https://my-blog.j8465383071.workers.dev/tags/%E4%BA%8B%E5%8A%A1/Recent content in 事务 on Kernel NotesHugo -- 0.147.7zh-cnMon, 17 Aug 2026 18:36:57 +0800https://my-blog.j8465383071.workers.dev/posts/postgresql/3-6-tans-ai/Fri, 03 Jul 2026 16:51:00 +0800https://my-blog.j8465383071.workers.dev/posts/postgresql/3-6-tans-ai/<h1 id="概述">概述</h1> <p>假如，2个会话，并发写入数据：</p> <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-sql" data-lang="sql"><span class="line"><span class="cl"><span class="k">BEGIN</span><span class="p">;</span><span class="w"> </span></span></span><span class="line"><span class="cl"><span class="w"></span><span class="k">INSERT</span><span class="w"> </span><span class="k">INTO</span><span class="w"> </span><span class="n">t1</span><span class="w"> </span><span class="k">VALUES</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="s1">&#39;data1&#39;</span><span class="p">);</span><span class="w"> </span></span></span><span class="line"><span class="cl"><span class="w"></span><span class="k">COMMIT</span><span class="p">;</span><span class="w"> </span></span></span><span class="line"><span class="cl"><span class="w"> </span></span></span><span class="line"><span class="cl"><span class="w"></span><span class="k">BEGIN</span><span class="p">;</span><span class="w"> </span></span></span><span class="line"><span class="cl"><span class="w"></span><span class="k">INSERT</span><span class="w"> </span><span class="k">INTO</span><span class="w"> </span><span class="n">t1</span><span class="w"> </span><span class="k">VALUES</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="s1">&#39;data2&#39;</span><span class="p">);</span><span class="w"> </span></span></span><span class="line"><span class="cl"><span class="w"></span><span class="k">COMMIT</span><span class="p">;</span><span class="w"> </span></span></span></code></pre></div><p>加锁逻辑：</p> <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-python" data-lang="python"><span class="line"><span class="cl"><span class="n">exec_simple_query</span> </span></span><span class="line"><span class="cl"> <span class="n">pg_parse_query</span> </span></span><span class="line"><span class="cl"> <span class="n">pg_analyze_and_rewrite</span> </span></span><span class="line"><span class="cl"> <span class="n">pg_plan_queries</span> </span></span><span class="line"><span class="cl"> <span class="n">PortalStart</span> </span></span><span class="line"><span class="cl"> <span class="n">PortalRun</span> </span></span><span class="line"><span class="cl"> <span class="n">PortalRunMulti</span> </span></span><span class="line"><span class="cl"> <span class="n">ProcessQuery</span> </span></span><span class="line"><span class="cl"> <span class="n">ExecutorStart</span> </span></span><span class="line"><span class="cl"> <span class="n">standard_ExecutorStart</span> </span></span><span class="line"><span class="cl"> <span class="n">InitPlan</span> </span></span><span class="line"><span class="cl"> <span class="n">ExecInitNode</span> </span></span><span class="line"><span class="cl"> <span class="n">ExecInitModifyTable</span> </span></span><span class="line"><span class="cl"> <span class="n">ExecInitResultRelation</span> </span></span><span class="line"><span class="cl"> <span class="n">ExecGetRangeTableRelation</span> </span></span><span class="line"><span class="cl"> <span class="n">table_open</span><span class="p">(</span><span class="n">relation</span><span class="p">,</span> <span class="s1">&#39;RowExclusiveLock&#39;</span><span class="p">)</span> <span class="c1"># 8级锁，行级排他锁</span> </span></span><span class="line"><span class="cl"> <span class="n">relation_open</span> </span></span><span class="line"><span class="cl"> <span class="n">LockRelationOid</span> </span></span><span class="line"><span class="cl"> <span class="n">LockAcquire</span> </span></span><span class="line"><span class="cl"> <span class="n">ExecutorRun</span> </span></span><span class="line"><span class="cl"> <span class="n">standard_ExecutorRun</span> </span></span><span class="line"><span class="cl"> <span class="n">ExecutePlan</span> </span></span><span class="line"><span class="cl"> <span class="o">...