[GitHub] [spark] rdblue commented on a change in pull request #25601: [SPARK-28856][SQL] Implement SHOW DATABASES for Data Source V2 Tables

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rdblue commented on a change in pull request #25601: [SPARK-28856][SQL] Implement SHOW DATABASES for Data Source V2 Tables
URL: https://github.com/apache/spark/pull/25601#discussion_r319315772
 
 

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 File path: sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/ShowDatabasesExec.scala
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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.execution.datasources.v2
+
+import scala.collection.mutable.ArrayBuffer
+
+import org.apache.spark.rdd.RDD
+import org.apache.spark.sql.catalog.v2.CatalogV2Implicits.NamespaceHelper
+import org.apache.spark.sql.catalog.v2.SupportsNamespaces
+import org.apache.spark.sql.catalyst.InternalRow
+import org.apache.spark.sql.catalyst.encoders.RowEncoder
+import org.apache.spark.sql.catalyst.expressions.{Attribute, GenericRowWithSchema}
+import org.apache.spark.sql.catalyst.util.StringUtils
+import org.apache.spark.sql.execution.LeafExecNode
+
+/**
+ * Physical plan node for showing databases.
+ */
+case class ShowDatabasesExec(
+    output: Seq[Attribute],
+    catalog: SupportsNamespaces,
+    pattern: Option[String])
+    extends LeafExecNode {
+  override protected def doExecute(): RDD[InternalRow] = {
+    val namespaces = catalog.listNamespaces().flatMap(getNamespaces(catalog, _))
 
 Review comment:
   This shouldn't list the entire space. It should only call `listNamespaces` once. If the current namespace is and empty array then call `listNamespaces()` and if it is anything else, call `listNamespaces(current)`.
   
   From the [SPIP](https://docs.google.com/document/d/1jEcvomPiTc5GtB9F7d2RTVVpMY64Qy7INCA_rFEd9HQ/edit#heading=h.dwrdqt8xe2wu):
   
   > For a given operation, Spark will call the corresponding catalog method once. For example, SHOW TABLES will return results from listTables(currentNamespace). Spark will not traverse nested namespaces with multiple calls to listNamespaces and listTables.

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