Fons de Leeuw created ARROW-8545:
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Summary: Allow fast writing of Decimal column to parquet
Key: ARROW-8545
URL: https://issues.apache.org/jira/browse/ARROW-8545
Project: Apache Arrow
Issue Type: Improvement
Components: Python
Affects Versions: 0.17.0
Reporter: Fons de Leeuw
Currently, when one wants to use a decimal datatype in Pandas, the only
possibility is to use the `decimal.Decimal` standard-libary type. This is then
an "object" column in the DataFrame.
Arrow can write a column of decimal type to Parquet, which is quite impressive
given that [fastparquet does not write
decimals|[https://fastparquet.readthedocs.io/en/latest/details.html#data-types]]
at all. However, the writing is *very* slow, in the code snippet below a
factor of 4.
*Improvements***
Of course the best outcome would be if the conversion of a decimal column can
be made faster, but I am not familiar enough with pandas internals to know if
that's possible. (This same behavior also applies to `.to_pickle` etc.)
It would be nice, if a warning is shown that object-typed columns are being
converted which is very slow. That would at least make this behavior more
explicit.
Now, if fast parsing of a decimal.Decimal object column is not possible, it
would be nice if a workaround is possible. For example, pass an int and then
shift the dot "x" places to the left. (It is already possible to pass an int
column and specify "decimal" dtype in the Arrow schema during
`pa.Table.from_pandas()` but then it simply becomes a decimal without
decimals.) Also, it might be nice if it can be encoded as a 128-bit byte string
in the pandas column and then directly interpreted by Arrow.
*Usecase*
I need to save large dataframes (~10GB) of geospatial data with
latitude/longitude. I can't use float as comparisons need to be exact, and the
BigQuery "clustering" feature needs either an integer or a decimal but not a
float. In the meantime, I have to do a workaround where I use only ints (the
original number multiplied by 1000.)
*Snippet*
{code:java}
{code}
*import decimal
from time import time
import numpy as np
import pandas as pd
d = dict()
for col in "abcdefghijklmnopqrstuvwxyz":
d[col] = np.random.rand(int(1E7)) * 100
df = pd.DataFrame(d)
t0 = time()
df.to_parquet("/tmp/testabc.pq", engine="pyarrow")
t1 = time()
df["a"] = df["a"].round(decimals=3).astype(str).map(decimal.Decimal)
t2 = time()
df.to_parquet("/tmp/testabc_dec.pq", engine="pyarrow")
t3 = time()
print(f"Saving the normal dataframe took \{t1-t0:.3f}s, with one decimal column
\{t3-t2:.3f}s")*
*# Saving the normal dataframe took 4.430s, with one decimal column 17.673s***
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