[SPARK-16433][SQL] Improve StreamingQuery.explain when no data arrives

## What changes were proposed in this pull request?

Display `No physical plan. Waiting for data.` instead of `N/A`  for StreamingQuery.explain when no data arrives because `N/A` doesn't provide meaningful information.

## How was this patch tested?

Existing unit tests.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #14100 from zsxwing/SPARK-16433.

2 files changed, 4 insertions, 4 deletions

diff --git a/sql/core/src/test/scala/org/apache/spark/sql/streaming/FileStreamSourceSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/streaming/FileStreamSourceSuite.scala
index 29ce578bcd..3d28d4f99c 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/streaming/FileStreamSourceSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/streaming/FileStreamSourceSuite.scala
@@ -672,8 +672,8 @@ class FileStreamSourceSuite extends FileStreamSourceTest {
       val q = df.writeStream.queryName("file_explain").format("memory").start()
         .asInstanceOf[StreamExecution]
       try {
-        assert("N/A" === q.explainInternal(false))
-        assert("N/A" === q.explainInternal(true))
+        assert("No physical plan. Waiting for data." === q.explainInternal(false))
+        assert("No physical plan. Waiting for data." === q.explainInternal(true))
 
         val tempFile = Utils.tempFileWith(new File(tmp, "text"))
         val finalFile = new File(src, tempFile.getName)
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/streaming/StreamSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/streaming/StreamSuite.scala
index 28170f3064..1caafb9d74 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/streaming/StreamSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/streaming/StreamSuite.scala
@@ -251,8 +251,8 @@ class StreamSuite extends StreamTest {
     val q = df.writeStream.queryName("memory_explain").format("memory").start()
       .asInstanceOf[StreamExecution]
     try {
-      assert("N/A" === q.explainInternal(false))
-      assert("N/A" === q.explainInternal(true))
+      assert("No physical plan. Waiting for data." === q.explainInternal(false))
+      assert("No physical plan. Waiting for data." === q.explainInternal(true))
 
       inputData.addData("abc")
       q.processAllAvailable()