Oracle 中的隔離等級

文章目錄

• Read Committed Isolation
• Serializable Isolation

隔離等級（isoation eve）

隔離等級定義了事務與事務之間的隔離程度。

隔離等級與並發性是互為矛盾的：隔離程度越高，資料庫的並發性越差；隔離程度越低，資料庫的並發性越好。

ANSI/ISO SQ92標準定義了一些資料庫操作的隔離等級：

• 未提交讀（read uncommitted）
• 提交讀（read committed）  
• 重複讀（repeatabe read）  
• 序列化（seriaizabe）

通過一些現象，可以反映出隔離等級的效果。這些現象有：

• 更新丟失（ost update）：當系統允許兩個事務同時更新同一資料是，發生更新丟失。  
• 髒讀（dirty read）：當一個事務讀取另一個事務尚未提交的修改時，產生髒讀。
• 非重複讀（nonrepeatabe read）：同一查詢在同一事務中多次進行，由於其他提交事務所做的修改或刪除，每次返回不同的結果集，此時發生非重複讀。(A transaction rereads data it has previousy read and finds that another committed transaction has modified or deeted the data.  )
• 幻像（phantom read）：同一查詢在同一事務中多次進行，由於其他提交事務所做的插入操作，每次返回不同的結果集，此時發生幻像讀。(A transaction reexecutes a query returning a set of rows that satisfies a search condition and finds that another committed transaction has inserted additiona rows that satisfy the condition.  )

下面是隔離等級及其對應的可能出現或不可能出現的現象

	Dirty Read	NonRepeatabe Read	Phantom Read
Read uncommitted	Possible	Possible	Possible
Read committed	not possible	Possible	Possible
Repeatabe read	not possible	not possible	Possible
Seriaizabe	not possible	not possible	not possible

 

ORACE的隔離等級

ORACE提供了SQ92標準中的read committed和seriaizabe，同時提供了非SQ92標準的read-ony。

read committed：

• 這是ORACE預設的交易隔離等級。
• 事務中的每一條語句都遵從語句級的讀一致性。
• 保證不會髒讀；但可能出現非重複讀和幻像。

seriaizabe：（串列執行事務，並發性最小）

• 簡單地說，seriaizabe就是使事務看起來象是一個接著一個地順序地執行。
• 僅僅能看見在本事務開始前由其它事務提交的更改和在本事務中所做的更改。
• 保證不會出現非重複讀和幻像。
• Seriaizabe隔離等級提供了read-ony事務所提供的讀一致性（事務級的讀一致性），同時又允許DM操作。

如果有在seriaizabe事務開始時未提交的事務在seriaizabe事務結束之前修改了seriaizabe事務將要修改的行並進行了提交，則seriaizabe事務不會讀到這些變更，因此發生無法序列化訪問的錯誤。（換一種解釋方法：只要在seriaizabe事務開始到結束之間有其他事務對seriaizabe事務要修改的東西進行了修改並提交了修改，則發生無法序列化訪問的錯誤。）

If a serializable transaction contains data manipulation language (DML) that attempts to update any resource that may have been updated in a transaction uncommitted at the start of the serializable transaction, （並且修改在後來被提交而沒有復原），then the DML statement fails. 返回的錯誤是ORA-08177: Cannot serialize access for this transaction。

ORACE在資料區塊中記錄最近對資料行執行修改操作的N個事務的資訊，目的是確定本事務開始時，是否存在未提交的事務修改了本事務將要修改的行。具體見英文：

Oracle permits a serializable transaction to modify a data row only if it can determine that prior changes to the row were made by transactions that had committed when the serializable transaction began.

To make this determination efficiently, Oracle uses control information stored in the data block that indicates which rows in the block contain committed and uncommitted changes. In a sense, the block contains a recent history of transactions that affected each row in the block. The amount of history that is retained is controlled by the INITRANS parameter of CREATE TABLE and ALTER TABLE. Under some circumstances, Oracle may have insufficient history information to determine whether a row has been updated by a "too recent" transaction. This can occur when many transactions concurrently modify the same data block, or do so in a very short period. You can avoid this situation by setting higher values of INITRANS for tables that will experience many transactions updating the same blocks. Doing so will enable Oracle to allocate sufficient storage in each block to record the history of recent transactions that accessed the block.

The INITRANS Parameter：Oracle stores control information in each data block to manage access by concurrent transactions. Therefore, if you set the transaction isolation level to serializable, you must use the ALTER TABLE command to set INITRANS to at least 3. This parameter will cause Oracle to allocate sufficient storage in each block to record the history of recent transactions that accessed the block. Higher values should be used for tables that will undergo many transactions updating the same blocks.

read-ony：

• 遵從事務級的讀一致性，僅僅能看見在本事務開始前由其它事務提交的更改。
• 不允許在本事務中進行DM操作。
• read ony是seriaizabe的子集。它們都避免了非重複讀和幻像。區別是在read ony中是唯讀；而在seriaizabe中可以進行DM操作。
• Export with CONSISTENT = Y sets the transaction to read-ony.

