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| <h2 title="接口 Metadata" class="title">接口 Metadata</h2> |
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| <dt>所有已知子接口:</dt> |
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| <pre>public interface <span class="typeNameLabel">Metadata</span></pre> |
| <div class="block">Metadata about a relational expression. |
| |
| <p>For particular types of metadata, a sub-class defines one of more methods |
| to query that metadata. Then a <a href="../../../../../org/apache/calcite/rel/metadata/RelMetadataProvider.html" title="org.apache.calcite.rel.metadata中的接口"><code>RelMetadataProvider</code></a> can offer those |
| kinds of metadata for particular sub-classes of <a href="../../../../../org/apache/calcite/rel/RelNode.html" title="org.apache.calcite.rel中的接口"><code>RelNode</code></a>. |
| |
| <p>User code (typically in a planner rule or an implementation of |
| <a href="../../../../../org/apache/calcite/rel/RelNode.html#computeSelfCost-org.apache.calcite.plan.RelOptPlanner-org.apache.calcite.rel.metadata.RelMetadataQuery-"><code>RelNode.computeSelfCost(org.apache.calcite.plan.RelOptPlanner, RelMetadataQuery)</code></a>) |
| acquires a <code>Metadata</code> instance by calling <a href="../../../../../org/apache/calcite/rel/RelNode.html#metadata-java.lang.Class-org.apache.calcite.rel.metadata.RelMetadataQuery-"><code>RelNode.metadata(java.lang.Class<M>, org.apache.calcite.rel.metadata.RelMetadataQuery)</code></a>. |
| |
| <p>A <code>Metadata</code> instance already knows which particular <code>RelNode</code> |
| it is describing, so the methods do not pass in the <code>RelNode</code>. In fact, |
| quite a few metadata methods have no extra parameters. For instance, you can |
| get the row-count as follows:</p> |
| |
| <blockquote><pre><code> |
| RelNode rel; |
| double rowCount = rel.metadata(RowCount.class).rowCount(); |
| </code></pre></blockquote></div> |
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| <td class="colFirst"><code><a href="../../../../../org/apache/calcite/rel/RelNode.html" title="org.apache.calcite.rel中的接口">RelNode</a></code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../org/apache/calcite/rel/metadata/Metadata.html#rel--">rel</a></span>()</code> |
| <div class="block">Returns the relational expression that this metadata is about.</div> |
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| <pre><a href="../../../../../org/apache/calcite/rel/RelNode.html" title="org.apache.calcite.rel中的接口">RelNode</a> rel()</pre> |
| <div class="block">Returns the relational expression that this metadata is about.</div> |
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