[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.

sql/catalyst/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBase.g4

@@ -136,7 +136,7 @@ statement
     | SHOW CREATE TABLE tableIdentifier                                #showCreateTable
     | (DESC | DESCRIBE) FUNCTION EXTENDED? describeFuncName            #describeFunction
     | (DESC | DESCRIBE) DATABASE EXTENDED? identifier                  #describeDatabase
-    | (DESC | DESCRIBE) option=(EXTENDED | FORMATTED)?
+    | (DESC | DESCRIBE) TABLE? option=(EXTENDED | FORMATTED)?
         tableIdentifier partitionSpec? describeColName?                #describeTable
     | REFRESH TABLE tableIdentifier                                    #refreshTable
     | REFRESH .*?                                                      #refreshResource

sql/core/src/main/scala/org/apache/spark/sql/execution/SparkSqlParser.scala

@@ -265,7 +265,9 @@ class SparkSqlAstBuilder(conf: SQLConf) extends AstBuilder {
     }
 
     val statement = plan(ctx.statement)
-    if (isExplainableStatement(statement)) {
+    if (statement == null) {
+      null  // This is enough since ParseException will raise later.
+    } else if (isExplainableStatement(statement)) {
       ExplainCommand(statement, extended = ctx.EXTENDED != null, codegen = ctx.CODEGEN != null)
     } else {
       ExplainCommand(OneRowRelation)

sql/core/src/test/resources/sql-tests/inputs/describe.sql

@@ -2,8 +2,12 @@ CREATE TABLE t (a STRING, b INT) PARTITIONED BY (c STRING, d STRING);
 
 ALTER TABLE t ADD PARTITION (c='Us', d=1);
 
+DESCRIBE t;
+
 DESC t;
 
+DESC TABLE t;
+
 -- Ignore these because there exist timestamp results, e.g., `Create Table`.
 -- DESC EXTENDED t;
 -- DESC FORMATTED t;

sql/core/src/test/resources/sql-tests/results/describe.sql.out

 -- Automatically generated by SQLQueryTestSuite
--- Number of queries: 8
+-- Number of queries: 10
 
 
 -- !query 0
@@ -19,7 +19,7 @@ struct<>
 
 
 -- !query 2
-DESC t
+DESCRIBE t
 -- !query 2 schema
 struct<col_name:string,data_type:string,comment:string>
 -- !query 2 output
@@ -34,7 +34,7 @@ d                   	string
 
 
 -- !query 3
-DESC t PARTITION (c='Us', d=1)
+DESC t
 -- !query 3 schema
 struct<col_name:string,data_type:string,comment:string>
 -- !query 3 output
@@ -49,30 +49,60 @@ d                   	string
 
 
 -- !query 4
-DESC t PARTITION (c='Us', d=2)
+DESC TABLE t
 -- !query 4 schema
-struct<>
+struct<col_name:string,data_type:string,comment:string>
 -- !query 4 output
+# Partition Information	                    	                    
+# col_name          	data_type           	comment             
+a                   	string              	                    
+b                   	int                 	                    
+c                   	string              	                    
+c                   	string              	                    
+d                   	string              	                    
+d                   	string
+
+
+-- !query 5
+DESC t PARTITION (c='Us', d=1)
+-- !query 5 schema
+struct<col_name:string,data_type:string,comment:string>
+-- !query 5 output
+# Partition Information	                    	                    
+# col_name          	data_type           	comment             
+a                   	string              	                    
+b                   	int                 	                    
+c                   	string              	                    
+c                   	string              	                    
+d                   	string              	                    
+d                   	string
+
+
+-- !query 6
+DESC t PARTITION (c='Us', d=2)
+-- !query 6 schema
+struct<>
+-- !query 6 output
 org.apache.spark.sql.catalyst.analysis.NoSuchPartitionException
 Partition not found in table 't' database 'default':
 c -> Us
 d -> 2;
 
 
--- !query 5
+-- !query 7
 DESC t PARTITION (c='Us')
--- !query 5 schema
+-- !query 7 schema
 struct<>
--- !query 5 output
+-- !query 7 output
 org.apache.spark.sql.AnalysisException
 Partition spec is invalid. The spec (c) must match the partition spec (c, d) defined in table '`default`.`t`';
 
 
--- !query 6
+-- !query 8
 DESC t PARTITION (c='Us', d)
--- !query 6 schema
+-- !query 8 schema
 struct<>
--- !query 6 output
+-- !query 8 output
 org.apache.spark.sql.catalyst.parser.ParseException
 
 PARTITION specification is incomplete: `d`(line 1, pos 0)
@@ -82,9 +112,9 @@ DESC t PARTITION (c='Us', d)
 ^^^
 
 
--- !query 7
+-- !query 9
 DROP TABLE t
--- !query 7 schema
+-- !query 9 schema
 struct<>
--- !query 7 output
+-- !query 9 output
 

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")
+  }
 }