Fokko commented on code in PR #7831:
URL: https://github.com/apache/iceberg/pull/7831#discussion_r1229173359


##########
python/pyiceberg/utils/file_stats.py:
##########
@@ -0,0 +1,164 @@
+#  Licensed to the Apache Software Foundation (ASF) under one
+#  or more contributor license agreements.  See the NOTICE file
+#  distributed with this work for additional information
+#  regarding copyright ownership.  The ASF licenses this file
+#  to you under the Apache License, Version 2.0 (the
+#  "License"); you may not use this file except in compliance
+#  with the License.  You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing,
+#  software distributed under the License is distributed on an
+#  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+#  KIND, either express or implied.  See the License for the
+#  specific language governing permissions and limitations
+#  under the License.
+
+from pyiceberg.manifest import DataFile, FileFormat
+import pyarrow.parquet as pq
+import pyarrow.compute as pc
+import pyarrow as pa
+import struct
+import datetime
+
+BOUND_TRUNCATED_LENGHT = 16
+
+# Serialization rules: 
https://iceberg.apache.org/spec/#binary-single-value-serialization
+#
+# Type      Binary serialization
+# boolean   0x00 for false, non-zero byte for true
+# int       Stored as 4-byte little-endian
+# long      Stored as 8-byte little-endian
+# float     Stored as 4-byte little-endian
+# double    Stored as 8-byte little-endian
+# date      Stores days from the 1970-01-01 in an 4-byte little-endian int
+# time      Stores microseconds from midnight in an 8-byte little-endian long
+# timestamp without zone       Stores microseconds from 1970-01-01 
00:00:00.000000 in an 8-byte little-endian long
+# timestamp with zone  Stores microseconds from 1970-01-01 00:00:00.000000 UTC 
in an 8-byte little-endian long
+# string    UTF-8 bytes (without length)
+# uuid      16-byte big-endian value, see example in Appendix B
+# fixed(L)  Binary value
+# binary    Binary value (without length)
+#
+def serialize_to_binary(scalar: pa.Scalar) -> bytes:

Review Comment:
   We could reuse existing logic by first converting the PyArrow DataType to an 
Iceberg type using `pyarrow_to_schema(datatype)`, and then reuse `to_bytes` in 
`conversion.py`.



##########
python/pyiceberg/utils/file_stats.py:
##########
@@ -0,0 +1,164 @@
+#  Licensed to the Apache Software Foundation (ASF) under one
+#  or more contributor license agreements.  See the NOTICE file
+#  distributed with this work for additional information
+#  regarding copyright ownership.  The ASF licenses this file
+#  to you under the Apache License, Version 2.0 (the
+#  "License"); you may not use this file except in compliance
+#  with the License.  You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing,
+#  software distributed under the License is distributed on an
+#  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+#  KIND, either express or implied.  See the License for the
+#  specific language governing permissions and limitations
+#  under the License.
+
+from pyiceberg.manifest import DataFile, FileFormat
+import pyarrow.parquet as pq
+import pyarrow.compute as pc
+import pyarrow as pa
+import struct
+import datetime
+
+BOUND_TRUNCATED_LENGHT = 16
+
+# Serialization rules: 
https://iceberg.apache.org/spec/#binary-single-value-serialization
+#
+# Type      Binary serialization
+# boolean   0x00 for false, non-zero byte for true
+# int       Stored as 4-byte little-endian
+# long      Stored as 8-byte little-endian
+# float     Stored as 4-byte little-endian
+# double    Stored as 8-byte little-endian
+# date      Stores days from the 1970-01-01 in an 4-byte little-endian int
+# time      Stores microseconds from midnight in an 8-byte little-endian long
+# timestamp without zone       Stores microseconds from 1970-01-01 
00:00:00.000000 in an 8-byte little-endian long
+# timestamp with zone  Stores microseconds from 1970-01-01 00:00:00.000000 UTC 
in an 8-byte little-endian long
+# string    UTF-8 bytes (without length)
+# uuid      16-byte big-endian value, see example in Appendix B
+# fixed(L)  Binary value
+# binary    Binary value (without length)
+#
+def serialize_to_binary(scalar: pa.Scalar) -> bytes:
+    value = scalar.as_py()
+    if isinstance(scalar, pa.BooleanScalar):
+        return struct.pack('?', value)

Review Comment:
   For performance reasons, it is best to re-use the `struct`'s, see 
`conversions.py`



