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<div class="subTitle"><span class="packageLabelInType">Package</span>&nbsp;<a href="package-summary.html">org.apache.calcite.rel.metadata</a></div>
<h1 title="Interface Metadata" class="title">Interface Metadata</h1>
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<dt>All Known Subinterfaces:</dt>
<dd><code><a href="BuiltInMetadata.AllPredicates.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.AllPredicates</a></code>, <code><a href="BuiltInMetadata.Collation.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.Collation</a></code>, <code><a href="BuiltInMetadata.ColumnOrigin.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.ColumnOrigin</a></code>, <code><a href="BuiltInMetadata.ColumnUniqueness.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.ColumnUniqueness</a></code>, <code><a href="BuiltInMetadata.CumulativeCost.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.CumulativeCost</a></code>, <code><a href="BuiltInMetadata.DistinctRowCount.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.DistinctRowCount</a></code>, <code><a href="BuiltInMetadata.Distribution.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.Distribution</a></code>, <code><a href="BuiltInMetadata.ExplainVisibility.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.ExplainVisibility</a></code>, <code><a href="BuiltInMetadata.ExpressionLineage.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.ExpressionLineage</a></code>, <code><a href="BuiltInMetadata.LowerBoundCost.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.LowerBoundCost</a></code>, <code><a href="BuiltInMetadata.MaxRowCount.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.MaxRowCount</a></code>, <code><a href="BuiltInMetadata.Memory.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.Memory</a></code>, <code><a href="BuiltInMetadata.MinRowCount.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.MinRowCount</a></code>, <code><a href="BuiltInMetadata.NodeTypes.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.NodeTypes</a></code>, <code><a href="BuiltInMetadata.NonCumulativeCost.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.NonCumulativeCost</a></code>, <code><a href="BuiltInMetadata.Parallelism.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.Parallelism</a></code>, <code><a href="BuiltInMetadata.PercentageOriginalRows.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.PercentageOriginalRows</a></code>, <code><a href="BuiltInMetadata.PopulationSize.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.PopulationSize</a></code>, <code><a href="BuiltInMetadata.Predicates.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.Predicates</a></code>, <code><a href="BuiltInMetadata.RowCount.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.RowCount</a></code>, <code><a href="BuiltInMetadata.Selectivity.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.Selectivity</a></code>, <code><a href="BuiltInMetadata.Size.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.Size</a></code>, <code><a href="BuiltInMetadata.TableReferences.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.TableReferences</a></code>, <code><a href="BuiltInMetadata.UniqueKeys.html" title="interface in org.apache.calcite.rel.metadata">BuiltInMetadata.UniqueKeys</a></code></dd>
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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="RelMetadataProvider.html" title="interface in org.apache.calcite.rel.metadata"><code>RelMetadataProvider</code></a> can offer those
kinds of metadata for particular sub-classes of <a href="../RelNode.html" title="interface in org.apache.calcite.rel"><code>RelNode</code></a>.
<p>User code (typically in a planner rule or an implementation of
<a href="../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="../RelNode.html#metadata(java.lang.Class,org.apache.calcite.rel.metadata.RelMetadataQuery)"><code>RelNode.metadata(java.lang.Class&lt;M&gt;, 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="../RelNode.html" title="interface in org.apache.calcite.rel">RelNode</a></code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#rel()">rel</a></span>()</code></th>
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<div class="block">Returns the relational expression that this metadata is about.</div>
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<div class="memberSignature"><span class="returnType"><a href="../RelNode.html" title="interface in org.apache.calcite.rel">RelNode</a></span>&nbsp;<span class="memberName">rel</span>()</div>
<div class="block">Returns the relational expression that this metadata is about.</div>
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