by yan 20220223

• ANSI/ISO Transaction Isolation Levels(ANSI/ISO標準的隔離級別)
  （1）未提交讀（read uncommitted）
  （2）提交讀 或 不可重復讀（read committed）
  （3）重復讀（repeatable read）
  （4）序列化（Serializable）

• isolation levels 的一些表現
  （1）Dirtyreads（臟讀）
  Atransaction reads data that has been written by another transaction that hasnot been committed yet.
  –一個事務讀取另一個事務已經修改但為提交的數據
  （2）Nonrepeatable (fuzzy) reads（模糊讀）
  Atransaction rereads data it has previously read and finds that anothercommitted transaction has modified or deleted the data. For example, a userqueries a row and then later queries the same row, onlyto discover that the data has changed.
  –當事務重讀之前的數據時，發現這個事務被另一個事務修改，如modified 或者deleted，并且已經提交。
  （3）Phantomreads
  Atransaction reruns a queryreturning a set of rows that satisfies a search condition and finds thatanother committed transaction has inserted additional rows that satisfy thecondition.
  –同一查詢在同一事務中多次進行，由于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。

• 隔離級別與并發性
  隔離級別與并發性是互為矛盾的：隔離程度越高，數據庫的并發性越差；隔離程度越低，數據庫的并發性越好。
  

• java里的定義

//java.sql.Connection.java/*** A constant indicating that transactions are not supported.*/int TRANSACTION_NONE = 0;/*** A constant indicating that* dirty reads, non-repeatable reads and phantom reads can occur.* This level allows a row changed by one transaction to be read* by another transaction before any changes in that row have been* committed (a "dirty read"). If any of the changes are rolled back,* the second transaction will have retrieved an invalid row.*/int TRANSACTION_READ_UNCOMMITTED = 1;/*** A constant indicating that* dirty reads are prevented; non-repeatable reads and phantom* reads can occur. This level only prohibits a transaction* from reading a row with uncommitted changes in it.*/int TRANSACTION_READ_COMMITTED = 2;/*** A constant indicating that* dirty reads and non-repeatable reads are prevented; phantom* reads can occur. This level prohibits a transaction from* reading a row with uncommitted changes in it, and it also* prohibits the situation where one transaction reads a row,* a second transaction alters the row, and the first transaction* rereads the row, getting different values the second time* (a "non-repeatable read").*/int TRANSACTION_REPEATABLE_READ = 4;/*** A constant indicating that* dirty reads, non-repeatable reads and phantom reads are prevented.* This level includes the prohibitions in* {@code TRANSACTION_REPEATABLE_READ} and further prohibits the* situation where one transaction reads all rows that satisfy* a {@code WHERE} condition, a second transaction inserts a row that* satisfies that {@code WHERE} condition, and the first transaction* rereads for the same condition, retrieving the additional* "phantom" row in the second read.*/int TRANSACTION_SERIALIZABLE = 8; 創作挑戰賽新人創作獎勵來咯，堅持創作打卡瓜分現金大獎

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