Github user gatorsmile commented on a diff in the pull request:
https://github.com/apache/spark/pull/17483#discussion_r109325537
--- Diff: R/pkg/R/catalog.R ---
@@ -0,0 +1,478 @@
+#
+# 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.
+#
+
+# catalog.R: SparkSession catalog functions
+
+#' Create an external table
+#'
+#' Creates an external table based on the dataset in a data source,
+#' Returns a SparkDataFrame associated with the external table.
+#'
+#' The data source is specified by the \code{source} and a set of
options(...).
+#' If \code{source} is not specified, the default data source configured by
+#' "spark.sql.sources.default" will be used.
+#'
+#' @param tableName a name of the table.
+#' @param path the path of files to load.
+#' @param source the name of external data source.
+#' @param schema the schema of the data for certain data source.
+#' @param ... additional argument(s) passed to the method.
+#' @return A SparkDataFrame.
+#' @rdname createExternalTable
+#' @export
+#' @examples
+#'\dontrun{
+#' sparkR.session()
+#' df <- createExternalTable("myjson", path="path/to/json", source="json",
schema)
+#' }
+#' @name createExternalTable
+#' @method createExternalTable default
+#' @note createExternalTable since 1.4.0
+createExternalTable.default <- function(tableName, path = NULL, source =
NULL, schema = NULL, ...) {
+ sparkSession <- getSparkSession()
+ options <- varargsToStrEnv(...)
+ if (!is.null(path)) {
+ options[["path"]] <- path
+ }
+ catalog <- callJMethod(sparkSession, "catalog")
+ if (!is.null(schema)) {
+ sdf <- callJMethod(catalog, "createExternalTable", tableName, source,
options)
+ } else {
+ sdf <- callJMethod(catalog, "createExternalTable", tableName, source,
schema$jobj, options)
+ }
+ dataFrame(sdf)
+}
+
+createExternalTable <- function(x, ...) {
--- End diff --
> If you drop a managed table both data and meta data will be deleted if
you drop an external table only metadata is deleted, external table is a way to
protect data against accidental drop commands.
Thus, it is a pretty important concept. It could be either Hive or Spark
native one.
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