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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>优化级别</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 6 页（共9 页）</nobr></font></td>
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<p>影响存取方案的最重要参数是<i>优化级别</i>。该字段告诉优化器应当花费多少开销以及使用什么技术来确定存取方案。级别越高，会使优化器使用的算法和代数分析越复杂，并使用更多时间来生成最终方案。</p>
<p>每个优化级别所使用的是整个规则和统计信息的不同部分。有关这些级别十分详尽的描述可在 DB2 文档 <i>SQL Reference</i> 第 5 章&ldquo;Statements&rdquo;的&ldquo;Set Current Query Optimization&rdquo;一节（请参阅<a href="db2cert7014-7-2.html">参考资料</a>）中找到。下面简要介绍了每个优化级别。</p>
<ul>
<li>
							
<b>0</b> — 使用最小优化</li>
<li>
							
<b>1</b> — 使用与 DB2/6000 V1 大致类似的优化程度，再加上 V1 中没有的其它一些低成本功能</li>
<li>
							
<b>2</b> — 使用与优化级别 5 相同的功能，但是简化了连接算法</li>
<li>
							
<b>3</b> — 执行适中优化；类似于 DB2 for MVS/ESA 的查询优化特征</li>
<li>
							
<b>5</b> — 使用大量优化；带有 Heuristic 规则（缺省值）</li>
<li>
							
<b>7</b> — 使用大量优化；不带 Heuristic 规则</li>
<li>
							
<b>9</b> — 使用所有可用的优化技术</li>
</ul>
<p>一般经验法则是，针对不同的查询工作负载使用下列优化级别：</p>
<ul>
<li>
							
<b>非常简单的 OLTP：0 或 1。</b>适用于那些不太需要优化并且主要依赖于主键索引搜索或非常简单的连接的查询。</li>
<li>
							
<b>OLTP：1。</b>适用于涉及少量表和使用表上的索引进行连接的简单查询。</li>
<li>
							
<b>混合的 OLTP 和报告：5。</b>适用于涉及复杂 OLTP 或报告的工作负载，而 OLTP 或报告涉及多个表上的许多复杂连接。</li>
<li>
							
<b>非常复杂的数据挖掘或决策支持：9。</b>那些需要进行大量数据统计信息分析，并且长时间（超过 1 分钟）运行的查询应当使用最高级别的优化。该优化将花费比较长的时间，但是，存取方案中的改进所产生的影响通常会超过额外编译时间所产生的影响。</li>
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