felixcheung commented on a change in pull request #23746: [SPARK-26761][SQL][R] 
Vectorized R gapply() implementation
URL: https://github.com/apache/spark/pull/23746#discussion_r255841911
 
 

 ##########
 File path: R/pkg/inst/worker/worker.R
 ##########
 @@ -181,17 +185,35 @@ if (isEmpty != 0) {
                     colNames, computeFunc, data)
        } else {
         # gapply mode
+        outputs <- list()
         for (i in 1:length(data)) {
           # Timing reading input data for execution
           inputElap <- elapsedSecs()
           output <- compute(mode, partition, serializer, deserializer, 
keys[[i]],
                       colNames, computeFunc, data[[i]])
           computeElap <- elapsedSecs()
-          outputResult(serializer, output, outputCon)
+          if (deserializer == "arrow") {
+            requireNamespace("arrow", quietly = TRUE)
+            outputs[[length(outputs) + 1L]] <- output
+          } else {
+            outputResult(serializer, output, outputCon)
+          }
           outputElap <- elapsedSecs()
           computeInputElapsDiff <-  computeInputElapsDiff + (computeElap - 
inputElap)
           outputComputeElapsDiff <- outputComputeElapsDiff + (outputElap - 
computeElap)
         }
+
+        if (deserializer == "arrow") {
+          requireNamespace("arrow", quietly = TRUE)
+          # See 
https://stat.ethz.ch/pipermail/r-help/2010-September/252046.html
+          # rbind.fill might be an anternative to make it faster if plyr is 
installed.
+          combined <- do.call("rbind", outputs)
 
 Review comment:
   shouldn't we turn it off (as `stringsAsFactors = FALSE`)? 
   
   If I recall, we don't handle factor to JVM correctly, and compute() from the 
user function doesn't **really** have to return a data.frame per se.

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