##########
python/pyiceberg/utils/file_stats.py:
##########
@@ -0,0 +1,164 @@
+#  Licensed to the Apache Software Foundation (ASF) under one
+#  or more contributor license agreements.  See the NOTICE file
+#  distributed with this work for additional information
+#  regarding copyright ownership.  The ASF licenses this file
+#  to you under the Apache License, Version 2.0 (the
+#  "License"); you may not use this file except in compliance
+#  with the License.  You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing,
+#  software distributed under the License is distributed on an
+#  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+#  KIND, either express or implied.  See the License for the
+#  specific language governing permissions and limitations
+#  under the License.
+
+from pyiceberg.manifest import DataFile, FileFormat
+import pyarrow.parquet as pq
+import pyarrow.compute as pc
+import pyarrow as pa
+import struct
+import datetime
+
+BOUND_TRUNCATED_LENGHT = 16
+
+# Serialization rules: 
https://iceberg.apache.org/spec/#binary-single-value-serialization
+#
+# Type      Binary serialization
+# boolean   0x00 for false, non-zero byte for true
+# int       Stored as 4-byte little-endian
+# long      Stored as 8-byte little-endian
+# float     Stored as 4-byte little-endian
+# double    Stored as 8-byte little-endian
+# date      Stores days from the 1970-01-01 in an 4-byte little-endian int
+# time      Stores microseconds from midnight in an 8-byte little-endian long
+# timestamp without zone       Stores microseconds from 1970-01-01 
00:00:00.000000 in an 8-byte little-endian long
+# timestamp with zone  Stores microseconds from 1970-01-01 00:00:00.000000 UTC 
in an 8-byte little-endian long
+# string    UTF-8 bytes (without length)
+# uuid      16-byte big-endian value, see example in Appendix B
+# fixed(L)  Binary value
+# binary    Binary value (without length)
+#
+def serialize_to_binary(scalar: pa.Scalar) -> bytes:
+    value = scalar.as_py()
+    if isinstance(scalar, pa.BooleanScalar):
+        return struct.pack('?', value)
+    elif isinstance(scalar, (pa.Int8Scalar, pa.UInt8Scalar)):
+        return struct.pack('<b', value)
+    elif isinstance(scalar, (pa.Int16Scalar, pa.UInt16Scalar)):
+        return struct.pack('<h', value)
+    elif isinstance(scalar, (pa.Int32Scalar, pa.UInt32Scalar)):
+        return struct.pack('<i', value)
+    elif isinstance(scalar, (pa.Int64Scalar, pa.UInt64Scalar)):
+        return struct.pack('<q', value)
+    elif isinstance(scalar, pa.FloatScalar):
+        return struct.pack('<f', value)
+    elif isinstance(scalar, pa.DoubleScalar):
+        return struct.pack('<d', value)
+    elif isinstance(scalar, pa.StringScalar):
+        return value.encode('utf-8')
+    elif isinstance(scalar, pa.BinaryScalar):
+        return value
+    elif isinstance(scalar, (pa.Date32Scalar, pa.Date64Scalar)):
+        epoch = datetime.date(1970, 1, 1)
+        days = (value - epoch).days
+        return struct.pack('<i', days)
+    elif isinstance(scalar, (pa.Time32Scalar, pa.Time64Scalar)):
+        microseconds = int(value.hour * 60 * 60 * 1e6 +
+                        value.minute * 60 * 1e6 +
+                        value.second * 1e6 +
+                        value.microsecond)
+        return struct.pack('<q', microseconds)
+    elif isinstance(scalar, pa.TimestampScalar):
+        epoch = datetime.datetime(1970, 1, 1)
+        microseconds = int((value - epoch).total_seconds() * 1e6)
+        return struct.pack('<q', microseconds)
+    else:
+        raise TypeError('Unsupported type: {}'.format(type(scalar)))
+
+
+def fill_parquet_file_metadata(df: DataFile, file_object: pa.NativeFile, 
table: pa.Table = None) -> None:
+    """
+    Computes and fills the following fields of the DataFile object:
+
+    - file_format
+    - record_count
+    - file_size_in_bytes
+    - column_sizes
+    - value_counts
+    - null_value_counts
+    - nan_value_counts
+    - lower_bounds
+    - upper_bounds
+    - split_offsets
+    
+    Args:
+        df (DataFile): A DataFile object representing the Parquet file for 
which metadata is to be filled.
+        file_object (pa.NativeFile): A pyarrow NativeFile object pointing to 
the location where the 
+            Parquet file is stored.
+        table (pa.Table, optional): If the metadata is computed while writing 
a pyarrow Table to parquet
+            the table can be passed to compute the column statistics. If 
absent the table will be read
+            from file_object using pyarrow.parquet.read_table.
+    """
+    
+    parquet_file = pq.ParquetFile(file_object)
+    metadata = parquet_file.metadata
+
+    column_sizes = {}
+    value_counts = {}
+
+    for r in range(metadata.num_row_groups):
+        for c in range(metadata.num_columns):
+            column_sizes[c+1] = column_sizes.get(c+1, 0) + 
metadata.row_group(r).column(c).total_compressed_size
+            value_counts[c+1] = value_counts.get(c+1, 0) + 
metadata.row_group(r).column(c).num_values
+
+
+    # References:
+    # 
https://github.com/apache/iceberg/blob/fc381a81a1fdb8f51a0637ca27cd30673bd7aad3/parquet/src/main/java/org/apache/iceberg/parquet/ParquetUtil.java#L232
+    # 
https://github.com/apache/parquet-mr/blob/ac29db4611f86a07cc6877b416aa4b183e09b353/parquet-hadoop/src/main/java/org/apache/parquet/hadoop/metadata/ColumnChunkMetaData.java#L184
+    split_offsets = []
+    for r in range(metadata.num_row_groups):
+        data_offset       = metadata.row_group(r).column(0).data_page_offset
+        dictionary_offset = 
metadata.row_group(r).column(0).dictionary_page_offset
+        if metadata.row_group(r).column(0).has_dictionary_page and 
dictionary_offset < data_offset:
+            split_offsets.append(dictionary_offset)
+        else:
+            split_offsets.append(data_offset)
+
+    split_offsets.sort()
+
+    if table is None:
+        table = pa.parquet.read_table(file_object)
+
+    null_value_counts = {}
+    nan_value_counts  = {}
+    lower_bounds      = {}
+    upper_bounds      = {}
+
+    for c in range(metadata.num_columns):
+        null_value_counts[c+1] = 
table.filter(pc.field(c).is_null(nan_is_null=False)).num_rows

