MetadataFactory (Apache Calcite API)

All Known Implementing Classes:

MetadataFactoryImpl
```
public interface MetadataFactory
```
Source of metadata about relational expressions.
The metadata is typically various kinds of statistics used to estimate costs.

Each kind of metadata has an interface that extends Metadata and has a method. Some examples: BuiltInMetadata.Selectivity, BuiltInMetadata.ColumnUniqueness.

Method Summary

All Methods Instance Methods Abstract Methods
Modifier and Type	Method	Description
`<M extends Metadata> M`	`query(RelNode rel, RelMetadataQuery mq, java.lang.Class<M> metadataClazz)`	Returns a metadata interface to get a particular kind of metadata from a particular relational expression.

- Method Detail
  - query
```
<M extends Metadata> M query(RelNode rel,
                             RelMetadataQuery mq,
                             java.lang.Class<M> metadataClazz)
```
    Returns a metadata interface to get a particular kind of metadata from a particular relational expression. Returns null if that kind of metadata is not available.
    
    Type Parameters:
    
    M - Metadata type
    
    Parameters:
    
    rel - Relational expression
    
    mq - Metadata query
    
    metadataClazz - Metadata class
    
    Returns:
    
    Metadata bound to rel and query