alamb commented on code in PR #14699:
URL: https://github.com/apache/datafusion/pull/14699#discussion_r1966502387
##########
datafusion/physical-expr-common/src/physical_expr.rs:
##########
@@ -144,6 +153,111 @@ pub trait PhysicalExpr: Send + Sync + Display + Debug +
DynEq + DynHash {
Ok(Some(vec![]))
}
+ /// Computes the output statistics for the expression, given the input
+ /// statistics.
+ ///
+ /// # Parameters
+ ///
+ /// * `children` are the statistics for the children (inputs) of this
+ /// expression.
+ ///
+ /// # Returns
+ ///
+ /// A `Result` containing the output statistics for the expression in
+ /// case of success, or an error object in case of failure.
+ ///
+ /// Expressions (should) implement this function and utilize the
independence
+ /// assumption, match on children distribution types and compute the output
+ /// statistics accordingly. The default implementation simply creates an
+ /// unknown output distribution by combining input ranges. This logic loses
+ /// distribution information, but is a safe default.
+ fn evaluate_statistics(&self, children: &[&StatisticsV2]) ->
Result<StatisticsV2> {
Review Comment:
I did some more research into the current code, which has:
1.
[`Statistics`](https://docs.rs/datafusion/latest/datafusion/common/struct.Statistics.html)
has table level statistics, such as statistics for columns and the row count
and distinct count
2.
[`ColumnStatistics`](https://docs.rs/datafusion/latest/datafusion/common/struct.ColumnStatistics.html)
which has column level statistics
In this PR
1. `[&StatisticsV2]` is equivalent to `Statistics` ( distribution of
multiple columns)
2. `StatisticsV2` is equivalent to `ColumnStatistics` (distribution of a
single column)
In order to have the names be consistent, I recommend:
1. Renaming `StatisticsV2` to `ColumnStatisticsV2`
2. Introducing `StatisticsV2` that holds a set of column statistics
UPDATE -- I think calling this `Distribution` might more accurately describe
what it is trying to do
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