[SPARK-26313][SQL] move newScanBuilder from Table to read related m…

…ix-in traits

## What changes were proposed in this pull request?

As discussed in https://github.com/apache/spark/pull/23208/files#r239684490 , we should put `newScanBuilder` in read related mix-in traits like `SupportsBatchRead`, to support write-only table.

In the `Append` operator, we should skip schema validation if not necessary. In the future we would introduce a capability API, so that data source can tell Spark that it doesn't want to do validation.

## How was this patch tested?

existing tests.

Closes apache#23266 from cloud-fan/ds-read.

Authored-by: Wenchen Fan <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

sql/core/src/main/java/org/apache/spark/sql/sources/v2/SupportsBatchRead.java

@@ -24,10 +24,10 @@
 /**
  * An empty mix-in interface for {@link Table}, to indicate this table supports batch scan.
  * <p>
- * If a {@link Table} implements this interface, its {@link Table#newScanBuilder(DataSourceOptions)}
- * must return a {@link ScanBuilder} that builds {@link Scan} with {@link Scan#toBatch()}
- * implemented.
+ * If a {@link Table} implements this interface, the
+ * {@link SupportsRead#newScanBuilder(DataSourceOptions)} must return a {@link ScanBuilder} that
+ * builds {@link Scan} with {@link Scan#toBatch()} implemented.
  * </p>
  */
 @Evolving
-public interface SupportsBatchRead extends Table { }
+public interface SupportsBatchRead extends SupportsRead { }

sql/core/src/main/java/org/apache/spark/sql/sources/v2/SupportsRead.java

@@ -0,0 +1,35 @@
+/*
+ * 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;
+
+import org.apache.spark.sql.sources.v2.reader.Scan;
+import org.apache.spark.sql.sources.v2.reader.ScanBuilder;
+
+/**
+ * An internal base interface of mix-in interfaces for readable {@link Table}. This adds
+ * {@link #newScanBuilder(DataSourceOptions)} that is used to create a scan for batch, micro-batch,
+ * or continuous processing.
+ */
+interface SupportsRead extends Table {
+
+  /**
+   * Returns a {@link ScanBuilder} which can be used to build a {@link Scan}. Spark will call this
+   * method to configure each scan.
+   */
+  ScanBuilder newScanBuilder(DataSourceOptions options);
+}

sql/core/src/main/java/org/apache/spark/sql/sources/v2/Table.java

@@ -18,8 +18,6 @@
 package org.apache.spark.sql.sources.v2;
 
 import org.apache.spark.annotation.Evolving;
-import org.apache.spark.sql.sources.v2.reader.Scan;
-import org.apache.spark.sql.sources.v2.reader.ScanBuilder;
 import org.apache.spark.sql.types.StructType;
 
 /**
@@ -43,17 +41,8 @@ public interface Table {
   String name();
 
   /**
-   * Returns the schema of this table.
+   * Returns the schema of this table. If the table is not readable and doesn't have a schema, an
+   * empty schema can be returned here.
    */
   StructType schema();
-
-  /**
-   * Returns a {@link ScanBuilder} which can be used to build a {@link Scan} later. Spark will call
-   * this method for each data scanning query.
-   * <p>
-   * The builder can take some query specific information to do operators pushdown, and keep these
-   * information in the created {@link Scan}.
-   * </p>
-   */
-  ScanBuilder newScanBuilder(DataSourceOptions options);
 }