ARROW-15409: [C++] The C++ API for writing datasets could be improved #13959
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…esystem, partitioning, basename_template
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AlvinJ15
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cpp/src/arrow/dataset/file_base.h
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| /// Options for individual fragment writing. | ||
| std::shared_ptr<FileWriteOptions> file_write_options; | ||
| std::shared_ptr<FileWriteOptions> file_write_options = | ||
| CsvFileFormat().DefaultWriteOptions(); |
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default to ArrowFileFormat, since CSV is optional? (Though I guess, maybe you can't build datasets without CSV.)
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I think parquet is probably the best default format for writing datasets as it is generally going to be friendlier on the disk size. Although I could be convinced that IPC is better since, as you point out, parquet is also an optional component. I very much agree it should not be CSV.
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It seems this discussion advocates for there not being a default at all. These file formats have quite different characteristics and users should probably make a conscious choice about them.
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In pyarrow we default to parquet but perhaps that is for legacy reasons. I'm fine with no default here. However, we should then get rid of the implicit no-arg constructor and add a single-argument constructor taking in write options since this will now have a required argument. E.g.
FileSystemDatasetWriteOptions(std::shared_ptr<FileWriteOptions> file_write_options) : file_write_options(std::move(file_write_options)) {}
cpp/src/arrow/dataset/file_base.h
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| /// The final row group size may be less than this value and other options such as | ||
| /// `max_open_files` or `max_rows_per_file` lead to smaller row group sizes. | ||
| uint64_t min_rows_per_group = 0; | ||
| uint64_t min_rows_per_group = 10; |
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Why 10 and not something larger? Alternatively, why set this by default at all?
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If partitioning, it is very possible to end up with tiny row groups. For example, if we partition by year, and a batch comes in with 1 million rows spread across 1000 years you would end up with 1000 row groups with 1000 rows which is undesirable.
However, the default here should be 1 << 20 (1Mi)
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According with the ticket https://issues.apache.org/jira/browse/ARROW-15409?filter=-1, it suggest to set a value higher than 0, I use the 10 as I saw in some tests, what value would be apropiate for this?
@westonpace the max_rows_per_group is already 1 << 20 maybe I need to put a lower value, for now I will put 1 << 18;
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It's fine for max_rows_per_group and min_rows_per_group to have the same value I think. 1 << 18 is probably ok. The unit tests used 10 because I needed to be able to test the various features without generating a whole bunch of data (which would be time consuming) but 10 would have poor performance in practice because that means we would need to write a big block of metadata every 10 rows.
cpp/src/arrow/dataset/file_base.h
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| /// Options for individual fragment writing. | ||
| std::shared_ptr<FileWriteOptions> file_write_options; | ||
| std::shared_ptr<FileWriteOptions> file_write_options = | ||
| CsvFileFormat().DefaultWriteOptions(); |
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I think parquet is probably the best default format for writing datasets as it is generally going to be friendlier on the disk size. Although I could be convinced that IPC is better since, as you point out, parquet is also an optional component. I very much agree it should not be CSV.
cpp/src/arrow/dataset/file_base.h
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| /// Template string used to generate fragment basenames. | ||
| /// {i} will be replaced by an auto incremented integer. | ||
| std::string basename_template; | ||
| std::string basename_template = "data_{i}.arrow"; |
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The extension should be based on the format. We should add a const std::string& default_extension() method to FileFormat. Then we should default this to the empty string. Then, in the dataset writer, if this is an empty string, we should use "part-{i}." + format.default_extension(). This mimics what is done in python (and we could probably remove the python logic too). Then we should update the docstring for this field to reflect this behavior.
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FileFormat has a funtion called type_name() which currentyle is returning the dataset-file-formats, so I think
default_extension() is not necessary
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type_name() almost works. However, for IpcFileFormat the type_name is ipc and the extension should be arrow. We could probably update the type_name to be arrow though. It would technically be a backwards incompatible change but I'm not sure if anyone uses this field today.
cpp/src/arrow/dataset/file_base.h
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| /// The final row group size may be less than this value and other options such as | ||
| /// `max_open_files` or `max_rows_per_file` lead to smaller row group sizes. | ||
| uint64_t min_rows_per_group = 0; | ||
| uint64_t min_rows_per_group = 10; |
There was a problem hiding this comment.
If partitioning, it is very possible to end up with tiny row groups. For example, if we partition by year, and a batch comes in with 1 million rows spread across 1000 years you would end up with 1000 row groups with 1000 rows which is undesirable.
However, the default here should be 1 << 20 (1Mi)
…_rows_per_group to higher value
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Add defaults to FileSystemDatasetWriteOptions file_write_options, filesystem, partitioning, basename_template