Github user gatorsmile commented on a diff in the pull request: https://github.com/apache/spark/pull/19136#discussion_r138253471 --- Diff: sql/core/src/main/java/org/apache/spark/sql/sources/v2/reader/DataSourceV2Reader.java --- @@ -0,0 +1,58 @@ +/* + * 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.spark.sql.sources.v2.reader; + +import java.util.List; + +import org.apache.spark.sql.Row; +import org.apache.spark.sql.types.StructType; + +/** + * A data source reader that can mix in various query optimization interfaces and implement these + * optimizations. The actual scan logic should be delegated to `ReadTask`s that are returned by + * this data source reader. + * + * There are mainly 3 kinds of query optimizations: + * 1. push operators downward to the data source, e.g., column pruning, filter push down, etc. + * 2. propagate information upward to Spark, e.g., report statistics, report ordering, etc. + * 3. special scans like columnar scan, unsafe row scan, etc. Note that a data source reader can + * at most implement one special scan. --- End diff -- `at most implement one` -> ` implement at most one`
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