Jorge Leitão created ARROW-13487:
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Summary: [C++][Parquet] Reading dict pages is not reading all
values?
Key: ARROW-13487
URL: https://issues.apache.org/jira/browse/ARROW-13487
Project: Apache Arrow
Issue Type: Bug
Components: C++, Parquet
Reporter: Jorge Leitão
Attachments: generated_dictionary.parquet
While round tripping dictionary-encoded arrays in dictionary-encoded parquet
files in arrow2, I have been unable to have pyarrow read all values from the
dictionary page. This contrasts with (py)spark, that can read them.
Attached to this issue is a parquet file that I wrote from rust's arrow2
whereby I read the IPC "generated_dictionary" file and write it into parquet
(v1) with dictionary-encoding. I.e. 2 pages, one with the values, the other
with the indices.
The expected result for the column 0, "dict0" is
{code:python}
import pyarrow
path = "generated_dictionary"
golden_path =
f"../testing/arrow-testing/data/arrow-ipc-stream/integration/1.0.0-littleendian/{path}.arrow_file"
column = ("dict0", 0)
table = pyarrow.ipc.RecordBatchFileReader(golden_path).read_all()
expected = next(c for i, c in enumerate(expected) if i == column[1])
expected = expected.combine_chunks().tolist()
print(expected)
# ['nwg€6d€', None, None, None, None, None, None, None, None, 'e£a5µ矢a', None,
None, 'rpc£µ£3', None, None, None, None]
# read with pyspark
spark = pyspark.sql.SparkSession.builder.config(
# see https://stackoverflow.com/a/62024670/931303
"spark.sql.parquet.enableVectorizedReader",
"false",
).getOrCreate()
df = spark.read.parquet(f"{path}.parquet")
r = df.select(column[0]).collect()
result = [r[column[0]] for r in r]
assert expected == result
{code}
However, I have been unable to correctly read it from pyarrow. The result I get:
{code:python}
table = pq.read_table(f"{path}.parquet")
result = table[0]
print(result.combine_chunks().dictionary)
print(result.combine_chunks().indices)
[
"2lf4µµr",
"",
"nwg€6d€",
"rpc£µ£3",
"e£a5µ矢a"
]
[
2,
null,
null,
null,
null,
null,
null,
null,
null,
8,
null,
null,
4,
null,
null,
null,
null
]
{code}
which is incorrect as the largest index (8) is larger than the len (5) of the
values.
The indices are being read correctly, but not all values are. For clarity, the
buffer in the dictionary page (PLAIN-encoded as per spec) on the attached
parquet is:
{code:python}
# ["2lf4µµr", "", "nwg€6d€", "", "rpc£µ£3", "", "", "", "e£a5µ矢a", ""]
[
9, 0, 0, 0, 50, 108, 102, 52, 194, 181, 194, 181, 114,
0, 0, 0, 0,
11, 0, 0, 0, 110, 119, 103, 226, 130, 172, 54, 100, 226, 130, 172,
0, 0, 0, 0,
10, 0, 0, 0, 114, 112, 99, 194, 163, 194, 181, 194, 163, 51,
0, 0, 0, 0,
0, 0, 0, 0,
0, 0, 0, 0,
11, 0, 0, 0, 101, 194, 163, 97, 53, 194, 181, 231, 159, 162, 97,
0, 0, 0, 0
]
{code}
and the reported number of values in the dict page header is 10. I would expect
all values to be read directly to the dictionary.
We cannot discard the possibility that I am doing something wrong in writing.
So far I was able to round-trip these within arrow2 and can read dict-encoded
from both pyarrow and pyspark, which suggests that the arrow2 reader is correct.
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