| /* |
| * 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 org.apache.calcite.rel.metadata; |
| |
| import org.apache.calcite.rel.RelNode; |
| |
| /** |
| * 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 {@link RelMetadataProvider} can offer those |
| * kinds of metadata for particular sub-classes of {@link RelNode}. |
| * |
| * <p>User code (typically in a planner rule or an implementation of |
| * {@link RelNode#computeSelfCost(org.apache.calcite.plan.RelOptPlanner, RelMetadataQuery)}) |
| * acquires a {@code Metadata} instance by calling {@link RelNode#metadata}. |
| * |
| * <p>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:</p> |
| * |
| * <blockquote><pre><code> |
| * RelNode rel; |
| * double rowCount = rel.metadata(RowCount.class).rowCount(); |
| * </code></pre></blockquote> |
| */ |
| public interface Metadata { |
| /** Returns the relational expression that this metadata is about. */ |
| RelNode rel(); |
| } |
| |
| // End Metadata.java |
