事務(wù)是怎么回事，之前已經(jīng)聊過(參見Oracle事務(wù)的隔離)，無關(guān)哪個(gè)數(shù)據(jù)庫，只要提到事務(wù)，都是這四種隔離級(jí)別。mysql的默認(rèn)隔離級(jí)別是3——可重復(fù)讀。更重要的是，是否支持事務(wù)，跟mysql的存儲(chǔ)引擎相關(guān)。登陸mysql，查詢存儲(chǔ)引擎：

mysql>show engines;+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+

| Engine | Support | Comment | Transactions | XA | Savepoints |

+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+

| InnoDB | DEFAULT | Supports transactions, row-level locking, and foreign keys | YES | YES | YES |

| MRG_MYISAM | YES | Collection of identical MyISAM tables | NO | NO | NO |

| MEMORY | YES | Hash based, stored in memory, useful for temporary tables | NO | NO | NO |

| BLACKHOLE | YES | /dev/null storage engine (anything you write to it disappears) | NO | NO | NO |

| MyISAM | YES | MyISAM storage engine | NO | NO | NO |

| CSV | YES | CSV storage engine | NO | NO | NO |

| ARCHIVE | YES | Archive storage engine | NO | NO | NO |

| PERFORMANCE_SCHEMA | YES | Performance Schema | NO | NO | NO |

| FEDERATED | NO | Federated MySQL storage engine | NULL | NULL | NULL |

+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+

9 rows in set (0.00 sec)

我們看到，這么多存儲(chǔ)引擎，也就一個(gè)InnoDB支持事務(wù)。再瞧一下默認(rèn)的事務(wù)隔離級(jí)別：

mysql> select@@tx_isolation;+-----------------+

| @@tx_isolation |

+-----------------+

| REPEATABLE-READ |

+-----------------+

1 row in set, 1 warning (0.00sec)

mysql> select@@global.tx_isolation;+-----------------------+

| @@global.tx_isolation |

+-----------------------+

| REPEATABLE-READ |

+-----------------------+

1 row in set, 1 warning (0.00 sec)

我們看到，不管是當(dāng)前會(huì)話也好，整個(gè)系統(tǒng)也好，默認(rèn)的隔離級(jí)別都是可重復(fù)讀。

再看下默認(rèn)的事務(wù)自動(dòng)提交開關(guān)：

mysql> show variables like ‘a(chǎn)utocommit‘;+---------------+-------+

| Variable_name | Value |

+---------------+-------+

| autocommit | ON |

+---------------+-------+

1 row in set, 1 warning (0.00 sec)

我們把上面的開關(guān)給關(guān)了，好測(cè)試：

mysql> set autocommit=off;

Query OK,0 rows affected (0.00sec)

mysql> show variables like ‘a(chǎn)utocommit‘;+---------------+-------+

| Variable_name | Value |

+---------------+-------+

| autocommit | OFF |

+---------------+-------+

1 row in set, 1 warning (0.00 sec)

我們建個(gè)表測(cè)試一下：

mysql>create table test_wlf->(-> id INT(11),-> name VARCHAR(25)->);

Query OK,0 rows affected (0.06 sec)

插入一條數(shù)據(jù)：

mysql> insert into test_wlf values(1,"wulf");

Query OK,1 row affected (0.03sec)

mysql> select *from test_wlf;+------+------+

| id | name |

+------+------+

| 1 | wulf |

+------+------+

1 row in set (0.00 sec)

再來一個(gè)窗口，同樣把a(bǔ)utocommit關(guān)掉，兩事務(wù)同時(shí)搞起：

因?yàn)楫?dāng)前的事務(wù)隔離級(jí)別是可重復(fù)讀，所以右邊的事務(wù)始終可以重復(fù)讀取到wulf這條數(shù)據(jù)，即使左邊的事務(wù)新增了wumanshu這條數(shù)據(jù)，按理應(yīng)該出現(xiàn)幻讀，可惜已經(jīng)被InnoDB用mvcc(多版本并發(fā)控制)解決了。換句話說，幻讀在InnoDB已經(jīng)死絕了。

接下來我們把兩個(gè)窗口都改成已提交讀：

mysql> set tx_isolation=‘READ-COMMITTED‘;

Query OK,0 rows affected, 1 warning (0.00sec)

mysql> select@@tx_isolation;+----------------+

| @@tx_isolation |

+----------------+

| READ-COMMITTED |

+----------------+

1 row in set, 1 warning (0.00 sec)

mysql> set global tx_isolation=‘READ-COMMITTED‘;

Query OK,0 rows affected, 1 warning (0.00sec)

mysql> select@@global.tx_isolation;+-----------------------+

| @@global.tx_isolation |

+-----------------------+

| READ-COMMITTED |

+-----------------------+

1 row in set, 1 warning (0.00 sec)

再試試修改數(shù)據(jù)：

以上可以看到已提交讀出現(xiàn)了不可重復(fù)讀。繼續(xù)，接下來我們把兩個(gè)窗口都改成未提交讀：

mysql> set tx_isolation=‘READ-UNCOMMITTED‘;

Query OK,0 rows affected, 1 warning (0.00sec)

mysql> set global tx_isolation=‘READ-UNCOMMITTED‘;

Query OK,0 rows affected, 1 warning (0.00sec)

mysql> select@@tx_isolation;+------------------+

| @@tx_isolation |

+------------------+

| READ-UNCOMMITTED |

+------------------+

1 row in set, 1 warning (0.00sec)

mysql> select@@global.tx_isolation;+-----------------------+

| @@global.tx_isolation |

+-----------------------+

| READ-UNCOMMITTED |

+-----------------------+

1 row in set, 1 warning (0.00 sec)

再次更新id為2的name值，事務(wù)的中間結(jié)果都被查出來了，臟讀出現(xiàn)：

原文：https://www.cnblogs.com/wuxun1997/p/14087208.html

與50位技術(shù)專家面對(duì)面20年技術(shù)見證，附贈(zèng)技術(shù)全景圖

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