[SPARK-51993] Support emptyDataFrame and listColumns

Sources/SparkConnect/Catalog.swift

@@ -273,6 +273,22 @@ public actor Catalog: Sendable {
     return try await df.collect()[0].getAsBool(0)
   }
 
+  /// Returns a list of columns for the given table/view or temporary view.
+  /// - Parameter tableName: a qualified or unqualified name that designates a table/view. It follows the same
+  /// resolution rule with SQL: search for temp views first then table/views in the current
+  /// database (namespace).
+  /// - Returns: A ``DataFrame`` of ``Column``.
+  public func listColumns(_ tableName: String) async throws -> DataFrame {
+    let df = getDataFrame({
+      var listColumns = Spark_Connect_ListColumns()
+      listColumns.tableName = tableName
+      var catalog = Spark_Connect_Catalog()
+      catalog.listColumns = listColumns
+      return catalog
+    })
+    return df
+  }
+
   /// Check if the function with the specified name exists. This can either be a temporary function
   /// or a function.
   /// - Parameter functionName: a qualified or unqualified name that designates a function. It follows the same

Sources/SparkConnect/Documentation.docc/SparkSession.md

@@ -37,6 +37,7 @@ let csvDf = spark.read.csv("path/to/file.csv")
 
 ### DataFrame Operations
 
+- ``emptyDataFrame``
 - ``range(_:_:_:)``
 - ``sql(_:)``
 

Sources/SparkConnect/SparkConnectClient.swift

@@ -234,6 +234,16 @@ public actor SparkConnectClient {
     }
   }
 
+  func getLocalRelation() -> Plan {
+    var localRelation = Spark_Connect_LocalRelation()
+    localRelation.schema = ""
+    var relation = Relation()
+    relation.localRelation = localRelation
+    var plan = Plan()
+    plan.opType = .root(relation)
+    return plan
+  }
+
   /// Create a `Plan` instance for `Range` relation.
   /// - Parameters:
   ///   - start: A start of the range.

Sources/SparkConnect/SparkSession.swift

@@ -83,6 +83,13 @@ public actor SparkSession {
   public func stop() async {
     await client.stop()
   }
+
+  /// Returns a `DataFrame` with no rows or columns.
+  public var emptyDataFrame: DataFrame {
+    get async {
+      return await DataFrame(spark: self, plan: client.getLocalRelation())
+    }
+  }
 
   /// Create a ``DataFrame`` with a single ``Int64`` column name `id`, containing elements in a
   /// range from 0 to `end` (exclusive) with step value 1.

Tests/SparkConnectTests/CatalogTests.swift

@@ -143,6 +143,40 @@ struct CatalogTests {
     await spark.stop()
   }
 
+  @Test
+  func listColumns() async throws {
+    let spark = try await SparkSession.builder.getOrCreate()
+
+    // Table
+    let tableName = "TABLE_" + UUID().uuidString.replacingOccurrences(of: "-", with: "")
+    let path = "/tmp/\(tableName)"
+    try await SQLHelper.withTable(spark, tableName)({
+      try await spark.range(2).write.orc(path)
+      let expected = if await spark.version.starts(with: "4.") {
+        [Row("id", nil, "bigint", true, false, false, false)]
+      } else {
+        [Row("id", nil, "bigint", true, false, false)]
+      }
+      #expect(try await spark.catalog.createTable(tableName, path, source: "orc").count() == 2)
+      #expect(try await spark.catalog.listColumns(tableName).collect() == expected)
+      #expect(try await spark.catalog.listColumns("default.\(tableName)").collect() == expected)
+    })
+
+    // View
+    let viewName = "VIEW_" + UUID().uuidString.replacingOccurrences(of: "-", with: "")
+    try await SQLHelper.withTempView(spark, viewName)({
+      try await spark.range(1).createTempView(viewName)
+      let expected = if await spark.version.starts(with: "4.") {
+        [Row("id", nil, "bigint", false, false, false, false)]
+      } else {
+        [Row("id", nil, "bigint", false, false, false)]
+      }
+      #expect(try await spark.catalog.listColumns(viewName).collect() == expected)
+    })
+
+    await spark.stop()
+  }
+
   @Test
   func functionExists() async throws {
     let spark = try await SparkSession.builder.getOrCreate()

Tests/SparkConnectTests/SparkSessionTests.swift

@@ -67,6 +67,15 @@ struct SparkSessionTests {
     await spark.stop()
   }
 
+  @Test
+  func emptyDataFrame() async throws {
+    let spark = try await SparkSession.builder.getOrCreate()
+    #expect(try await spark.emptyDataFrame.count() == 0)
+    #expect(try await spark.emptyDataFrame.dtypes.isEmpty)
+    #expect(try await spark.emptyDataFrame.isLocal())
+    await spark.stop()
+  }
+
   @Test
   func range() async throws {
     let spark = try await SparkSession.builder.getOrCreate()