Possible bug in df.describe

[velicanu]

What happened?

df.describe crashes when two columns have different decimal types. Here's the file you can reproduce this with: bug.parquet

import ibis
df = ibis.read_parquet("bug.parquet")
df.describe()

What version of ibis are you using?

9.5.0

What backend(s) are you using, if any?

DuckDB

Relevant log output

$ python
Python 3.12.1 (main, Feb 27 2024, 08:10:07) [Clang 15.0.0 (clang-1500.0.40.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import ibis
>>> df = ibis.read_parquet("bug.parquet")
>>> df.describe()
Traceback (most recent call last):
  File ".direnv/python-3.12/lib/python3.12/site-packages/ibis/expr/operations/relations.py", line 339, in __init__
    missing_from_left = right.schema - left.schema
                        ~~~~~~~~~~~~~^~~~~~~~~~~~~
  File ".direnv/python-3.12/lib/python3.12/site-packages/ibis/common/collections.py", line 247, in __sub__
    common_keys = self._check_conflict(other)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".direnv/python-3.12/lib/python3.12/site-packages/ibis/common/collections.py", line 212, in _check_conflict
    raise ConflictingValuesError(conflicts)
ibis.common.exceptions.ConflictingValuesError: Conflicting values for keys:
  `mean`: decimal(38, 2) != decimal(20, 2)
  `std`: decimal(38, 2) != decimal(20, 2)

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".direnv/python-3.12/lib/python3.12/site-packages/ibis/expr/types/relations.py", line 2968, in describe
    t = ibis.union(*aggs)
        ^^^^^^^^^^^^^^^^^
  File ".direnv/python-3.12/lib/python3.12/site-packages/ibis/expr/api.py", line 1951, in union
    return table.union(*rest, distinct=distinct) if rest else table
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".direnv/python-3.12/lib/python3.12/site-packages/ibis/expr/types/relations.py", line 1674, in union
    return self._assemble_set_op(ops.Union, table, *rest, distinct=distinct)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".direnv/python-3.12/lib/python3.12/site-packages/ibis/expr/types/relations.py", line 1599, in _assemble_set_op
    node = opcls(left, right, distinct=distinct)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".direnv/python-3.12/lib/python3.12/site-packages/ibis/common/bases.py", line 72, in __call__
    return cls.__create__(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".direnv/python-3.12/lib/python3.12/site-packages/ibis/common/grounds.py", line 120, in __create__
    return super().__create__(**kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".direnv/python-3.12/lib/python3.12/site-packages/ibis/expr/operations/relations.py", line 342, in __init__
    raise RelationError(err_msg + "\n" + str(e)) from e
ibis.common.exceptions.RelationError: Table schemas must be equal for set operations.
Conflicting values for keys:
  `mean`: decimal(38, 2) != decimal(20, 2)
  `std`: decimal(38, 2) != decimal(20, 2)

Code of Conduct

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[cpcloud]

This can be reproduced without a file using memtable:

In [12]: t = ibis.memtable({'a': [1, 2, 3], 'b': [4, 5, 6]}).cast({'a': 'decimal(38,2)', 'b': 'decimal(20,2)'})

In [13]: t
Out[13]:
┏━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┓
┃ a              ┃ b              ┃
┡━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━┩
│ decimal(38, 2) │ decimal(20, 2) │
├────────────────┼────────────────┤
│           1.00 │           4.00 │
│           2.00 │           5.00 │
│           3.00 │           6.00 │
└────────────────┴────────────────┘

In [14]: t.describe()