[FEA/Discussion] Make left joins have no order guarantees by default

[Matt711]

Description

Since the 1.17 release, polars has supported different types of order preserving joins via the maintain_order parameter.

This option was also exposed in the python IR. We've started adding support for maintain_order in joins in the GPU executor, but we've had to skip some of the polars tests because I think left joins are order preserving by default. Is that true? In one of the tests in test_join.py, there's a comment about it. Is that comment accurate?

Here are the tests we skip:

    "tests/unit/operations/test_join.py::test_update"
    "tests/unit/operations/test_join.py::test_join_preserve_order_left",
    "tests/unit/streaming/test_streaming.py::test_streaming_generic_left_and_inner_join_from_disk",
    "tests/unit/streaming/test_streaming_join.py::test_join_null_matches[False]",
    "tests/unit/streaming/test_streaming_join.py::test_join_null_matches_multiple_keys[False]",
    "tests/unit/test_cse.py::test_cse_rename_cross_join_5405"

cc. @wence-

[MarcoGorelli]

Hi @Matt711 - I think it's already not guaranteed. It used to be, but that was changed in #20048

[orlp]

Feel free to rewrite any test which currently assumes joins are order-preserving to explicitly specify the order they expect preserved with the new maintain_order argument.