[SPARK-6212][SQL] Fix bug of explain CTAS

[chenghao-intel]

As we assume the CreateTableAsSelect is the leaf node, so it breaks the transformation rules applied recursively, while we do the explain command.

To execute the explain for CTAS like:

explain extended create table temp__b as select key, 1+4 from src limit 2;

W/o this PR:

== Parsed Logical Plan ==
'CreateTableAsSelect HiveTable(None,temp__b,List(),List(),Map(),Map(),ManagedTable,None,Some(org.apache.hadoop.mapred.TextInputFormat),Some(org.apache.hadoop.hive.ql.io.IgnoreKeyTextOutputFormat),None,None), false
 'Limit 2
  'Project [unresolvedalias('key),unresolvedalias((1 + 4))]
   'UnresolvedRelation [src], None

== Analyzed Logical Plan ==

CreateTableAsSelect [Database:default, TableName: temp__b, InsertIntoHiveTable]
Limit 2
 Project [key#1,(1 + 4) AS _c1#3]
  MetastoreRelation default, src, None

== Optimized Logical Plan ==
CreateTableAsSelect [Database:default, TableName: temp__b, InsertIntoHiveTable]
Limit 2
 Project [key#1,(1 + 4) AS _c1#3]
  MetastoreRelation default, src, None

== Physical Plan ==
ExecutedCommand (CreateTableAsSelect [Database:default, TableName: temp__b, InsertIntoHiveTable]
Limit 2
 Project [key#1,(1 + 4) AS _c1#3]
  MetastoreRelation default, src, None)

Code Generation: true
== RDD ==

W/ this PR:

== Parsed Logical Plan ==
'CreateTableAsSelect HiveTable(None,temp__b,List(),List(),Map(),Map(),ManagedTable,None,Some(org.apache.hadoop.mapred.TextInputFormat),Some(org.apache.hadoop.hive.ql.io.IgnoreKeyTextOutputFormat),None,None), false
 'Limit 2
  'Project [unresolvedalias('key),unresolvedalias((1 + 4))]
   'UnresolvedRelation [src], None

== Analyzed Logical Plan ==

CreateTableAsSelect [AllowExisting: false, HiveTable: HiveTable(Some(default),temp__b,List(),List(),Map(),Map(),ManagedTable,None,Some(org.apache.hadoop.mapred.TextInputFormat),Some(org.apache.hadoop.hive.ql.io.IgnoreKeyTextOutputFormat),Some(org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe),None)]
 Limit 2
  Project [key#1,(1 + 4) AS _c1#3]
   MetastoreRelation default, src, None

== Optimized Logical Plan ==
CreateTableAsSelect [AllowExisting: false, HiveTable: HiveTable(Some(default),temp__b,List(),List(),Map(),Map(),ManagedTable,None,Some(org.apache.hadoop.mapred.TextInputFormat),Some(org.apache.hadoop.hive.ql.io.IgnoreKeyTextOutputFormat),Some(org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe),None)]
 Limit 2
  Project [key#1,5 AS _c1#3]
   MetastoreRelation default, src, None

== Physical Plan ==
ExecutedCommand [cmd:CreateTableAsSelect [AllowExisting: false, HiveTable: HiveTable(Some(default),temp__b,List(),List(),Map(),Map(),ManagedTable,None,Some(org.apache.hadoop.mapred.TextInputFormat),Some(org.apache.hadoop.hive.ql.io.IgnoreKeyTextOutputFormat),Some(org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe),None)]
 Limit 2
  Project [key#1,5 AS _c1#3]
   MetastoreRelation default, src, None
]

Code Generation: true
== RDD ==

[SparkQA]

Test build #39963 has finished for PR 7980 at commit f6961e5.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #40003 has finished for PR 7980 at commit 8dc37e9.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.