max() on empty LazyFrame returned by scan_delta() causes PanicException

[ramonvermeulen]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

When I create a new delta table via Databricks with the following SQL:

CREATE TABLE IF NOT EXISTS does.not.matter (
  ts TIMESTAMP_NTZ,
  loc STRING
) USING delta
LOCATION 'gs://path/to/delta/table'
TBLPROPERTIES ('delta.enableDeletionVectors' = false);

And when I then try to get the max timestamp via Polars:

a = pl.scan_delta(
    source="gs://path/to/delta/table",
    storage_options=self.storage_options,
)
.select(pl.col("ts").max())
.collect()

I get a PanicException

When I then insert one record via:

INSERT INTO does.not.matter (ts, loc) VALUES ("2024-11-20 12:34:56", "A");

And try it again, it works fine.

print(a)
shape: (1, 1)
┌─────────────────────┐
│ ts                  │
│ ---                 │
│ datetime[μs]        │
╞═════════════════════╡
│ 2024-11-20 12:34:56 │
└─────────────────────┘

Also, when I try to do this on an entirely fresh LazyFrame (not created via scan_delta) it seems to work fine:

lf = pl.LazyFrame({"ts": pl.Series([], dtype=pl.Datetime)})
a = lf.select(pl.col("ts").max()).collect()

print(a)
shape: (1, 1)
┌──────────────┐
│ ts           │
│ ---          │
│ datetime[μs] │
╞══════════════╡
│ null         │
└──────────────┘

So probably it is related to how scan_delta() creates a LazyFrame? In code I could check for records first as a workaround, however the PanicException seems not like desired behavior to me.

Log output

Traceback (most recent call last):
  File "/home/ramon/.pycharm_helpers/pydev/_pydevd_bundle/pydevd_exec2.py", line 3, in Exec
    exec(exp, global_vars, local_vars)
  File "<input>", line 7, in <module>
  File "/home/ramon/projects/work/xxxx/.venv/lib/python3.12/site-packages/polars/lazyframe/frame.py", line 2029, in collect
    return wrap_df(ldf.collect(callback))
                   ^^^^^^^^^^^^^^^^^^^^^
pyo3_runtime.PanicException: called `Option::unwrap()` on a `None` value
thread '<unnamed>' panicked at /home/runner/xxxx/polars/polars/crates/polars-core/src/utils/mod.rs:743:34:
called `Option::unwrap()` on a `None` value

Issue description

Aggregation (at least max() but probably it is the same for other aggregation functions) on top of an empty LazyFrame returned from scan_delta() throws a PanicException.

Expected behavior

I would expect to return an empty DataFrame, just like the 2nd example where I created my own LazyFrame instead of throwing a PanicException. Want to check first if this is indeed a bug and not desired functionality, most likely it is specific to scan_delta().

Installed versions

pl.show_versions()
--------Version info---------
Polars:              1.14.0
Index type:          UInt32
Platform:            Linux-5.15.167.4-microsoft-standard-WSL2-x86_64-with-glibc2.39
Python:              3.12.7 (main, Oct  7 2024, 22:29:45) [GCC 13.2.0]
LTS CPU:             False
----Optional dependencies----
adbc_driver_manager  <not installed>
altair               <not installed>
boto3                <not installed>
cloudpickle          <not installed>
connectorx           <not installed>
deltalake            0.21.0
fastexcel            <not installed>
fsspec               <not installed>
gevent               <not installed>
google.auth          2.36.0
great_tables         <not installed>
matplotlib           <not installed>
nest_asyncio         <not installed>
numpy                1.26.4
openpyxl             3.1.5
pandas               2.2.3
pyarrow              16.1.0
pydantic             <not installed>
pyiceberg            <not installed>
sqlalchemy           <not installed>
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           <not installed>