https://hangyan.github.io/tags/postgresql/Recent content in Postgresql on 涯余的博客Hugozhhang.yan@hotmail.com (涯余)hang.yan@hotmail.com (涯余)Tue, 27 Oct 2020 15:33:02 +0800https://hangyan.github.io/post/2020-10-27-postgres-tran/Tue, 27 Oct 2020 15:33:02 +0800hang.yan@hotmail.com (涯余)https://hangyan.github.io/post/2020-10-27-postgres-tran/<p>虽然日常工作需要涉及到数据库的底层部分并不多，但Postgresql作为一个数据库实现的范本是很值得研究的。可以通过它的实现来探索很多通用的数据库设计以及系统设计的理念。本文主要关注于事物设计方面。</p> <h2 id="mvcc">MVCC</h2> <p>pg底层使用<code>MVCC</code>，修改数据时会直接创建新版本，而不是直接修改旧数据。这部分有一点需要注意的是在pg中，所有的语句都是在事物中执行的，不管是不是明确地用了<code>BEGIN/COMMIT</code>。</p> <h2 id="transactions-tuples-and-snapshots">Transactions, tuples, and snapshots</h2> <p>先看一下 Transaction 的主要数据结构:</p> <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-c" data-lang="c"><span class="line"><span class="cl"><span class="k">typedef</span> <span class="k">struct</span> <span class="n">PGXACT</span> </span></span><span class="line"><span class="cl"><span class="p">{</span> </span></span><span class="line"><span class="cl"> <span class="n">TransactionId</span> <span class="n">xid</span><span class="p">;</span> <span class="cm">/* id of top-level transaction currently being </span></span></span><span class="line"><span class="cl"><span class="cm"> * executed by this proc, if running and XID </span></span></span><span class="line"><span class="cl"><span class="cm"> * is assigned; else InvalidTransactionId */</span> </span></span><span class="line"><span class="cl"> </span></span><span class="line"><span class="cl"> <span class="n">TransactionId</span> <span class="n">xmin</span><span class="p">;</span> <span class="cm">/* minimal running XID as it was when we were </span></span></span><span class="line"><span class="cl"><span class="cm"> * starting our xact, excluding LAZY VACUUM: </span></span></span><span class="line"><span class="cl"><span class="cm"> * vacuum must not remove tuples deleted by </span></span></span><span class="line"><span class="cl"><span class="cm"> * xid &gt;= xmin ! */</span> </span></span><span class="line"><span class="cl"> </span></span><span class="line"><span class="cl"> <span class="p">...</span> </span></span><span class="line"><span class="cl"><span class="p">}</span> <span class="n">PGXACT</span><span class="p">;</span> </span></span></code></pre></div><p>事物以<code>xid</code>作为标识.pg针对它做了很多优化，仅在真正开始写数据的时候才分配xid,如果是只读的事物就完全不分配。 <code>xmin</code>表示当这个事物开始的时候仍然处在运行中的事物列表中最小的那个<code>xid</code></p>