</span> </span></span><span class="line"><span class="cl"> <span class="n">ExecInsert</span> </span></span><span class="line"><span class="cl"> <span class="n">table_tuple_insert</span> </span></span><span class="line"><span class="cl"> <span class="n">heapam_tuple_insert</span> </span></span><span class="line"><span class="cl"> <span class="n">heap_insert</span> </span></span><span class="line"><span class="cl"> <span class="n">RelationGetBufferForTuple</span> </span></span><span class="line"><span class="cl"> <span class="n">LockBuffer</span> </span></span><span class="line"><span class="cl"> <span class="n">LWLockAcquire</span><span class="p">(</span><span class="n">buffer</span><span class="p">,</span> <span class="s1">&#39;LW_EXCLUSIVE&#39;</span><span class="p">)</span> <span class="c1"># 读写锁</span> </span></span><span class="line"><span class="cl"> <span class="n">RelationPutHeapTuple</span> </span></span><span class="line"><span class="cl"> <span class="n">log_heap_insert</span> </span></span></code></pre></div><ul> <li> <p>锁</p> </li> <li> <p>HWLock</p> <ul> <li>8级</li> <li>relation, index, buffer</li> <li>死锁检测</li> </ul> </li> <li> <h2 id="lwlocks">LWLocks</h2> <ul> <li>buffer</li> </ul> </li> <li> <p>SpinLock</p> </li> <li> <p>PresicateLock</p> </li> </ul> <h2 id="hwlock">HWLock</h2> <table> <thead> <tr> <th>锁</th> <th>取值</th> <th>操作</th> </tr> </thead> <tbody> <tr> <td>AccessShareLock</td> <td>1</td> <td>SELECT</td> </tr> <tr> <td>RowShareLock</td> <td>2</td> <td>SELECT FOR UPDATE/SHARE</td> </tr> <tr> <td>RowExclusiveLock</td> <td>3</td> <td>INSERT/UPDATE/DELETE</td> </tr> <tr> <td>ShareUpdateExclusiveLock</td> <td>4</td> <td>VACUUM/ANALYZE/CRATE INDEX CONCURRENTLY</td> </tr> <tr> <td>ShareLock</td> <td>5</td> <td>CREATE INDEX</td> </tr> <tr> <td>ShareRowExclusiveLock</td> <td>6</td> <td>row share</td> </tr> <tr> <td>ExclusiveLock</td> <td>7</td> <td>block ROW SHARE/SELECT FOR UPDATE</td> </tr> <tr> <td>AccessExclusiveLock</td> <td>8</td> <td>ALTER TABLE/DROP TABLE/VACUUM FULL</td> </tr> </tbody> </table> <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-bash" data-lang="bash"><span class="line"><span class="cl">LockRelationOid </span></span><span class="line"><span class="cl"> LockAcquireExtended </span></span><span class="line"><span class="cl"> hash_search </span></span></code></pre></div><p>所有进程共享</p>https://my-blog.j8465383071.workers.dev/posts/postgresql/3-6-transaction/Tue, 21 Jan 2025 13:55:00 +0800https://my-blog.j8465383071.workers.dev/posts/postgresql/3-6-transaction/<h1 id="1-事务的概念">1 事务的概念</h1> <h2 id="11-事务的基本特性">1.1 事务的基本特性</h2> <p>事务的4个特性：</p> <ul> <li>原子性 Automicity</li> <li>一致性 Consistency</li> <li>隔离性 Isolation</li> <li>持久性 Durability</li> </ul> <h2 id="12-事务的隔离性">1.2 事务的隔离性</h2> <p>不同隔离级别下，可能出现的异常如下：（T 表示可能）</p> <table> <thead> <tr> <th>异常</th> <th>读未提交</th> <th>读已提交</th> <th>可重复读</th> <th>可串行化</th> <th>解释</th> </tr> </thead> <tbody> <tr> <td>脏读</td> <td>T</td> <td>.</td> <td>.</td> <td>.</td> <td>读其他事务未提交数据</td> </tr> <tr> <td>不可重复读</td> <td>T</td> <td>T</td> <td>.</td> <td>.</td> <td>2次读取数据不一致</td> </tr> <tr> <td>幻读</td> <td>T</td> <td>T</td> <td>T</td> <td>.</td> <td>2次读取数据条数不一致</td> </tr> </tbody> </table> <p>postgresql默认的隔离级别是读已提交</p> <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-sql" data-lang="sql"><span class="line"><span class="cl"><span class="c1">-- 查看默认事务隔离级别 </span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="k">show</span><span class="w"> </span><span class="n">default_transaction_isolation</span><span class="p">;</span><span class="w"> </span></span></span><span class="line"><span class="cl"><span class="w"> </span><span class="n">default_transaction_isolation</span><span class="w"> </span></span></span><span class="line"><span class="cl"><span class="w"></span><span class="c1">------------------------------- </span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="w"> </span><span class="k">read</span><span class="w"> </span><span class="k">committed</span><span class="w"> </span></span></span><span class="line"><span class="cl"><span class="w"> </span></span></span><span class="line"><span class="cl"><span class="w"></span><span class="c1">-- 查询当前的事务快照 pg 10 </span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="k">SELECT</span><span class="w"> </span><span class="n">txid_current_snapshot</span><span class="p">();</span><span class="w"> </span></span></span><span class="line"><span class="cl"><span class="w"> </span><span class="n">txid_current_snapshot</span><span class="w"> </span></span></span><span class="line"><span class="cl"><span class="w"></span><span class="c1">----------------------- </span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="w"> </span><span class="mi">1081</span><span class="p">:</span><span class="mi">1081</span><span class="p">:</span><span class="w"> </span></span></span><span class="line"><span class="cl"><span class="w"> </span></span></span><span class="line"><span class="cl"><span class="w"></span><span class="c1">-- 查询所有正在活跃的进程 </span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="k">SELECT</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="k">FROM</span><span class="w"> </span><span class="n">pg_stat_activity</span><span class="p">;</span><span class="w"> </span></span></span><span class="line"><span class="cl"><span class="w"> </span></span></span><span class="line"><span class="cl"><span class="w"></span><span class="c1">-- </span></span></span></code></pre></div><h2 id="13-事务的实现">1.3 事务的实现</h2> <p>为实现事务，有3种并发控制机制：</p>