以使用索引，也可以為所使用的列建立另外一個索引，同時應絕對避免在order by子句中使用表示式。

●在頻繁進行排序或分組（即進行group by或order by操作）的列上建立索引。

●如果待排序的列有多個，可以在這些列上建立複合索引（compound index）。

磁碟排序的開銷是很大的，有幾個方面的原因。首先，和記憶體排序相比較，它們特別慢；而且磁碟排序會消耗臨時表空間中的資源。Oracle還必須分配緩衝池塊來保持臨時表空間中的塊。無論什麼時候，記憶體排序都比磁碟排序好，磁碟排序將會令任務變慢，並且會影響Oracle例項的當前任務的執行。還有，過多的磁碟排序將會令free buffer waits的值變高，從而令其它任務的資料塊由緩衝中移走。

3.1 總結

（1）採用索引避免排序：排序資料較多時

（2）去掉不必要的distinct，很多distinct是由於程式設計師對資料的瞭解不自信而多加的。

總而言之，排序是非常耗費CPU資源的，能不排序就不要排序，如果非得排序，可以考慮在排序列上建立合適的索引。

記得之前有個SQL，不加排序的話，秒級可以出結果，即響應速度很快，但是加上排序後得5或6分鐘才可以，看了下是結果集很大，又得排序造成的。

這裡簡單舉個例子吧：

3.2 例子

[oracle@rhel6_lhr ~]$ sqlplus / as sysdba

SQL*Plus: Release 11.2.0.3.0 Production on Thu May 14 10:55:26 2015

Connected to:

Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production

With the Partitioning, Automatic Storage Management, OLAP, Data Mining

and Real Application Testing options

10:55:26 SQL> conn lhr/lhr

Connected.

12:08:08 SQL> create table test_index_lhr as select * from dba_objects;

Table created.

Elapsed: 00:00:03.70

12:08:27 SQL> insert into test_index_lhr select * from test_index_lhr;

77241 rows created.

12:08:39 SQL> commit;

Commit complete.

Elapsed: 00:00:00.00

12:08:41 SQL> set autot traceonly explain stat

12:08:41 SQL> select object_name from test_index_lhr where object_name is not null order by object_name;

154482 rows selected.

Elapsed: 00:00:01.18

Execution Plan

----------------------------------------------------------

Plan hash value: 1466335622

---------------------------------------------------------------------------------------------

---------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 155K| 9M| | 3078 (1)| 00:00:37 |

| 1 | SORT ORDER BY | | 155K| 9M| 10M| 3078 (1)| 00:00:37 |

|* 2 | TABLE ACCESS FULL| TEST_INDEX_LHR | 155K| 9M| | 623 (1)| 00:00:08 |

---------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

2 - filter("OBJECT_NAME" IS NOT NULL)

Note

-----

- dynamic sampling used for this statement (level=2)

Statistics

----------------------------------------------------------

10 recursive calls

6 db block gets

2808 consistent gets

614 physical reads

34996 redo size

3787521 bytes sent via SQL*Net to client

113801 bytes received via SQL*Net from client

10300 SQL*Net roundtrips to/from client

1 sorts (memory)

0 sorts (disk)

154482 rows processed

12:08:48 SQL> create index ind_test_inde on test_index_lhr(object_name) ;

Index created.

Elapsed: 00:00:02.45

12:08:58 SQL> EXEC dbms_stats.gather_table_stats(ownname => 'LHR', tabname=> 'test_index_lhr', cascade => TRUE );

PL/SQL procedure successfully completed.

Elapsed: 00:00:02.62

12:09:04 SQL> select object_name from test_index_lhr where object_name is not null order by object_name;

154482 rows selected.

Elapsed: 00:00:01.35

Execution Plan

----------------------------------------------------------

Plan hash value: 712275200

----------------------------------------------------------------------------------

----------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 154K| 3771K| 766 (1)| 00:00:10 |

|* 1 | INDEX FULL SCAN | IND_TEST_INDE | 154K| 3771K| 766 (1)| 00:00:10 |

----------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

1 - filter("OBJECT_NAME" IS NOT NULL)

Note

-----

- SQL plan baseline "SQL_PLAN_8kcy12j8f3s5n2a6f2b1f" used for this statement

Statistics

----------------------------------------------------------

704 recursive calls

64 db block gets

11715 consistent gets

37 physical reads

33236 redo size

3787521 bytes sent via SQL*Net to client

113801 bytes received via SQL*Net from client

10300 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

154482 rows processed

12:09:09 SQL> select owner, object_name from test_index_lhr where object_name is not null order by object_name;

154482 rows selected.

Elapsed: 00:00:01.28

Execution Plan

----------------------------------------------------------

Plan hash value: 1466335622

---------------------------------------------------------------------------------------------

---------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 154K| 4676K| | 1947 (1)| 00:00:24 |

| 1 | SORT ORDER BY | | 154K| 4676K| 6072K| 1947 (1)| 00:00:24 |

|* 2 | TABLE ACCESS FULL| TEST_INDEX_LHR | 154K| 4676K| | 623 (1)| 00:00:08 |

---------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

2 - filter("OBJECT_NAME" IS NOT NULL)

Statistics

----------------------------------------------------------

6 recursive calls

6 db block gets

2241 consistent gets

895 physical reads

680 redo size

4232382 bytes sent via SQL*Net to client

113801 bytes received via SQL*Net from client

10300 SQL*Net roundtrips to/from client

1 sorts (memory)

0 sorts (disk)

154482 rows processed

12:09:22 SQL> select /*+index(a,IND_TEST_INDE)*/ owner, object_name from test_index_lhr a where object_name is not null order by object_name;

154482 rows selected.

Elapsed: 00:00:09.59

Execution Plan

----------------------------------------------------------

Plan hash value: 880046030

----------------------------------------------------------------------------------------------

----------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 154K| 4676K| 109K (1)| 00:21:57 |

| 1 | TABLE ACCESS BY INDEX ROWID| TEST_INDEX_LHR | 154K| 4676K| 109K (1)| 00:21:57 |

|* 2 | INDEX FULL SCAN | IND_TEST_INDE | 154K| | 766 (1)| 00:00:10 |

----------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

2 - filter("OBJECT_NAME" IS NOT NULL)

Statistics

----------------------------------------------------------

6 recursive calls

4 db block gets

122955 consistent gets

2198 physical reads

724 redo size

4392715 bytes sent via SQL*Net to client

113801 bytes received via SQL*Net from client

10300 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

154482 rows processed

12:14:23 SQL>

相關連線：

【書評:Oracle查詢最佳化改寫】第一章 http://blog.itpub.net/26736162/viewspace-1652985/

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本文作者：小麥苗，只專注於資料庫的技術，更注重技術的運用

ITPUB BLOG：http://blog.itpub.net/26736162

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【書評:Oracle查詢最佳化改寫】第二章

1 translate用法

2 按數字和字母混合字串中的字母排序，採用translate函式來實現

3 關於order by 排序的最佳化

3.1 總結

3.2 例子

相關文章