zero323 commented on a change in pull request #29813:
URL: https://github.com/apache/spark/pull/29813#discussion_r491707650
##########
File path: R/pkg/R/DataFrame.R
##########
@@ -2863,11 +2863,19 @@ setMethod("unionAll",
#' \code{UNION ALL} and \code{UNION DISTINCT} in SQL as column positions are
not taken
#' into account. Input SparkDataFrames can have different data types in the
schema.
#'
+#' When the parameter `allowMissingColumns` is `TRUE`, this function allows
+#' different set of column names between two `SparkDataFrames`.
+#' Missing columns at each side, will be filled with null values.
+#' The missing columns at left `SparkDataFrame` will be added at the end in
the schema
+#' of the union result.
+#'
#' Note: This does not remove duplicate rows across the two SparkDataFrames.
#' This function resolves columns by name (not by position).
#'
#' @param x A SparkDataFrame
#' @param y A SparkDataFrame
+#' @param allowMissingColumns logical
+#' @param ... further arguments to be passed to or from other methods.
Review comment:
That's correct, but I am not sure if there is a better way of handling
that.
Right now we have generic as follows:
```R
setGeneric("unionByName", function(x, y, ...) {
standardGeneric("unionByName") })
```
‒ as far as I am aware this is the convention for handling optional
arguments we use in SparkR.
Technically speaking we could have
```R
setGeneric("unionByName", function(x, y, allowMissingColumns) {
standardGeneric("unionByName") })
```
but then we'd have to support
```R
signature(x = "SparkDataFrame", y = "SparkDataFrame", allowMissingColumns =
"missing")
```
and
```R
signature(x = "SparkDataFrame", y = "SparkDataFrame", allowMissingColumns =
"logical")
```
if I am not mistaken, and in the past I've been told that's too much.
Do I miss something?
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