PERF: Bypass chunking/validation logic in StringDtype__from_arrow__
State: closed

Instead of converting each chunk to a StringArray after casting to array and then concatenating, instead use pyarrow to  concatenate chunks and convert to numpy.

Finally, bypass validation logic (unneeded as validated on parquet write) by initializing NDArrayBacked instead of StringArray.

This removes most of the performance overhead seen in #47345. There is still a slight overhead when comparing to `object` string arrays because of None -> NA conversion. I found that leaving that out still results in NA types in the example I gave (and would actually improve performance over the `object` case), but this is not consistent and thus conversion is left in.

- [x] closes #47345 
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.


Labels:
name: Performance
description: Memory or execution speed performance
name: Strings
description: String extension data type and string data
name: Arrow
description: pyarrow functionality

Comments:
Good point, I hadn't considered this. No - afaict this code requires pyarrow 3.0 (`pyarrow.concat_arrays` as well as `array.to_numpy(zero_copy_only=False)` were both introduced in 3.0, whereas https://pandas.pydata.org/pandas-docs/stable/getting_started/install.html states pyarrow 1.0.1 as the minimum version.

I understand this performance issue alone may not be sufficient reason to bump a version, but, in general, what would be the requirements for that? There's just 5 months between the two releases.
Edit: Looking at how `pyarrow` has been bumped in accordance with new pandas versions in the past, it feels like moving to pyarrow 3 for pandas 1.5 could be reasonable (current pyarrow is version 8).
Could you open an issue about bumping pyarrow? The we can discuss there and move forward from that
Any way doing this without requiring 3.0? Otherwise would have to wait for a bit
I think it's possible to implement something that's already a little better than what's on 1.4 without requiring pyarrow 3.
However, it's probably wise to switch to how it's done in this PR once pandas does require pa3.
I could make another PR later this week, if that's not too late for this release - and this one could be kept open for 1.5.1.
Depends on the nature of the change, we don’t backport anything big to a release candidate.

this one would have to wait for 1.6, we avoid Performance things on 1.5.x
In that case, I think it's fine to just wait for 1.6 and make this change directly. One can work around the performance impact by using `object` strings until then.
This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this.
We just increased the minimum version to 6.0, so we could finish this
Excellent, I'll revisit this soon!

Edit: I recently found that pyarrow's `to_pandas()` method can be the bottleneck when loading large parquet files that are read as large chunked arrays. I think implementing a similar logic (using pyarrow's own methods over concatenating lists of numpy arrays) for other datatypes might drastically improve read performance. Would it make sense to open a larger PR containing all those changes (if I can show improvements), or add those here?
>  Would it make sense to open a larger PR containing all those changes (if I can show improvements), or add those here?

Smaller, singular topic scoped PRs would be preferred
I think this is ready then - I just changed the whatsnew edit, the code change stays the same.

I also briefly checked what I thought might have improved performance across the other dtypes, but this wasn't so. There may be some parts where one could switch to pyarrow concatenation, but those that I checked (integer numerical) didn't yield performance improvements (and may result in some memory overhead).
Can you merge main?
@timlod there is a merge conflict here but since the rc is now cut this would probably need the release note moved to 2.1
Thanks for sticking with this @timlod