Rich-T-kid commented on code in PR #10654:
URL: https://github.com/apache/arrow-rs/pull/10654#discussion_r3766718883
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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:
worth noting that turning the flag on does cause a 1.4x-8.8x regressions
for the arrow writer.
Happy to make a separate issue to improve the perf of the current approach.
I suspect this will also cause a decent regression when turned on since we need
an exact count which requires a hash set which is both expensive in term of
compute and memory.
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