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<a name="navskip"></a><td height="25" colspan="4"><img alt="4.利用 Explain 分析 SQL" src="imagemaster/titlebar4.jpg" border="0" height="25" width="562"></td>
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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>Visual Explain</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 3 页（共9 页）</nobr></font></td>
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<p>Visual Explain 工具是从 DB2 控制中心（Control Center）调用的。要使用该工具，请打开 DB2 控制中心，找到并打开您希望对其运行 SQL 的数据库。用鼠标右键单击数据库名，然后选择 <b>Explain SQL</b>，如下图所示。</p>
<img src="images/s7_i4_1_ccVEMenu.gif" alt="Visual Explain 链接"><p>新弹出的菜单会要求您输入您希望 Explain 的 SQL。菜单上还有其它一些可用选项：</p>
<ul>
<li>
							
<b>Get：</b>允许您打开存储 SQL 的文本文件。</li>
<li>
							
<b>Save：</b>将正在使用的 SQL 语句保存到文本文件。</li>
<li>
							
<b>Query number：</b>允许您为查询分配具体的标号。</li>
<li>
							
<b>Query tag：</b>查询的字母标记。</li>
<li>
							
<b>Optimization level：</b>设置查询的优化级别（请参阅<a href="db2cert7014-4-6.html">优化级别</a>）。</li>
<li>
							
<b>Populate columns：</b>该选项将把存取方案信息存储在数据库中的一系列表中。这允许您以后无须重新编译 SQL 语句就能查看存取方案和解释信息。</li>
</ul>
<img src="images/s7_i4_2_sqlExplain.gif" alt="SQL Explain 菜单"><p>一旦成功执行了 SQL，将会打开一个新菜单，它将向您显示该查询的存取方案。还将创建一个更小的标记为 <b>Overview</b> 的窗口。该窗口利用蓝色矩形框来突出显示您正在查看的那部分存取方案。当您正在使用非常大而且复杂的查询，并且很难知道当前正在查看哪一部分时，该功能就很便捷。主窗口的左边有一个滚动条，它允许您控制视图的缩放。这只会影响视图大小，而不会为您提供任何其它信息。</p>
<img src="images/s7_i4_3_accessPlan.gif" alt="存取方案"><p>可以单击存取方案的每个组成部分，以显示有关该部分的更多详细信息。不妨尝试单击所有不同的对象类型，以充分了解可以利用的查询信息。</p>
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