Union of DuckDB `DuckDBPyRelation`s based on Polars `DataFrame`s fails on internal error

[Dutcho]

What happens?

This issue occurs for relations created from data frames.
While duckdb.sql("... UNION ALL ...") works, the equivalent t1.union(t2) fails when queried/show()n.

If the relations are first stored as tables (.to_table), the issue disappears.

To Reproduce

import duckdb, polars

print(duckdb.version())
df1, df2 = polars.DataFrame(dict(a=(1, 2, 3))), polars.DataFrame(dict(a=(3, 4, 5)))
t1, t2 = duckdb.sql('FROM df1'), duckdb.sql('FROM df2')

duckdb.sql('FROM t1 UNION ALL FROM t2').show()  # works as expected
t1.union(t2).show()  # equivalent fails on _duckdb.InternalException: INTERNAL Error: Failed to cast bound query node to type - query node type mismatch

gives as output

1.4.2 (with duckdb 1.4.2)
┌───────┐
│   a   │
│ int64 │
├───────┤
│     1 │
│     2 │
│     3 │
│     3 │
│     4 │
│     5 │
└───────┘
Traceback (most recent call last):
  File "C:\...\duckdb-minimal\main.py", line 8, in <module>
    t1.union(t2).show()  # fails on _duckdb.InternalException: INTERNAL Error: Failed to cast bound query node to type - query node type mismatch
    ~~~~~~~~~~~~~~~~~^^
_duckdb.InternalException: INTERNAL Error: Failed to cast bound query node to type - query node type mismatch
This error signals an assertion failure within DuckDB. This usually occurs due to unexpected conditions or errors in the program's logic.
For more information, see https://duckdb.org/docs/stable/dev/internal_errors

OS:

Windows 11

DuckDB Package Version:

1.4.2

Python Version:

1.4.2

Full Name:

Olaf G

Affiliation:

private

What is the latest build you tested with? If possible, we recommend testing with the latest nightly build.

I have not tested with any build

Did you include all relevant data sets for reproducing the issue?

Not applicable - the reproduction does not require a data set

Did you include all code required to reproduce the issue?

• Yes, I have

Did you include all relevant configuration to reproduce the issue?

• Yes, I have