Table$to_arrow_batch_reader

Table$to_arrow_batch_reader(max_results=NULL, variables=NULL) → RecordBatchReader

Returns a reader that mimics the Arrow RecordBatchStreamReader, which can then be consumed to process batches of rows in a streaming fashion. By streaming data, you can avoid the need to load more than a small chunk of data into memory at a time. This approach is highly scalable, since it won't be limited by available memory or disk.

The batch reader exposes a $read_next_batch() method, which should be repeatedly called to fetch the next Arrow RecordBatch. If next() returns NULL, this signals that there is no more data left to read.

Also callable on Queries and Uploads →

 query$to_arrow_batch_reader(...) | upload$to_arrow_batch_reader(...)

Parameters:

max_results : int, default NULL The max number of records to load into the dataframe. If not specified, the entire table will be loaded.

variables : list(str) | character vector The specific variables to return, e.g., variables = c("name", "date") . If not specified, all variables in the table will be returned.

Returns:

class RecordBatchReader()

Methods:

$read_next_batch()→ Arrow RecordBatch | NULL Read the next batch of records. If NULL is returned, indicates that there is nothing left to read.

$close()→ void Close the underlying connection and releases all resources.

Notes:

The returned instance largely mirrors the behavior of the Arrow RecordBatchStreamReader, though only the $read_next_batch() method is implemented.

Make sure to call on.exit(batch_reader$close()) to ensure resources are released if your application prematurely exits.

Examples

batch_reader = redivis$table("test_scores")$to_arrow_batch_reader()

# Make sure the batch_reader gets closed when we exit this function,
# including if an error occurs
on.exit(batch_reader$close())

count <- 0
total <- 0

# Streaming works especially well when performing various aggregations
while (!is.null(batch <- reader$read_next_batch())){
  # batch is an instance of an Arrow RecordBatch -> https://arrow.apache.org/docs/r/reference/record_batch.html
  scores <- batch$scores
  count <- count + length(scores)
  total <- sum(scores)
}

print(str_interp("The average of all test cores was ${total/count}"))