xudong963 commented on code in PR #15330:
URL: https://github.com/apache/datafusion/pull/15330#discussion_r2005013876
##########
datafusion/datasource-parquet/src/file_format.rs:
##########
@@ -797,10 +797,34 @@ pub async fn fetch_statistics(
statistics_from_parquet_meta_calc(&metadata, table_schema)
}
-/// Convert statistics in [`ParquetMetaData`] into [`Statistics`] using
['StatisticsConverter`]
+/// Convert statistics in [`ParquetMetaData`] into [`Statistics`] using
[`StatisticsConverter`]
///
/// The statistics are calculated for each column in the table schema
/// using the row group statistics in the parquet metadata.
+///
+/// # Key behaviors:
+///
+/// 1. Extracts row counts and byte sizes from all row groups
+/// 2. Applies schema type coercions to align file schema with table schema
+/// 3. Collects and aggregates statistics across row groups when available
+///
+/// # When there are no statistics:
+///
+/// If the Parquet file doesn't contain any statistics (has_statistics is
false), the function returns a Statistics object with:
+/// - Exact row count
+/// - Exact byte size
+/// - All column statistics marked as unknown via
Statistics::unknown_column(&table_schema)
+/// # When only some columns have statistics:
+///
+/// For columns with statistics:
+/// - Min/max values are properly extracted and represented as Precision::Exact
+/// - Null counts are calculated by summing across row groups
+///
+/// For columns without statistics,
+/// - For min/max, there are two situations:
+/// 1. The column isn't in arrow schema, then min/max values are set to
Precision::Absent
+/// 2. The column is in arrow schema, but not in parquet schema due to
schema revolution, min/max values are set to Precision::Exact(null)
Review Comment:
In fact, I have questions about this behavior, shouldn't it be
`Precision::Absent`?
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