[SPARK-15862][SQL] Better Error Message When Having Database Name in CACHE TABLE AS SELECT

#### What changes were proposed in this pull request?
~~If the temp table already exists, we should not silently replace it when doing `CACHE TABLE AS SELECT`. This is inconsistent with the behavior of `CREAT VIEW` or `CREATE TABLE`. This PR is to fix this silent drop.~~

~~Maybe, we also can introduce new syntax for replacing the existing one. For example, in Hive, to replace a view, the syntax should be like `ALTER VIEW AS SELECT` or `CREATE OR REPLACE VIEW AS SELECT`~~

The table name in `CACHE TABLE AS SELECT` should NOT contain database prefix like "database.table". Thus, this PR captures this in Parser and outputs a better error message, instead of reporting the view already exists.

In addition, refactoring the `Parser` to generate table identifiers instead of returning the table name string.

#### How was this patch tested?
- Added a test case for caching and uncaching qualified table names
- Fixed a few test cases that do not drop temp table at the end
- Added the related test case for the issue resolved in this PR

Author: gatorsmile <gatorsmile@gmail.com>
Author: xiaoli <lixiao1983@gmail.com>
Author: Xiao Li <xiaoli@Xiaos-MacBook-Pro.local>

Closes #13572 from gatorsmile/cacheTableAsSelect.

1 files changed, 1 insertions, 1 deletions

diff --git a/sql/hive/src/main/scala/org/apache/spark/sql/hive/test/TestHive.scala b/sql/hive/src/main/scala/org/apache/spark/sql/hive/test/TestHive.scala
index 1d1d5e3f7b..b45be0251d 100644
--- a/sql/hive/src/main/scala/org/apache/spark/sql/hive/test/TestHive.scala
+++ b/sql/hive/src/main/scala/org/apache/spark/sql/hive/test/TestHive.scala
@@ -449,7 +449,7 @@ private[hive] class TestHiveQueryExecution(
 
   override lazy val analyzed: LogicalPlan = {
     val describedTables = logical match {
-      case CacheTableCommand(tbl, _, _) => tbl :: Nil
+      case CacheTableCommand(tbl, _, _) => tbl.table :: Nil
       case _ => Nil
     }