feat: select into supports dataframe with schema but no data

java/openmldb-batch/src/main/scala/com/_4paradigm/openmldb/batch/nodes/SelectIntoPlan.scala

@@ -35,7 +35,7 @@ object SelectIntoPlan {
 
     // write options don't need deepCopy
     val (format, options, mode, _) = HybridseUtil.parseOptions(outPath, node)
-    if (input.getDf().isEmpty) {
+    if (input.getSchema.size == 0 && input.getDf().isEmpty) {
       throw new Exception("select empty, skip save")
     }
 

java/openmldb-batch/src/test/scala/com/_4paradigm/openmldb/batch/end2end/TestSelectInto.scala

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2021 4Paradigm
+ *
+ * Licensed 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 com._4paradigm.openmldb.batch.end2end
+
+import com._4paradigm.openmldb.batch.SparkTestSuite
+import com._4paradigm.openmldb.batch.api.OpenmldbSession
+import com._4paradigm.openmldb.batch.utils.SparkUtil
+import org.apache.spark.sql.Row
+import org.apache.spark.sql.types.{IntegerType, StringType, StructField, StructType}
+
+
+class TestSelectInto extends SparkTestSuite {
+
+  test("Test end2end select into") {
+    val spark = getSparkSession
+    val sess = new OpenmldbSession(spark)
+
+    val csvFilePath = "file:///tmp/openmldb_output/"
+    val sqlText = s"SELECT 1 INTO OUTFILE '$csvFilePath' OPTIONS (mode='overwrite')"
+    sess.sql(sqlText)
+
+    val csvDf = spark.read.option("header", true).csv(csvFilePath)
+    assert(csvDf.isEmpty)
+    assert(csvDf.schema.size == 1)
+    assert(csvDf.schema.fields(0).name.equals("1"))
+  }
+
+}