Github user sun-rui commented on a diff in the pull request:
https://github.com/apache/spark/pull/10480#discussion_r48517634
--- Diff: R/pkg/R/SQLContext.R ---
@@ -556,3 +556,61 @@ createExternalTable <- function(sqlContext, tableName,
path = NULL, source = NUL
sdf <- callJMethod(sqlContext, "createExternalTable", tableName, source,
options)
dataFrame(sdf)
}
+
+#' Create a DataFrame representing the database table accessible via JDBC
URL
+#'
+#' Additional JDBC database connection properties can be set (...)
+#'
+#' Only one of partitionColumn or predicates should be set. Partitions of
the table will be
+#' retrieved in parallel based on the `numPartitions` or by the predicates.
+#'
+#' Don't create too many partitions in parallel on a large cluster;
otherwise Spark might crash
+#' your external database systems.
+#'
+#' @param sqlContext SQLContext to use
+#' @param url JDBC database url of the form `jdbc:subprotocol:subname`
+#' @param tableName the name of the table in the external database
+#' @param partitionColumn the name of a column of integral type that will
be used for partitioning
+#' @param lowerBound the minimum value of `partitionColumn` used to decide
partition stride
+#' @param upperBound the maximum value of `partitionColumn` used to decide
partition stride
+#' @param numPartitions the number of partitions, This, along with
`lowerBound` (inclusive),
+#' `upperBound` (exclusive), form partition strides
for generated WHERE
+#' clause expressions used to split the column
`partitionColumn` evenly.
+#' This defaults to SparkContext.defaultParallelism
when unset.
+#' @param predicates a list of conditions in the where clause; each one
defines one partition
+#' @return DataFrame
+#' @rdname read.jdbc
+#' @name read.jdbc
+#' @export
+#' @examples
+#'\dontrun{
+#' sc <- sparkR.init()
+#' sqlContext <- sparkRSQL.init(sc)
+#' jdbcUrl <- "jdbc:mysql://localhost:3306/databasename"
+#' df <- read.jdbc(sqlContext, jdbcUrl, "table", predicates =
list("field<=123"), user = "username")
+#' df2 <- read.jdbc(sqlContext, jdbcUrl, "table2", partitionColumn =
"index", lowerBound = 0,
+#' upperBound = 10000, user = "username", password =
"password")
+#' }
+
+read.jdbc <- function(sqlContext, url, tableName,
+ partitionColumn = NULL, lowerBound = NULL,
upperBound = NULL,
+ numPartitions = 0L, predicates = list(), ...) {
--- End diff --
default parameter for predicates can be NULL?
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