[SPARK-16374][SQL] Remove Alias from MetastoreRelation and SimpleCatalogRelation

#### What changes were proposed in this pull request?
Different from the other leaf nodes, `MetastoreRelation` and `SimpleCatalogRelation` have a pre-defined `alias`, which is used to change the qualifier of the node. However, based on the existing alias handling, alias should be put in `SubqueryAlias`.

This PR is to separate alias handling from `MetastoreRelation` and `SimpleCatalogRelation` to make it consistent with the other nodes. It simplifies the signature and conversion to a `BaseRelation`.

For example, below is an example query for `MetastoreRelation`,  which is converted to a `LogicalRelation`:
```SQL
SELECT tmp.a + 1 FROM test_parquet_ctas tmp WHERE tmp.a > 2
```

Before changes, the analyzed plan is
```
== Analyzed Logical Plan ==
(a + 1): int
Project [(a#951 + 1) AS (a + 1)#952]
+- Filter (a#951 > 2)
   +- SubqueryAlias tmp
      +- Relation[a#951] parquet
```
After changes, the analyzed plan becomes
```
== Analyzed Logical Plan ==
(a + 1): int
Project [(a#951 + 1) AS (a + 1)#952]
+- Filter (a#951 > 2)
   +- SubqueryAlias tmp
      +- SubqueryAlias test_parquet_ctas
         +- Relation[a#951] parquet
```

**Note: the optimized plans are the same.**

For `SimpleCatalogRelation`, the existing code always generates two Subqueries. Thus, no change is needed.

#### How was this patch tested?
Added test cases.

Author: gatorsmile <gatorsmile@gmail.com>

Closes #14053 from gatorsmile/removeAliasFromMetastoreRelation.

3 files changed, 4 insertions, 5 deletions

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/SessionCatalog.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/SessionCatalog.scala
index e1d49912c3..ffaefeb09a 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/SessionCatalog.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/SessionCatalog.scala
@@ -403,7 +403,7 @@ class SessionCatalog(
       val relation =
         if (name.database.isDefined || !tempTables.contains(table)) {
           val metadata = externalCatalog.getTable(db, table)
-          SimpleCatalogRelation(db, metadata, alias)
+          SimpleCatalogRelation(db, metadata)
         } else {
           tempTables(table)
         }
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/interface.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/interface.scala
index 6197acab33..b12606e17d 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/interface.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/interface.scala
@@ -244,8 +244,7 @@ trait CatalogRelation {
  */
 case class SimpleCatalogRelation(
     databaseName: String,
-    metadata: CatalogTable,
-    alias: Option[String] = None)
+    metadata: CatalogTable)
   extends LeafNode with CatalogRelation {
 
   override def catalogTable: CatalogTable = metadata
@@ -261,7 +260,7 @@ case class SimpleCatalogRelation(
         CatalystSqlParser.parseDataType(f.dataType),
         // Since data can be dumped in randomly with no validation, everything is nullable.
         nullable = true
-      )(qualifier = Some(alias.getOrElse(metadata.identifier.table)))
+      )(qualifier = Some(metadata.identifier.table))
     }
   }
 
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/catalog/SessionCatalogSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/catalog/SessionCatalogSuite.scala
index c8e7c5103b..05eb302c3c 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/catalog/SessionCatalogSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/catalog/SessionCatalogSuite.scala
@@ -407,7 +407,7 @@ class SessionCatalogSuite extends SparkFunSuite {
     val relationWithAlias =
       SubqueryAlias(alias,
         SubqueryAlias("tbl1",
-          SimpleCatalogRelation("db2", tableMetadata, Some(alias))))
+          SimpleCatalogRelation("db2", tableMetadata)))
     assert(catalog.lookupRelation(
       TableIdentifier("tbl1", Some("db2")), alias = None) == relation)
     assert(catalog.lookupRelation(