Review Comment:
   This combines comment re-using `to_bytes` and fetching the field-id above.
   
   Here we also have to index the field IDs. When we get the field from the 
Iceberg schema, we also know the type. We could invoke `to_bytes` directly with 
the Iceberg type.



##########
python/pyiceberg/utils/file_stats.py:
##########
@@ -0,0 +1,164 @@
+#  Licensed to the Apache Software Foundation (ASF) under one
+#  or more contributor license agreements.  See the NOTICE file
+#  distributed with this work for additional information
+#  regarding copyright ownership.  The ASF licenses this file
+#  to you under the Apache License, Version 2.0 (the
+#  "License"); you may not use this file except in compliance
+#  with the License.  You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing,
+#  software distributed under the License is distributed on an
+#  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+#  KIND, either express or implied.  See the License for the
+#  specific language governing permissions and limitations
+#  under the License.
+
+from pyiceberg.manifest import DataFile, FileFormat
+import pyarrow.parquet as pq
+import pyarrow.compute as pc
+import pyarrow as pa
+import struct
+import datetime
+
+BOUND_TRUNCATED_LENGHT = 16
+
+# Serialization rules: 
https://iceberg.apache.org/spec/#binary-single-value-serialization
+#
+# Type      Binary serialization
+# boolean   0x00 for false, non-zero byte for true
+# int       Stored as 4-byte little-endian
+# long      Stored as 8-byte little-endian
+# float     Stored as 4-byte little-endian
+# double    Stored as 8-byte little-endian
+# date      Stores days from the 1970-01-01 in an 4-byte little-endian int
+# time      Stores microseconds from midnight in an 8-byte little-endian long
+# timestamp without zone       Stores microseconds from 1970-01-01 
00:00:00.000000 in an 8-byte little-endian long
+# timestamp with zone  Stores microseconds from 1970-01-01 00:00:00.000000 UTC 
in an 8-byte little-endian long
+# string    UTF-8 bytes (without length)
+# uuid      16-byte big-endian value, see example in Appendix B
+# fixed(L)  Binary value
+# binary    Binary value (without length)
+#
+def serialize_to_binary(scalar: pa.Scalar) -> bytes:
+    value = scalar.as_py()
+    if isinstance(scalar, pa.BooleanScalar):
+        return struct.pack('?', value)
+    elif isinstance(scalar, (pa.Int8Scalar, pa.UInt8Scalar)):
+        return struct.pack('<b', value)
+    elif isinstance(scalar, (pa.Int16Scalar, pa.UInt16Scalar)):
+        return struct.pack('<h', value)
+    elif isinstance(scalar, (pa.Int32Scalar, pa.UInt32Scalar)):
+        return struct.pack('<i', value)
+    elif isinstance(scalar, (pa.Int64Scalar, pa.UInt64Scalar)):
+        return struct.pack('<q', value)
+    elif isinstance(scalar, pa.FloatScalar):
+        return struct.pack('<f', value)
+    elif isinstance(scalar, pa.DoubleScalar):
+        return struct.pack('<d', value)
+    elif isinstance(scalar, pa.StringScalar):
+        return value.encode('utf-8')
+    elif isinstance(scalar, pa.BinaryScalar):
+        return value
+    elif isinstance(scalar, (pa.Date32Scalar, pa.Date64Scalar)):
+        epoch = datetime.date(1970, 1, 1)
+        days = (value - epoch).days
+        return struct.pack('<i', days)
+    elif isinstance(scalar, (pa.Time32Scalar, pa.Time64Scalar)):
+        microseconds = int(value.hour * 60 * 60 * 1e6 +
+                        value.minute * 60 * 1e6 +
+                        value.second * 1e6 +
+                        value.microsecond)
