Don't guarantee left join ordering.

[ritchie46]

Description

This shouldn't have been guaranteed, but left as an implementation detail.

polars/py-polars/polars/lazyframe/frame.py

Line 4443 in c3c38a9

A left join preserves the row order of the left DataFrame.

Link

No response

[s-banach]

Hoo boy, this one is going to break some code.

[orlp]

There should be a preserve_order attribute added, defaulting to None, which can be set to "left" or "right".

@s-banach Without breaking this promise the streaming join will be slow by default, because you can't do a partitioned join if you must preserve order. Or at least, it would require a slow re-combining and re-sorting step afterwards.

And if order is preserved we can't switch which side of the join is a build and probe side either, in streaming. That's something we'd like to be able to do in the future as you'd much rather have a small build side.

[orlp]

I think we can already add this preserve_order parameter and implement it before 2.0 hits.

[ritchie46]

Yes, maintain_order it's called then. We already use that.