org.apache.calcite.rel.metadata

Interface Metadata

All Known Subinterfaces:

BuiltInMetadata.Collation, BuiltInMetadata.ColumnOrigin, BuiltInMetadata.ColumnUniqueness, BuiltInMetadata.CumulativeCost, BuiltInMetadata.DistinctRowCount, BuiltInMetadata.Distribution, BuiltInMetadata.ExplainVisibility, BuiltInMetadata.Memory, BuiltInMetadata.NonCumulativeCost, BuiltInMetadata.Parallelism, BuiltInMetadata.PercentageOriginalRows, BuiltInMetadata.PopulationSize, BuiltInMetadata.Predicates, BuiltInMetadata.RowCount, BuiltInMetadata.Selectivity, BuiltInMetadata.Size, BuiltInMetadata.UniqueKeys

public interface Metadata

Metadata about a relational expression.

For particular types of metadata, a sub-class defines one of more methods to query that metadata. Then a RelMetadataProvider can offer those kinds of metadata for particular sub-classes of RelNode.

User code (typically in a planner rule or an implementation of RelNode.computeSelfCost(org.apache.calcite.plan.RelOptPlanner)) acquires a Metadata instance by calling RelNode.metadata(java.lang.Class<M>).

A Metadata instance already knows which particular RelNode it is describing, so the methods do not pass in the RelNode. In fact, quite a few metadata methods have no extra parameters. For instance, you can get the row-count as follows:

 RelNode rel;
 double rowCount = rel.metadata(RowCount.class).rowCount();
 

Method Summary

Methods

Modifier and Type	Method and Description
RelNode	rel() Returns the relational expression that this metadata is about.

Method Detail

rel

RelNode rel()

Returns the relational expression that this metadata is about.