[SPARK-29753][SQL] refine the default catalog config

### What changes were proposed in this pull request?

rename the config to address the comment: apache#24594 (comment)

improve the config description, provide a default value to simplify the code.

### Why are the changes needed?

make the config more understandable.

### Does this PR introduce any user-facing change?

no

### How was this patch tested?

existing tests

Closes apache#26395 from cloud-fan/config.

Authored-by: Wenchen Fan <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: cloud-fan

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

@@ -53,18 +53,6 @@ class CatalogManager(
     }
   }
 
-  private def defaultCatalog: Option[CatalogPlugin] = {
-    conf.defaultV2Catalog.flatMap { catalogName =>
-      try {
-        Some(catalog(catalogName))
-      } catch {
-        case NonFatal(e) =>
-          logError(s"Cannot load default v2 catalog: $catalogName", e)
-          None
-      }
-    }
-  }
-
   private def loadV2SessionCatalog(): CatalogPlugin = {
     Catalogs.load(SESSION_CATALOG_NAME, conf) match {
       case extension: CatalogExtension =>
@@ -127,9 +115,7 @@ class CatalogManager(
   private var _currentCatalogName: Option[String] = None
 
   def currentCatalog: CatalogPlugin = synchronized {
-    _currentCatalogName.map(catalogName => catalog(catalogName))
-      .orElse(defaultCatalog)
-      .getOrElse(v2SessionCatalog)
+    catalog(_currentCatalogName.getOrElse(conf.getConf(SQLConf.DEFAULT_CATALOG)))
   }
 
   def setCurrentCatalog(catalogName: String): Unit = synchronized {

sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala

@@ -37,7 +37,6 @@ import org.apache.spark.sql.catalyst.expressions.CodegenObjectFactoryMode
 import org.apache.spark.sql.catalyst.expressions.codegen.CodeGenerator
 import org.apache.spark.sql.catalyst.plans.logical.HintErrorHandler
 import org.apache.spark.sql.connector.catalog.CatalogManager.SESSION_CATALOG_NAME
-import org.apache.spark.sql.internal.SQLConf.StoreAssignmentPolicy
 import org.apache.spark.unsafe.array.ByteArrayMethods
 import org.apache.spark.util.Utils
 
@@ -2019,10 +2018,11 @@ object SQLConf {
     .booleanConf
     .createWithDefault(false)
 
-  val DEFAULT_V2_CATALOG = buildConf("spark.sql.default.catalog")
-    .doc("Name of the default v2 catalog, used when a catalog is not identified in queries")
+  val DEFAULT_CATALOG = buildConf("spark.sql.defaultCatalog")
+    .doc("Name of the default catalog. This will be the current catalog if users have not " +
+      "explicitly set the current catalog yet.")
     .stringConf
-    .createOptional
+    .createWithDefault(SESSION_CATALOG_NAME)
 
   val V2_SESSION_CATALOG_IMPLEMENTATION =
     buildConf(s"spark.sql.catalog.$SESSION_CATALOG_NAME")
@@ -2556,8 +2556,6 @@ class SQLConf extends Serializable with Logging {
 
   def castDatetimeToString: Boolean = getConf(SQLConf.LEGACY_CAST_DATETIME_TO_STRING)
 
-  def defaultV2Catalog: Option[String] = getConf(DEFAULT_V2_CATALOG)
-
   def ignoreDataLocality: Boolean = getConf(SQLConf.IGNORE_DATA_LOCALITY)
 
   /** ********************** SQLConf functionality methods ************ */

sql/catalyst/src/test/scala/org/apache/spark/sql/connector/catalog/CatalogManagerSuite.scala

@@ -43,7 +43,7 @@ class CatalogManagerSuite extends SparkFunSuite {
     assert(catalogManager.currentNamespace.sameElements(Array("default")))
 
     conf.setConfString("spark.sql.catalog.dummy", classOf[DummyCatalog].getName)
-    conf.setConfString(SQLConf.DEFAULT_V2_CATALOG.key, "dummy")
+    conf.setConfString(SQLConf.DEFAULT_CATALOG.key, "dummy")
 
     // The current catalog should be changed if the default catalog is set.
     assert(catalogManager.currentCatalog.name() == "dummy")
@@ -60,7 +60,7 @@ class CatalogManagerSuite extends SparkFunSuite {
     assert(catalogManager.currentNamespace.sameElements(Array("a", "b")))
 
     conf.setConfString("spark.sql.catalog.dummy2", classOf[DummyCatalog].getName)
-    conf.setConfString(SQLConf.DEFAULT_V2_CATALOG.key, "dummy2")
+    conf.setConfString(SQLConf.DEFAULT_CATALOG.key, "dummy2")
     // The current catalog shouldn't be changed if it's set before.
     assert(catalogManager.currentCatalog.name() == "dummy")
   }

sql/core/src/test/scala/org/apache/spark/sql/connector/DataSourceV2SQLSuite.scala

@@ -222,7 +222,7 @@ class DataSourceV2SQLSuite
   }
 
   test("CreateTable: use default catalog for v2 sources when default catalog is set") {
-    spark.conf.set("spark.sql.default.catalog", "testcat")
+    spark.conf.set(SQLConf.DEFAULT_CATALOG.key, "testcat")
     spark.sql(s"CREATE TABLE table_name (id bigint, data string) USING foo")
 
     val testCatalog = catalog("testcat").asTableCatalog
@@ -489,7 +489,7 @@ class DataSourceV2SQLSuite
   }
 
   test("CreateTableAsSelect: use default catalog for v2 sources when default catalog is set") {
-    spark.conf.set("spark.sql.default.catalog", "testcat")
+    spark.conf.set(SQLConf.DEFAULT_CATALOG.key, "testcat")
 
     val df = spark.createDataFrame(Seq((1L, "a"), (2L, "b"), (3L, "c"))).toDF("id", "data")
     df.createOrReplaceTempView("source")
@@ -710,7 +710,7 @@ class DataSourceV2SQLSuite
   }
 
   test("ShowTables: namespace is not specified and default v2 catalog is set") {
-    spark.conf.set("spark.sql.default.catalog", "testcat")
+    spark.conf.set(SQLConf.DEFAULT_CATALOG.key, "testcat")
     spark.sql("CREATE TABLE testcat.table (id bigint, data string) USING foo")
 
     // v2 catalog is used where default namespace is empty for TestInMemoryTableCatalog.
@@ -872,7 +872,7 @@ class DataSourceV2SQLSuite
   }
 
   test("ShowNamespaces: show root namespaces with default v2 catalog") {
-    spark.conf.set("spark.sql.default.catalog", "testcat")
+    spark.conf.set(SQLConf.DEFAULT_CATALOG.key, "testcat")
 
     testShowNamespaces("SHOW NAMESPACES", Seq())
 
@@ -915,7 +915,7 @@ class DataSourceV2SQLSuite
     spark.conf.set(
       "spark.sql.catalog.testcat_no_namspace",
       classOf[BasicInMemoryTableCatalog].getName)
-    spark.conf.set("spark.sql.default.catalog", "testcat_no_namspace")
+    spark.conf.set(SQLConf.DEFAULT_CATALOG.key, "testcat_no_namspace")
 
     val exception = intercept[AnalysisException] {
       sql("SHOW NAMESPACES")