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https://issues.apache.org/jira/browse/ARROW-3020?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16726349#comment-16726349
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Tanya Schlusser commented on ARROW-3020:
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I looked into this and do not believe the Parquet code permits this at the
moment despite the comment in the OP's hyperlink saying they thought it did.
pyarrow's {{ParquetWriter}} eventually uses this {{FileWriter}} class, and
here's the current code (also [linked
here|https://github.com/apache/parquet-cpp/blob/642da055adf009652689b20e68a198cffb857651/src/parquet/arrow/writer.cc#L1110-L1129]).
{code:title=from "parquet/arrow/writer.h"}
Status FileWriter::WriteTable(const Table& table, int64_t chunk_size) {
if (chunk_size <= 0) {
return Status::Invalid("chunk size per row_group must be greater than 0");
} else if (chunk_size > impl_->properties().max_row_group_length()) {
chunk_size = impl_->properties().max_row_group_length();
}
for (int chunk = 0; chunk * chunk_size < table.num_rows(); chunk++) {
int64_t offset = chunk * chunk_size;
int64_t size = std::min(chunk_size, table.num_rows() - offset);
RETURN_NOT_OK_ELSE(NewRowGroup(size), PARQUET_IGNORE_NOT_OK(Close()));
for (int i = 0; i < table.num_columns(); i++) {
auto chunked_data = table.column(i)->data();
RETURN_NOT_OK_ELSE(WriteColumnChunk(chunked_data, offset, size),
PARQUET_IGNORE_NOT_OK(Close()));
}
}
return Status::OK();
}
{code}
> [Python] Addition of option to allow empty Parquet row groups
> -------------------------------------------------------------
>
> Key: ARROW-3020
> URL: https://issues.apache.org/jira/browse/ARROW-3020
> Project: Apache Arrow
> Issue Type: New Feature
> Components: C++, Python
> Reporter: Alex Mendelson
> Priority: Major
> Labels: parquet
> Fix For: 0.12.0
>
>
> While our use case is not common, I was able to find one related request from
> roughly a year ago. Could this be added as a feature?
> https://issues.apache.org/jira/browse/PARQUET-1047
> *Motivation*
> We have an application where each row is associated with one of N contexts,
> though a minority of contexts may have no associated rows. When encountering
> the Nth context, we will wish to retrieve all the associated rows. Row groups
> would provide a natural way to index the data, as the nth context could
> naturally relate to the nth row group.
> Unfortunately, this is not possible at the present time, as pyarrow does not
> support writing empty row groups. If one writes a pyarrow.Table containing
> zero rows using pyarrow.parquet.ParquetWriter, it is omitted from the final
> file, and this distorts the indexing.
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