Merge with [SPARK-29039][SQL] centralize the catalog and table lookup logic

Author: imback82

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/ResolveCatalogs.scala

@@ -165,13 +165,11 @@ class ResolveCatalogs(val catalogManager: CatalogManager)
         s"Can not specify catalog `${catalog.name}` for view ${viewName.quoted} " +
           s"because view support in catalog has not been implemented yet")
 
-    case ShowNamespacesStatement(Some(NonSessionCatalog(catalog, nameParts)), pattern) =>
-      val namespace = if (nameParts.isEmpty) None else Some(nameParts)
+    case ShowNamespacesStatement(Some(CurrentCatalogAndNamespace(catalog, namespace)), pattern) =>
       ShowNamespaces(catalog.asNamespaceCatalog, namespace, pattern)
 
-    // TODO (SPARK-29014): we should check if the current catalog is not session catalog here.
-    case ShowNamespacesStatement(None, pattern) if defaultCatalog.isDefined =>
-      ShowNamespaces(defaultCatalog.get.asNamespaceCatalog, None, pattern)
+    case ShowNamespacesStatement(None, pattern) =>
+      ShowNamespaces(currentCatalog.asNamespaceCatalog, None, pattern)
 
     case ShowTablesStatement(Some(NonSessionCatalog(catalog, nameParts)), pattern) =>
       ShowTables(catalog.asTableCatalog, nameParts, pattern)
@@ -185,8 +183,7 @@ class ResolveCatalogs(val catalogManager: CatalogManager)
         SetCatalogAndNamespace(catalogManager, None, Some(nameParts))
       } else {
         val CurrentCatalogAndNamespace(catalog, namespace) = nameParts
-        val ns = if (namespace.isEmpty) { None } else { Some(namespace) }
-        SetCatalogAndNamespace(catalogManager, Some(catalog.name()), ns)
+        SetCatalogAndNamespace(catalogManager, Some(catalog.name()), namespace)
       }
   }
 

sql/catalyst/src/main/scala/org/apache/spark/sql/connector/catalog/LookupCatalog.scala

@@ -102,7 +102,7 @@ private[sql] trait LookupCatalog extends Logging {
     }
   }
 
-  type CurrentCatalogAndNamespace = (CatalogPlugin, Seq[String])
+  type CurrentCatalogAndNamespace = (CatalogPlugin, Option[Seq[String]])
 
   /**
    * Extract catalog and namespace from a multi-part identifier with the current catalog if needed.
@@ -112,10 +112,11 @@ private[sql] trait LookupCatalog extends Logging {
     def unapply(parts: Seq[String]): Some[CurrentCatalogAndNamespace] = parts match {
       case Seq(catalogName, tail @ _*) =>
         try {
-          Some((catalogManager.catalog(catalogName), tail))
+          Some(
+            (catalogManager.catalog(catalogName), if (tail.isEmpty) { None } else { Some(tail) }))
         } catch {
           case _: CatalogNotFoundException =>
-            Some((currentCatalog, parts))
+            Some((currentCatalog, Some(parts)))
         }
     }
   }

sql/core/src/main/scala/org/apache/spark/sql/catalyst/analysis/ResolveSessionCatalog.scala

@@ -256,15 +256,6 @@ class ResolveSessionCatalog(
     case DropViewStatement(SessionCatalog(catalog, viewName), ifExists) =>
       DropTableCommand(viewName.asTableIdentifier, ifExists, isView = true, purge = false)
 
-    case ShowNamespacesStatement(Some(SessionCatalog(catalog, nameParts)), pattern) =>
-      throw new AnalysisException(
-        "SHOW NAMESPACES is not supported with the session catalog.")
-
-    // TODO (SPARK-29014): we should check if the current catalog is session catalog here.
-    case ShowNamespacesStatement(None, pattern) if defaultCatalog.isEmpty =>
-      throw new AnalysisException(
-        "SHOW NAMESPACES is not supported with the session catalog.")
-
     case ShowTablesStatement(Some(SessionCatalog(catalog, nameParts)), pattern) =>
       if (nameParts.length != 1) {
         throw new AnalysisException(