 

read committed和seriaizabe的區別和聯絡：

事務1先於事務2開始，並保持未提交狀態。事務2想要修改正被事務1修改的行。事務2等待。如果事務1復原，則事務2（不論是read committed還是seriaizabe方式）進行它想要做的修改。如果事務1提交，則當事務2是read committed方式時，進行它想要做的修改；當事務2是seriaizabe方式時，失敗並報錯“Cannot seriaize access”，因為事務2看不見事務1提交的修改，且事務2想在事務1修改的基礎上再做修改。

即seriaizabe不允許存在事務嵌套

具體見英文：

Both read committed and serializable transactions use row-level locking, and both will wait if they try to change a row updated by an uncommitted concurrent transaction. The second transaction that tries to update a given row waits for the other transaction to commit or roll back and release its lock. If that other transaction rolls back, the waiting transaction (regardless of its isolation mode) can proceed to change the previously locked row, as if the other transaction had not existed. However, if the other (blocking) transaction commits and releases its locks, a read committed transaction proceeds with its intended update. A serializable transaction, however, fails with the error "Cannot serialize access", because the other transaction has committed a change that was made since the serializable transaction began.

read committed和seriaizabe可以在ORACE並行伺服器中使用。

關於SET TRANSACTION READ WRITE：read write和read committed 應該是一樣的。在讀方面，它們都避免了髒讀，但都無法實現重複讀。雖然沒有文檔說明read write在寫方面與read committed一致，但顯然它在寫的時候會加獨佔鎖定以避免更新丟失。在加鎖的過程中，如果遇到待鎖定資源無法鎖定，應該是等待而不是放棄。這與read committed一致。

語句級的讀一致性

• ORACE保證語句級的讀一致性，即一個語句所處理的資料集是在單一時間點上的資料集，這個時間點是這個語句開始的時間。
• 一個語句看不見在它開始執行後提交的修改。
• 對於DM語句，它看不見由自己所做的修改，即DM語句看見的是它本身開始執行以前存在的資料。

事務級的讀一致性

• 事務級的讀一致性保證了可重複讀，並保證不會出現幻像。

設定隔離等級

設定一個事務的隔離等級

• SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
• SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
• SET TRANSACTION READ ONLY;

設定增個會話的隔離等級

• ATER SESSION SET ISOLATION_LEVE SERIALIZABLE;
• ATER SESSION SET ISOLATION_LEVE READ COMMITTED;

 

Choice of Isolation Level

Application designers and developers should choose an isolation level based on application performance and consistency needs as well as application coding requirements.

For environments with many concurrent users rapidly submitting transactions, designers must assess transaction performance requirements in terms of the expected transaction arrival rate and response time demands. Frequently, for high-performance environments, the choice of isolation levels involves a trade-off between consistency and concurrency.

Read Committed Isolation

For many applications, read committed is the most appropriate isolation level. Read committed isolation can provide considerably more concurrency with a somewhat increased risk of inconsistent results due to phantoms and non-repeatable reads for some transactions.

Many high-performance environments with high transaction arrival rates require more throughput and faster response times than can be achieved with serializable isolation. Other environments that supports users with a very low transaction arrival rate also face very low risk of incorrect results due to phantoms and nonrepeatable reads. Read committed isolation is suitable for both of these environments.

兩種情況：（1）在事務量大、高效能的計算環境，需要更高的輸送量和回應時間；（2）事務數少，並且發生幻影和不可重複讀取的幾率的比較低

Oracle read committed isolation provides transaction set consistency for every query. That is, every query sees data in a consistent state. Therefore, read committed isolation will suffice for many applications that might require a higher degree of isolation if run on other database management systems that do not use multiversion concurrency control.

Read committed isolation mode does not require application logic to trap the "Cannot serialize access" error and loop back to restart a transaction. In most applications, few transactions have a functional need to issue the same query twice, so for many applications protection against phantoms and non-repeatable reads is not important. Therefore many developers choose read committed to avoid the need to write such error checking and retry code in each transaction.

Serializable Isolation

Oracle's serializable isolation is suitable for environments where there is a relatively low chance that two concurrent transactions will modify the same rows and the long-running transactions are primarily read-only. It is most suitable for environments with large databases and short transactions that update only a few rows.

（1）適合於很少存在兩個事務同時修改同一條記錄的情況

（2）長事務以唯讀為主

（3）大型資料庫並且每個短事務只修改很少的記錄

Serializable isolation mode provides somewhat more consistency by protecting against phantoms and nonrepeatable reads and can be important where a read/write transaction executes a query more than once.

Unlike other implementations of serializable isolation, which lock blocks for read as well as write, Oracle provides nonblocking queries and the fine granularity of row-level locking, both of which reduce write/write contention. For applications that experience mostly read/write contention, Oracle serializable isolation can provide significantly more throughput than other systems. Therefore, some applications might be suitable for serializable isolation on Oracle but not on other systems.

All queries in an Oracle serializable transaction see the database as of a single point in time, so this isolation level is suitable where multiple consistent queries must be issued in a read/write transaction. A report-writing application that generates summary data and stores it in the database might use serializable mode because it provides the consistency that a READ ONLY transaction provides, but also allows INSERT, UPDATE, and DELETE.