[SPARK-17328][SQL] Fix NPE with EXPLAIN DESCRIBE TABLE

## What changes were proposed in this pull request?

This PR fixes the following NPE scenario in two ways.

**Reported Error Scenario**
```scala
scala> sql("EXPLAIN DESCRIBE TABLE x").show(truncate = false)
INFO SparkSqlParser: Parsing command: EXPLAIN DESCRIBE TABLE x
java.lang.NullPointerException
```

- **DESCRIBE**: Extend `DESCRIBE` syntax to accept `TABLE`.
- **EXPLAIN**: Prevent NPE in case of the parsing failure of target statement, e.g., `EXPLAIN DESCRIBE TABLES x`.

## How was this patch tested?

Pass the Jenkins test with a new test case.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #15357 from dongjoon-hyun/SPARK-17328.

1 files changed, 16 insertions, 2 deletions

diff --git a/sql/core/src/test/scala/org/apache/spark/sql/execution/SparkSqlParserSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/execution/SparkSqlParserSuite.scala
index 8161c08b2c..6712d32924 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/execution/SparkSqlParserSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/execution/SparkSqlParserSuite.scala
@@ -17,11 +17,12 @@
 
 package org.apache.spark.sql.execution
 
-import org.apache.spark.sql.catalyst.FunctionIdentifier
+import org.apache.spark.sql.catalyst.{FunctionIdentifier, TableIdentifier}
 import org.apache.spark.sql.catalyst.parser.ParseException
 import org.apache.spark.sql.catalyst.plans.PlanTest
 import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
-import org.apache.spark.sql.execution.command.{DescribeFunctionCommand, ShowFunctionsCommand}
+import org.apache.spark.sql.execution.command.{DescribeFunctionCommand, DescribeTableCommand,
+  ShowFunctionsCommand}
 import org.apache.spark.sql.internal.SQLConf
 
 /**
@@ -72,4 +73,17 @@ class SparkSqlParserSuite extends PlanTest {
       DescribeFunctionCommand(FunctionIdentifier("bar", database = Option("f")), isExtended = true))
   }
 
+  test("SPARK-17328 Fix NPE with EXPLAIN DESCRIBE TABLE") {
+    assertEqual("describe table t",
+      DescribeTableCommand(
+        TableIdentifier("t"), Map.empty, isExtended = false, isFormatted = false))
+    assertEqual("describe table extended t",
+      DescribeTableCommand(
+        TableIdentifier("t"), Map.empty, isExtended = true, isFormatted = false))
+    assertEqual("describe table formatted t",
+      DescribeTableCommand(
+        TableIdentifier("t"), Map.empty, isExtended = false, isFormatted = true))
+
+    intercept("explain describe tables x", "Unsupported SQL statement")
+  }
 }