+        return struct.pack('<q', microseconds)
+    elif isinstance(scalar, pa.TimestampScalar):
+        epoch = datetime.datetime(1970, 1, 1)
+        microseconds = int((value - epoch).total_seconds() * 1e6)
+        return struct.pack('<q', microseconds)
+    else:
+        raise TypeError('Unsupported type: {}'.format(type(scalar)))
+
+
+def fill_parquet_file_metadata(df: DataFile, file_object: pa.NativeFile, 
table: pa.Table = None) -> None:
+    """
+    Computes and fills the following fields of the DataFile object:
+
+    - file_format
+    - record_count
+    - file_size_in_bytes
+    - column_sizes
+    - value_counts
+    - null_value_counts
+    - nan_value_counts
+    - lower_bounds
+    - upper_bounds
+    - split_offsets
+    
+    Args:
+        df (DataFile): A DataFile object representing the Parquet file for 
which metadata is to be filled.
+        file_object (pa.NativeFile): A pyarrow NativeFile object pointing to 
the location where the 
+            Parquet file is stored.
+        table (pa.Table, optional): If the metadata is computed while writing 
a pyarrow Table to parquet
+            the table can be passed to compute the column statistics. If 
absent the table will be read
+            from file_object using pyarrow.parquet.read_table.
+    """
+    
+    parquet_file = pq.ParquetFile(file_object)
+    metadata = parquet_file.metadata
+
+    column_sizes = {}
+    value_counts = {}
+
+    for r in range(metadata.num_row_groups):
+        for c in range(metadata.num_columns):
+            column_sizes[c+1] = column_sizes.get(c+1, 0) + 
metadata.row_group(r).column(c).total_compressed_size

Review Comment:
   FieldIDs are a fundamental concept of Iceberg. Instead of relying on column 
names (or positions), each of the fields gets a unique field-id assigned that 
is monotonically increasing when new fields are being added (and are never 
re-used). These field IDs are used for example:
   
   - To safely rename a column (you change the name, but the ID is still 
pointing at the existing field).
   - To drop a column, and create a new column with the same name. Give this a 
try in Spark/Hive on a plain Parquet table.
   
   The `column_sizes` is also indexed by the field-id. What we need to do is:
   
   ```
   >>> metadata_collector[0].row_group(0).to_dict()
   {
        'num_columns': 2,
        'num_rows': 6,
        'total_byte_size': 256,
        'columns': [{
                'file_offset': 119,
                'file_path': 'c569c5eaf90c4395885f31e012068b69-0.parquet',
                'physical_type': 'INT64',
                'num_values': 6,
                'path_in_schema': 'n_legs',
                'is_stats_set': True,
                'statistics': {
                        'has_min_max': True,
                        'min': 2,
                        'max': 100,
                        'null_count': 0,
                        'distinct_count': 0,
                        'num_values': 6,
                        'physical_type': 'INT64'
                },
                'compression': 'SNAPPY',
                'encodings': ('PLAIN_DICTIONARY', 'PLAIN', 'RLE'),
                'has_dictionary_page': True,
                'dictionary_page_offset': 4,
                'data_page_offset': 46,
                'total_compressed_size': 115,
                'total_uncompressed_size': 117
        }
   }
   ```
   Take the `path_in_schema`, in this case, `n_legs`. Using the current table 
schema you can do `schema.find_field('n_legs')` and that will return the 
`Field` that contains the field-id `schema.field_id`.



