safe_merge: cuDF dtype coercion mutates input DataFrame #892

Problem

In PR #886, we added cuDF dtype coercion to safe_merge() in Engine.py:

if engine_concrete == Engine.CUDF and len(left) > 0:
    merge_cols = []
    if on is not None:
        merge_cols = [on] if isinstance(on, str) else list(on)
    elif left_on is not None:
        left_cols = [left_on] if isinstance(left_on, str) else list(left_on)
        right_cols = [right_on] if isinstance(right_on, str) else list(right_on)
        merge_cols = list(zip(left_cols, right_cols))

    for col_spec in merge_cols:
        if isinstance(col_spec, tuple):
            left_col, right_col = col_spec
        else:
            left_col = right_col = col_spec

        if left_col in left.columns and right_col in right.columns:
            left_dtype = left[left_col].dtype
            right_dtype = right[right_col].dtype
            # Cast right column to match left column type if they differ
            if left_dtype \!= right_dtype:
                try:
                    right[right_col] = right[right_col].astype(left_dtype)
                except (ValueError, TypeError):
                    pass  # Let the merge fail naturally if cast is impossible

This has several issues:

1. Mutates input DataFrame - right[right_col] = ... modifies the caller's DataFrame in place
2. cuDF-only - pandas/dask/dask_cudf don't get this fix even though they may have similar issues
3. Silent failure - the pass on exception hides potential problems

Why we backed it out

The mutation bug is serious - callers don't expect safe_merge to modify their input DataFrames. This can cause subtle bugs where a DataFrame is reused after a merge and has unexpectedly changed dtypes.

Context

The fix was added to handle empty cuDF DataFrames that have float64 columns due to type inference issues. When merging with a non-empty DataFrame, the dtype mismatch causes cuDF to fail.

Proposed Fix

1. Use .copy() before modifying: right = right.copy() before the dtype coercion
2. Consider if this should apply to all engines, not just cuDF
3. Add logging/warning instead of silent pass
4. Add tests to verify the fix works and doesn't mutate inputs

Related

• PR feat(gfql): WHERE clause with df_executor #886 - WHERE clause feature
• Commit c3d6ea9 - original fix
• Commit 18dbc4c - reverted the fix

The fix was reverted in PR #886 to avoid shipping a mutation bug. cuDF tests may fail until this is properly addressed.