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<div class="sub-title"><span class="package-label-in-type">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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<section class="class-description" id="class-description">
<dl class="notes">
<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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<div class="type-signature"><span class="modifiers">public interface </span><span class="element-name type-name-label">Metadata</span></div>
<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>
</section>
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<h2>Method Summary</h2>
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<div class="table-header col-first">Modifier and Type</div>
<div class="table-header col-second">Method</div>
<div class="table-header col-last">Description</div>
<div class="col-first even-row-color method-summary-table method-summary-table-tab2 method-summary-table-tab3"><code><a href="../RelNode.html" title="interface in org.apache.calcite.rel">RelNode</a></code></div>
<div class="col-second even-row-color method-summary-table method-summary-table-tab2 method-summary-table-tab3"><code><a href="#rel()" class="member-name-link">rel</a>()</code></div>
<div class="col-last even-row-color method-summary-table method-summary-table-tab2 method-summary-table-tab3">
<div class="block">Returns the relational expression that this metadata is about.</div>
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<h3>rel</h3>
<div class="member-signature"><span class="return-type"><a href="../RelNode.html" title="interface in org.apache.calcite.rel">RelNode</a></span>&nbsp;<span class="element-name">rel</span>()</div>
<div class="block">Returns the relational expression that this metadata is about.</div>
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