Carl Boettiger created ARROW-15151:
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Summary: write_dataset() never increments {i} in partitions
part-{i}
Key: ARROW-15151
URL: https://issues.apache.org/jira/browse/ARROW-15151
Project: Apache Arrow
Issue Type: Bug
Components: R
Affects Versions: 6.0.1
Environment: Ubuntu 21.04
Reporter: Carl Boettiger
Introducing partitioning in write_dataset() creates sub-folders just fine, but
the lowest-level subfolder only ever contains a part-0.parquet. I don't see
how to get write_dataset() to ever generate output with multiple part-filenames
in a single directory, like part-0.parquet, part-1.parquet, etc. e.g. the
documentation for open_dataset() implies we should get three `Z` level parts:
{code:java}
# You can also partition by the values in multiple columns
# (here: "cyl" and "gear").
# This creates a structure of the form cyl=X/gear=Y/part-Z.parquet.
two_levels_tree <- tempfile()
write_dataset(mtcars, two_levels_tree, partitioning = c("cyl", "gear"))
list.files(two_levels_tree, recursive = TRUE)
# In the two previous examples we would have:
# X = {4,6,8}, the number of cylinders.
# Y = {3,4,5}, the number of forward gears.
# Z = {0,1,2}, the number of saved parts, starting from 0. {code}
But I only get the expected structure with part-0.parquet files.
Context: I frequently need to partition large files that lack any natural
grouping variable; I merely want a bunch of small parts of equal size. It
would be great if there was an automatic way of doing this; currently I can
hack this by creating a partition column with integers 1...n where n is my
desired number of partitions, and partition on that. I'd then like to write
these to a flat structure with part-0.parquet, part-1.parquet etc, not a nested
folder structure, if possible.
(Or better yet, it would be amazing if write_dataset() just let us set a
maximum partition file size and could automate the sharding into parts while
preserving the existing behavior for actually semantically meaningful groups.
Maybe that is already the intent but I cannot see how to activate it!)
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