etseidl commented on code in PR #10654:
URL: https://github.com/apache/arrow-rs/pull/10654#discussion_r3769034284
##########
parquet/src/file/properties.rs:
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@@ -880,6 +894,24 @@ impl WriterPropertiesBuilder {
self
}
+ /// Enable or disable writing the distinct value count
(`num_distinct_values`) into
+ /// row group column chunk statistics (defaults to `false` via
+ /// [`DEFAULT_WRITE_ROW_GROUP_NUMBER_DISTINCT_VALUES`]).
+ ///
+ /// When enabled, the [`ArrowWriter`] scans each column's values before
encoding
+ /// and stores the number of distinct non-null values in the row group
statistics
+ /// footer.
+ ///
+ /// # Performance
+ ///
+ /// Computing the distinct count requires hashing every non-null value in
the column.
+ /// For large row groups or columns with many values this adds measurable
overhead.
+ /// Benchmark your workload before enabling this globally.
Review Comment:
I think I'd prefer "significant" to "measurable".
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