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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>Explain 表格式化命令</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 7 页（共9 页）</nobr></font></td>
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<p>
<code>db2exfmt</code> 工具用于处理插入到 Explain 表中的信息。DB2 使用这些表来记录有关 SQL 不同组成部分的所有信息以及优化信息。当您使用 Visual Explain 工具时，会自动为您创建这些表。如果您最初使用文本工具，那么您首先必须运行一个脚本来创建这些表。在您所连接的 db2 实例主目录下的 <code>misc</code> 子目录中执行下列命令。将为您创建所有表和索引。</p>
<p>下面是 Explain 表出错消息：</p>
<img src="images/s7_i8_1_explainErr.gif" alt="Explain 表错误"><p>
下面是适当的命令：</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
db2 connect to databaseName
db2 -tvf EXPLAIN.DDL
</code>
</pre>
<p>
<code>db2exfmt</code> 工具的不同之处在于，它使用已经捕获的信息，然后用一种易于读取的格式来表示它。该工作完成后，填充 Explain 表。</p>
<p>
						
<b>捕获 Explain 信息</b>
					
</p>
<p>捕获 Explain 信息的最简单方法是将 SQL 存储在文本文件中，然后可以从 DB2 命令行处理器（CLP）（在 Windows 中）或从命令行（在 UNIX 或 Linux 中）运行它。DB2 有一个变量 CURRENT EXPLAIN MODE，它确定是否捕获 Explain 信息。可以对该变量进行设置，使得每当执行查询时，就让 DB2 填充 Explain 表。下面是执行这一工作的适当命令：</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
db2 SET CURRENT EXPLAIN MODE [no | yes | explain]
</code>
</pre>
<p>下面是 CURRENT EXPLAIN MODE 可能使用的值：</p>
<ul>
<li>
							
<code>NO:</code> 该值是缺省值；查询正常执行</li>
<li>
							
<code>YES:</code> 查询正常执行，同时捕获 Explain 信息</li>
<li>
<code>EXPLAIN:</code> 不执行查询，但是将 Explain 信息填入表</li>
</ul>
<p>
下面是一个示例：</p>
<img src="images/s7_i8_2_explainCmd.gif" alt="Explain 示例"><p>
						
<b>使用工具</b>
					
</p>
<p>
						
<code>db2exfmt</code> 有许多选项，这些选项允许您选择您希望使用的 Explain 语句。您还可以对显示哪些信息进行控制。最简单的方法是全部使用缺省值，这样就会使该工具返回有关最近解释过的语句的信息：</p>
<img src="images/s7_i8_3_db2exfmt.gif" alt="db2exfmt"><p>
下面是样本查询摘要输出：</p>
<img src="images/s7_i8_4_out1.gif" alt="查询输出 1"><p>
下面是样本查询树信息：</p>
<img src="images/s7_i8_5_out2.gif" alt="查询输出 2"><br>
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