##########
python/pyiceberg/utils/file_stats.py:
##########
@@ -0,0 +1,164 @@
+#  Licensed to the Apache Software Foundation (ASF) under one
+#  or more contributor license agreements.  See the NOTICE file
+#  distributed with this work for additional information
+#  regarding copyright ownership.  The ASF licenses this file
+#  to you under the Apache License, Version 2.0 (the
+#  "License"); you may not use this file except in compliance
+#  with the License.  You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing,
+#  software distributed under the License is distributed on an
+#  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+#  KIND, either express or implied.  See the License for the
+#  specific language governing permissions and limitations
+#  under the License.
+
+from pyiceberg.manifest import DataFile, FileFormat
+import pyarrow.parquet as pq
+import pyarrow.compute as pc
+import pyarrow as pa
+import struct
+import datetime
+
+BOUND_TRUNCATED_LENGHT = 16
+
+# Serialization rules: 
https://iceberg.apache.org/spec/#binary-single-value-serialization
+#
+# Type      Binary serialization
+# boolean   0x00 for false, non-zero byte for true
+# int       Stored as 4-byte little-endian
+# long      Stored as 8-byte little-endian
+# float     Stored as 4-byte little-endian
+# double    Stored as 8-byte little-endian
+# date      Stores days from the 1970-01-01 in an 4-byte little-endian int
+# time      Stores microseconds from midnight in an 8-byte little-endian long
+# timestamp without zone       Stores microseconds from 1970-01-01 
00:00:00.000000 in an 8-byte little-endian long
+# timestamp with zone  Stores microseconds from 1970-01-01 00:00:00.000000 UTC 
in an 8-byte little-endian long
+# string    UTF-8 bytes (without length)
+# uuid      16-byte big-endian value, see example in Appendix B
+# fixed(L)  Binary value
+# binary    Binary value (without length)
+#
+def serialize_to_binary(scalar: pa.Scalar) -> bytes:
+    value = scalar.as_py()
+    if isinstance(scalar, pa.BooleanScalar):
+        return struct.pack('?', value)
+    elif isinstance(scalar, (pa.Int8Scalar, pa.UInt8Scalar)):
+        return struct.pack('<b', value)
+    elif isinstance(scalar, (pa.Int16Scalar, pa.UInt16Scalar)):
+        return struct.pack('<h', value)
+    elif isinstance(scalar, (pa.Int32Scalar, pa.UInt32Scalar)):
+        return struct.pack('<i', value)
+    elif isinstance(scalar, (pa.Int64Scalar, pa.UInt64Scalar)):
+        return struct.pack('<q', value)
+    elif isinstance(scalar, pa.FloatScalar):
+        return struct.pack('<f', value)
+    elif isinstance(scalar, pa.DoubleScalar):
+        return struct.pack('<d', value)
+    elif isinstance(scalar, pa.StringScalar):
+        return value.encode('utf-8')
+    elif isinstance(scalar, pa.BinaryScalar):
+        return value
+    elif isinstance(scalar, (pa.Date32Scalar, pa.Date64Scalar)):
+        epoch = datetime.date(1970, 1, 1)
+        days = (value - epoch).days
+        return struct.pack('<i', days)
+    elif isinstance(scalar, (pa.Time32Scalar, pa.Time64Scalar)):
+        microseconds = int(value.hour * 60 * 60 * 1e6 +
+                        value.minute * 60 * 1e6 +
+                        value.second * 1e6 +
+                        value.microsecond)
+        return struct.pack('<q', microseconds)
+    elif isinstance(scalar, pa.TimestampScalar):
+        epoch = datetime.datetime(1970, 1, 1)
+        microseconds = int((value - epoch).total_seconds() * 1e6)
+        return struct.pack('<q', microseconds)
+    else:
+        raise TypeError('Unsupported type: {}'.format(type(scalar)))
+
+
+def fill_parquet_file_metadata(df: DataFile, file_object: pa.NativeFile, 
table: pa.Table = None) -> None:
+    """
+    Computes and fills the following fields of the DataFile object:
+
+    - file_format
+    - record_count
+    - file_size_in_bytes
+    - column_sizes
+    - value_counts
+    - null_value_counts
+    - nan_value_counts
+    - lower_bounds
+    - upper_bounds
+    - split_offsets
+    
+    Args:
+        df (DataFile): A DataFile object representing the Parquet file for 
which metadata is to be filled.
+        file_object (pa.NativeFile): A pyarrow NativeFile object pointing to 
the location where the 
+            Parquet file is stored.
+        table (pa.Table, optional): If the metadata is computed while writing 
a pyarrow Table to parquet
+            the table can be passed to compute the column statistics. If 
absent the table will be read
+            from file_object using pyarrow.parquet.read_table.
+    """
+    
+    parquet_file = pq.ParquetFile(file_object)

Review Comment:
   This is my main concern, we read the file here, and I would like to see if 
we can collect the statistics while writing.



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