Github user maropu commented on a diff in the pull request:
https://github.com/apache/spark/pull/20929#discussion_r186593019
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/types/TypePlaceholder.scala ---
@@ -0,0 +1,23 @@
+/*
+ * 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.types
+
+/**
+ * An internal type that is a not yet available and will be replaced by an
actual type later.
+ */
+case object TypePlaceholder extends StringType
--- End diff --
In the latest fix, I used `NullType` for unresolved types and
`FileStreamSource` would resolve the types when we see non-null values. As
suggested in `SPARK-12436`, I think we might need to fallback into `StringType`
just before `Sink`s, e.g.,
https://github.com/apache/spark/blob/76ecd095024a658bf68e5db658e4416565b30c17/sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/MicroBatchExecution.scala#L492
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