Github user marmbrus commented on a diff in the pull request:
https://github.com/apache/spark/pull/2475#discussion_r19646282
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/sources/package.scala ---
@@ -0,0 +1,95 @@
+/*
+ * 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
+
+import org.apache.spark.annotation.DeveloperApi
+import org.apache.spark.rdd.RDD
+import org.apache.spark.sql.catalyst.expressions.{Expression, Attribute}
+
+/**
+ * A set of APIs for adding data sources to Spark SQL.
+ */
+package object sources {
+
+ /**
+ * 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'
+ */
+ 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.
+ */
+ @DeveloperApi
+ abstract class BaseRelation {
+ def sqlContext: SQLContext
+ def schema: StructType
+ }
+
+ /**
+ * Mixed into a BaseRelation that can produce all of its tuples as an
RDD of Row objects.
+ */
+ @DeveloperApi
+ trait TableScan {
+ self: BaseRelation =>
+
+ def buildScan(): RDD[Row]
+ }
+
+ /**
+ * Mixed into a BaseRelation that can eliminate unneeded columns before
producing an RDD
+ * containing all of its tuples as Row objects.
+ */
+ @DeveloperApi
+ trait PrunedScan {
+ self: BaseRelation =>
+
+ def buildScan(requiredColumns: Seq[Attribute]): RDD[Row]
--- End diff --
I guess it could be limited to `AttributeReference` at this point in query
execution. `Attribute` is just the generic version that can also be
unresolved. From a usage standpoint they have essentially the same interfaces
so all internal APIs are typed as `Attribute`
I considered using field names instead of `Attribute`, but that seems a
little weird with `Expression` being exposed below. This does not support
nested data access as that is represented by `GetField` expressions.
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