tustvold opened a new issue, #4155:
URL: https://github.com/apache/arrow-rs/issues/4155
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A common request is to be able to combine parquet files together without
re-encoding data (#557) (#4150). However, correctly translating the metadata is
non-trivial, and requires care to ensure the relevant file offsets are
correctly updated.
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I would like an API on `SerializedRowGroupWriter` that lets me append an
existing ColumnChunk from another source. For example,
```
/// Splice a column from another file without decoding it
///
/// This can be used for efficiently concatenating or projecting parquet data
pub fn splice_column<R: ChunkReader>(&mut self, reader: &R, metadata:
&ColumnChunkMetaData) -> Result<()> {
```
I originally debated making the signature
```
pub fn splice_column(&mut self, column: &dyn PageReader) -> Result<()> {
```
But this runs into a couple of problems
* The PageReader returns uncompressed, decoded pages (although the value
data is still encoded)
* It isn't clear how to preserve the page index or any bloom filter
information
I also debated allowing appending pages individually, however, in addition
to the above problems it runs into:
* A column chunk can only have a single dictionary page
* The compression codec is specified at the column chunk level
The downside of the `ChunkReader` API is that potentially someone could pass
a reader that doesn't match the `ColumnChunkMetaData`, which would result in an
inconsistent parquet file. I'm inclined to think this isn't a problem, as there
are plenty of other ways to generate an invalid "parquet" file :sweat_smile:
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