Github user liancheng commented on a diff in the pull request:
https://github.com/apache/spark/pull/2475#discussion_r19658936
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/sources/interfaces.scala ---
@@ -0,0 +1,86 @@
+/*
+* 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
+
+import org.apache.spark.annotation.DeveloperApi
+import org.apache.spark.rdd.RDD
+import org.apache.spark.sql.catalyst.expressions.{Expression, Attribute}
+
+/**
+ * Implemented by objects that produce relations for a specific kind of
data source. When
+ * Spark SQL is given a DDL operation with a USING clause specified, this
interface is used to
+ * pass in the parameters specified by a user.
+ *
+ * Users may specify the fully qualified class name of a given data
source. When that class is
+ * not found Spark SQL will append the class name `DefaultSource` to the
path, allowing for
+ * less verbose invocation. For example, 'org.apache.spark.sql.json'
would resolve to the
+ * data source 'org.apache.spark.sql.json.DefaultSource'
+ *
+ * A new instance of this class with be instantiated each time a DDL call
is made.
+ */
+@DeveloperApi
+trait RelationProvider {
+ /** Returns a new base relation with the given parameters. */
+ def createRelation(sqlContext: SQLContext, parameters: Map[String,
String]): BaseRelation
+}
+
+/**
+ * Represents a collection of tuples with a known schema. Classes that
extend BaseRelation must
+ * be able to produce the schema of their data in the form of a
[[StructType]] In order to be
+ * executed, a BaseRelation must also mix in at least one of the Scan
traits.
+ *
+ * BaseRelations must also define a equality function that only returns
true when the two
+ * instances will return the same data. This equality function is used
when determining when
+ * it is safe to substitute cached results for a given relation.
--- End diff --
Need a default `sameResult: BaseRelation => Boolean` implementation here
and hook it in `LogicalRelation`, e.g.:
```scala
def sameResult(otherRelation: BaseRelation) = this eq otherRelation
```
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