Github user gatorsmile commented on a diff in the pull request:
https://github.com/apache/spark/pull/19424#discussion_r142800670
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/PushDownOperatorsToDataSource.scala
---
@@ -0,0 +1,99 @@
+/*
+ * 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.execution.datasources.v2
+
+import org.apache.spark.sql.catalyst.expressions.{And, AttributeMap,
AttributeSet, Expression, ExpressionSet}
+import org.apache.spark.sql.catalyst.planning.PhysicalOperation
+import org.apache.spark.sql.catalyst.plans.logical.{Filter, LogicalPlan,
Project}
+import org.apache.spark.sql.catalyst.rules.Rule
+import org.apache.spark.sql.execution.datasources.DataSourceStrategy
+import org.apache.spark.sql.sources
+import org.apache.spark.sql.sources.v2.reader._
+
+/**
+ * Pushes down various operators to the underlying data source for better
performance. We classify
+ * operators into different layers, operators in the same layer are
orderless, i.e. the query result
+ * won't change if we switch the operators within a layer(e.g. we can
switch the order of predicates
+ * and required columns). The operators in layer N can only be pushed down
if operators in layer N-1
+ * that above the data source relation are all pushed down. As an example,
you can't push down limit
+ * if a filter below limit is not pushed down.
--- End diff --
> As an example, given a LIMIT has a FILTER child, you can't push down
LIMIT if FILTER is not completely pushed down. When both are pushed down, the
data source should execute FILTER before LIMIT.
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