com.hazelcast.org.apache.calcite.rel.metadata.Metadata Maven / Gradle / Ivy
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to you under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.hazelcast.org.apache.calcite.rel.metadata;
import com.hazelcast.org.apache.calcite.rel.RelNode;
/**
* Metadata about a relational expression.
*
* For particular types of metadata, a sub-class defines one of more methods
* to query that metadata. Then a {@link RelMetadataProvider} can offer those
* kinds of metadata for particular sub-classes of {@link RelNode}.
*
*
User code (typically in a planner rule or an implementation of
* {@link RelNode#computeSelfCost(com.hazelcast.org.apache.calcite.plan.RelOptPlanner, RelMetadataQuery)})
* acquires a {@code Metadata} instance by calling {@link RelNode#metadata}.
*
*
A {@code Metadata} instance already knows which particular {@code RelNode}
* it is describing, so the methods do not pass in the {@code 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();
*
*/
public interface Metadata {
/** Returns the relational expression that this metadata is about. */
RelNode rel();
}