BUG: read_parquet wrongly returns empty index if asked to read empty column list

[batterseapower]

BUG: read_parquet wrongly returns empty index if asked to read empty column list

[batterseapower]

My guess based on the observable behaviour is that there are two seperate bugs here:

1. Pandas has a bug in the code for pd.read_parquet in the columns=[] case
2. pq.write_table has a bug in the case where the Table it is asked to serialize is empty, causing it to write a parquet file with 0 rows rather than the true row count. The similar bug in DataFrame.to_parquet is a consequence of this pyarrow bug.

[batterseapower]

Actually, maybe the bug is purely in pyarrow since engine='fastparquet' fixes both things?

pd.DataFrame(index=pd.RangeIndex(2), columns=['C', 'D']).to_parquet('temp.parquet')

# Prints 2
print(len(pd.read_parquet('temp.parquet', columns=[], engine='fastparquet').index))

pd.DataFrame(index=pd.RangeIndex(2), columns=[]).to_parquet('temp.parquet', engine='fastparquet')

# Prints 2
print(pq.read_table('temp.parquet', columns=[]).num_rows)