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http://mysql.taobao.org/monthly/2018/11/04/"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"[^4]: https://dev.mysql.com/doc/refman/8.0/en/innodb-consistent-read.html"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"[^5]: 極客時間- MySQL 專欄--事務到底是隔離的還是不隔離的"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/e9/e93d657aa663979cfa352fb10334cc05.jpeg","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"0"}]}]}
MySQL 可重複讀，差點就我背上了一個 P0 事故！
{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"小黑黑的碎碎念"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"哎，最近有點忙，備考複習不利，明天還要搬家，好難啊！！ 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事故：餘額多扣！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這是一個真實的生產事件，事件起因如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"現有一個交易系統，每次產生交易都會更新相應賬戶的餘額，出賬扣減餘額，入賬增加餘額。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"爲了保證資金安全，餘額發生扣減時，需要比較現有餘額與扣減金額大小，若扣減金額大於現有餘額，扣減餘額不足，扣減失敗。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"賬戶表（"},{"type":"text","marks":[{"type":"italic"}],"text":"省去其他字段"},{"type":"text","text":"）結構如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"sql"},"content":[{"type":"text","text":"CREATE 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會在事務開始後建立一個一致性視圖（*"},{"type":"text","marks":[{"type":"strong"}],"text":"並不是立刻建立"},{"type":"text","marks":[{"type":"italic"}],"text":"），在這個視圖中，會保存所有活躍的事務("},{"type":"text","marks":[{"type":"strong"}],"text":"還未提交的事務"},{"type":"text","text":"*)。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"假設當前事務保存活躍事務數組爲如下圖。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/5e/5ec1815995a43323f89247732cbce108.jpeg","alt":"視圖數組","title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"判斷版本對於當前事務是否可見時，基於以下規則判斷："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"若版本事務 id 小於當前活躍事務 id 數組最小值,比如版本 id 爲 40，小於活躍數組最小值 45。這就代表當前版本的事務已提交，當前版本對於當前事務可見。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"2","normalizeStart":"2"},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"若版本事務 id 大於當前活躍事務數組的最大值，如版本事務 id 爲 100， 大於數組最大事務 id 90。說明了這個版本是當前事務創建之後生成，所以這個版本對於當前事務不可見。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"3","normalizeStart":"3"},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"若版本事務 id 是當前活躍數組事務之一，比如版本事務 id 爲 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id，代表該行數據是當前事務變更的，當然得可見。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"4 這個規則可能比較繞，結合上面圖片比較好理解。"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"以上判斷規則可能比較抽象，看不懂，沒事，我們再用大白話解釋一下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"未提交事務生成的記錄版本，不可見。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"2","normalizeStart":"2"},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"視圖生成前，已提交事務生成記錄版本可見。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"3","normalizeStart":"3"},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"視圖生成後，新事務生成記錄版本不可見。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"4","normalizeStart":"4"},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","text":"自身事務更新永遠可見。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"一致性視圖只會在 RR 與 RC 下才會生成，對於 RR 來說，一致性視圖會在"},{"type":"text","marks":[{"type":"strong"}],"text":"第一個查詢語句"},{"type":"text","text":"的時候生成。而對於 RC 來說，每個查詢語句都會重新生成視圖。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"當前讀與快照讀"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"MySQL 使用 MVCC 機制，可以讀取之前版本數據。這些舊版本記錄不會且也無法再去修改，就像快照一樣。所以我們將這種查詢稱爲"},{"type":"text","marks":[{"type":"strong"}],"text":"快照讀"},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當然並不是所有查詢都是快照讀，select .... for update/ in share mode 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RR，t2，t3 時刻兩個事務由於查詢語句，分別建立了一致性視圖。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"t4 時刻，由於事務 1 使用 "},{"type":"codeinline","content":[{"type":"text","text":"select.. for update"}]},{"type":"text","text":" 爲 id=1 這一行上了一把鎖，然後獲取到最新結果。而 t5 時刻，由於該行已被上鎖，事務 2 必須等待事務 1 釋放鎖才能繼續執行。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"t6 時刻根據一致性視圖，不能讀取到其他事務提交的版本，所以數據沒變。t8 時刻餘額扣減 100，t9 時刻提交事務。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"此時最新版本記錄爲 "},{"type":"text","marks":[{"type":"strong"}],"text":"id=1 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通過一致性視圖確保數據版本的可見性，相關規則總結如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"對於 RR 事務隔離等級，普通查詢僅能查到事務啓動前就已經提交完成的版本數據。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"對於 RC 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http://mysql.taobao.org/monthly/2018/11/04/"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"[^4]: https://dev.mysql.com/doc/refman/8.0/en/innodb-consistent-read.html"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"[^5]: 極客時間- MySQL 專欄--事務到底是隔離的還是不隔離的"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/e9/e93d657aa663979cfa352fb10334cc05.jpeg","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"0"}]}]}
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