A? table lock , also called a? TM lock , is acquired by a transaction when a table is modified by an? INSERT ,? UPDATE ,? DELETE ,? MERGE ,? SELECT ?with the? FOR UPDATE ?clause, or? LOCK ? TABLE ?statement. DML operations require table locks to reserve DML access to the table on behalf of a transaction and to prevent DDL operations that would conflict with the transaction.

A table lock can be held in any of the following modes:

表級(jí)鎖包含了以下幾種模式：

• Row Share (RS)

  This lock, also called a? subshare table lock (SS) , indicates that the transaction holding the lock on the table has locked rows in the table and intends to update them. A row share lock is the least restrictive mode of table lock, offering the highest degree of concurrency for a table.

  行共享：表級(jí)鎖中最低限度的鎖表模式，能夠提供最高的并發(fā)度。此時(shí)不希望其他用戶(hù)或繪畫(huà)對(duì)改行進(jìn)行更新操作。

• Row Exclusive Table Lock (RX)

  This lock, also called a? subexclusive table lock (SX) , generally indicates that the transaction holding the lock has updated table rows or issued SELECT ... FOR UPDATE . An SX lock allows other transactions to query, insert, update, delete, or lock rows concurrently in the same table. Therefore, SX locks allow multiple transactions to obtain simultaneous SX and subshare table locks for the same table.

  行級(jí)排他鎖，表示獲得該鎖的事務(wù)將對(duì)表進(jìn)行DML操作，SX鎖允許其他事務(wù)對(duì)相同的表進(jìn)行查詢(xún)、插入、更新、修改或者鎖其他行。因此SX鎖允許多個(gè)事務(wù)獲得SX鎖和S鎖，但是不能獲得X鎖

• Share Table Lock (S)

  A share table lock held by a transaction allows other transactions to query the table (without using? SELECT ... FOR UPDATE ), but updates are allowed only if a single transaction holds the share table lock. Because multiple transactions may hold a share table lock concurrently, holding this lock is not sufficient to ensure that a transaction can modify the table.

  共享鎖，不允許任何用戶(hù)更新表，但是允許其他用戶(hù)發(fā)出select ... for update添加RS鎖

• Share Row Exclusive Table Lock (SRX)

  This lock, also called a? share-subexclusive table lock (SSX) , is more restrictive than a share table lock. Only one transaction at a time can acquire an SSX lock on a given table. An SSX lock held by a transaction allows other transactions to query the table (except for? SELECT ... ? FOR UPDATE ) but not to update the table.

  同時(shí)只有一個(gè)事務(wù)可以獲得該共享行級(jí)排他鎖，此時(shí)只允許其他事務(wù)查詢(xún)表，但不能進(jìn)行DML操作，也不能使用select ... for update。

• Exclusive Table Lock (X)

  This lock is the most restrictive, prohibiting other transactions from performing any type of DML statement or placing any type of lock on the table.

  排他鎖，其他事務(wù)不能對(duì)該表進(jìn)行任何DML和DDL操作，也不能添加任何類(lèi)型的鎖。

  各個(gè)模式的鎖之間的兼容性如下：