thinkharderdev opened a new issue, #7444:
URL: https://github.com/apache/arrow-rs/issues/7444
**Is your feature request related to a problem or challenge? Please describe
what you are trying to do.**
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Add a method
```
impl ArrowReaderBuilder<T> {
pub fn with_rowid(self, field_name: impl Into<String>) -> Self {...}
}
```
that, will project a column with name `field_name` into the output of the
reader that contains the row offset in the parquet file of each row
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Prototype implementation can be found here
https://github.com/coralogix/arrow-rs/commit/3d4a09fc9f7f25d0b6a32ca03a582308950cf716
If this seems like something we can merge upstream I can create a PR to
master in the upstream repo
**Describe alternatives you've considered**
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Not do it :)
**Additional context**
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I'm trying to implement something like
https://clickhouse.com/blog/clickhouse-gets-lazier-and-faster-introducing-lazy-materialization
in a way that does not require re-scanning metadata or re-scanning fields that
have already been read and decoded.
The basic idea is that you have a parquet file with some projections and a
TopK sort on some (ideally small) subset of those projections. So you can:
1. Read the columns required for the topk sort along with their row offsets
2. Build the topk and discard everything else
3. Use the rowids from the topk rows to build a `RowSelection` to read
remaining columns
4. Read remaining columns using row selection.
The current implementation of parquet reader can't support this if you have
row filters you are pushing down to the scan since the offset of rows from the
scan in `1` will not align with the offset of rows in the file.
But it is relatively straightforward to keep track of the offsets during
scan and just return them.
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