Fix row slice bug in Union column decoding with many columns

[friendlymatthew]

Which issue does this PR close?

• Closes Fix panic when decoding multiple Union columns in RowConverter #8999

Rationale for this change

This PR fixes a bug in the row-to-column conversion for Union types when multiple union columns are present in the same row converter

Previously, the row slice was being consumed from reading their data correctly. The fix tracks bytes consumed per row across all union fields, this way it properly advances row slices

[friendlymatthew]

@Jefffrey would you have time to review this

[Jefffrey]

@Jefffrey would you have time to review this

I'll try take a look when I have time

[Jefffrey]

@friendlymatthew when I check this PR out locally, it seems to fail to compile regarding the tests; I assume it's because it depends on #8891?

Funny that the CI didn't fail, presumably because it merged in from main? 🤔

[Jefffrey]

This is a nice pickup, and the fix makes sense to me.

One minor note:

This PR fixes a bug in the row-to-column conversion for Union types when multiple union columns are present in the same row converter

I think the bug would occur as long as there are multiple columns and there is a union column that isn't last; so something like [Union, Int64] array input would also fail, not necessarily about multiple union columns.

[Jefffrey]

Suggested change

	// track bytes consumed for rows that belong to this field
	for (row_idx, child_row) in field_rows.iter() {
	let remaining_len = sparse_data[*row_idx].len();
	bytes_consumed[*row_idx] = 1 + child_row.len() - remaining_len;
	}
	// ensure we advance pass consumed bytes in rows
	for (row_idx, child_row) in field_rows.iter() {
	let remaining_len = sparse_data[*row_idx].len();
	let consumed_length = 1 + child_row.len() - remaining_len;
	rows[*row_idx] = &rows[*row_idx][consumed_length..];
	}

Thoughts of inlining it like this, which can remove the need for a separate bytes_consumed vec?

[friendlymatthew]

I like it!