xudong963 commented on code in PR #19957:
URL: https://github.com/apache/datafusion/pull/19957#discussion_r2910738802
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datafusion/common/src/stats.rs:
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@@ -660,7 +637,14 @@ impl Statistics {
col_stats.max_value =
col_stats.max_value.max(&item_col_stats.max_value);
col_stats.min_value =
col_stats.min_value.min(&item_col_stats.min_value);
col_stats.sum_value =
col_stats.sum_value.add(&item_col_stats.sum_value);
- col_stats.distinct_count = Precision::Absent;
+ // Use max as a conservative lower bound for distinct count
+ // (can't accurately merge NDV since duplicates may exist across
partitions)
Review Comment:
HyperLogLog is a classic way to process this, but for a large dataset, it
takes a long time to process.
IIRC, the merge method may be called during query execution, so using
`HyperLogLog` here isn't ideal.
So I'm looking forward to hearing some other thoughts!
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