org.eigenbase.rel.metadata
Class ChainedRelMetadataProvider

java.lang.Object
  org.eigenbase.rel.metadata.ChainedRelMetadataProvider

All Implemented Interfaces:

RelMetadataProvider

Direct Known Subclasses:

DefaultRelMetadataProvider

public class ChainedRelMetadataProvider
extends Object
implements RelMetadataProvider

ChainedRelMetadataProvider implements the RelMetadataProvider interface via the Glossary.ChainOfResponsibilityPattern.

Version:

$Id: //open/dev/farrago/src/org/eigenbase/rel/metadata/ChainedRelMetadataProvider.java#8 $

Author:

John V. Sichi

Field Summary

private List<RelMetadataProvider>

providers

Constructor Summary

ChainedRelMetadataProvider()
Creates a new empty chain.

Method Summary

void	addProvider(RelMetadataProvider provider) Adds a provider, giving it higher priority than all those already in chain.
Object	getRelMetadata(RelNode rel, String metadataQueryName, Object[] args) Retrieves metadata about a relational expression.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

providers

private List<RelMetadataProvider> providers

Constructor Detail

ChainedRelMetadataProvider

public ChainedRelMetadataProvider()

Creates a new empty chain.

Method Detail

addProvider

public void addProvider(RelMetadataProvider provider)

Adds a provider, giving it higher priority than all those already in chain. Chain order matters, since the first provider which answers a query is used.

Parameters:

provider - provider to add

getRelMetadata

public Object getRelMetadata(RelNode rel,
                             String metadataQueryName,
                             Object[] args)

Description copied from interface: RelMetadataProvider

Retrieves metadata about a relational expression.

Specified by:

getRelMetadata in interface RelMetadataProvider

Parameters:

rel - relational expression of interest

metadataQueryName - name of metadata query to invoke

args - arguments to metadata query (expected number and type depend on query name; must have well-defined hashCode/equals for use by caching); null can be used instead of empty array

Returns:

metadata result (actual type depends on query name), or null if the provider cannot answer the given query/rel combination; it is better to return null than to return a possibly incorrect answer