gabotechs commented on code in PR #19957:
URL: https://github.com/apache/datafusion/pull/19957#discussion_r2726984397
##########
datafusion/physical-expr/src/projection.rs:
##########
@@ -660,9 +660,25 @@ impl ProjectionExprs {
}
}
} else {
- // TODO stats: estimate more statistics from expressions
- // (expressions should compute their statistics themselves)
- ColumnStatistics::new_unknown()
+ // TODO: expressions should compute their own statistics
Review Comment:
Do you have any thoughts about how the current `Distribution`-based API
could be used for propagating NDV stats across expressions?
##########
datafusion/common/src/stats.rs:
##########
@@ -632,7 +632,14 @@ 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)
+ col_stats.distinct_count = col_stats
+ .distinct_count
+ .get_value()
+ .max(item_col_stats.distinct_count.get_value())
+ .map(|&v| Precision::Inexact(v))
+ .unwrap_or(Precision::Absent);
Review Comment:
👍 this sounds pretty reasonable to me
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