我對ORACLE資料鎖的一點體會，我對oracle

我對ORACLE資料鎖的一點體會，我對oracle
該貼的部分內容是針對帖中帶的附件（一位大俠關於鎖的理解），提的理解/疑問，
大家在讀此貼前，請先閱讀該文。
先說說各類型的鎖：共用鎖定，排它說，共用排它鎖（對錶定義共用，對錶操作的記錄排它）。
1、我對文章中意圖鎖定的理解，就是：對錶的記錄進行操作之前，先對錶定義（包括表結構、約束等） 
 加了共用鎖定，這是為了避免對錶的DDL操作。 
 比如：當你往TAB1插入一條記錄時，該表的一個欄位COL8是允許為空白的，插入這條記錄的該欄位 
 的值是空的，此時，若不對該表定義加共用鎖定，則另外一SESSION對TAB1.COL8加非空約束， 
 那該表結構修改就與插入的記錄發生衝突了。所以，當執行DML操作是，必定對操作的表加DDL 
 共用鎖定。  這是我理解的意圖鎖定。 2、文章中提及，記錄行是無共用鎖定。我對此觀點表示不同：當插入/修改子表時，對應的父表的主鍵 
 記錄應該被加共用鎖定，因為此時要保證父鍵的存在。經測試，當插入/修改/刪除子表時，父表 
 確實被加了SS鎖，此時可以對父表的主鍵做插入動作，但不允許做修改/刪除。這符合邏輯。因為 
 插入操作並不影響主外鍵的關係。但刪除/修改則可能。 
3、SHARE VS ROW SHARE有什麼區別？ 
 我猜想：ROW SHARE是（層級=2）對錶加表定義共用鎖定，SHARE（層級=4）是對錶定義及表“所有”的 
 記錄加共用鎖定。但這樣就有問題：1、SELECT FOR UPDATE產生TM=2、對查詢出來的記錄產生TX=6的 
 鎖，那這樣應該與：表“所有”的記錄加共用鎖定 相衝突了？但實際並不是，實際上，當對錶加SHARE 
 鎖時，還是可以該表執行 SELECT FOR UPDATE，但卻不可以執行DML操作！
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