ozankabak commented on code in PR #14699:
URL: https://github.com/apache/datafusion/pull/14699#discussion_r1966506841
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datafusion/physical-expr-common/src/physical_expr.rs:
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@@ -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:
> if it is only meant to represent distributions of values
This is indeed the case. It will replace `Precision` in the current code.
The hierarchy we had in mind was
1. `Statistics(V2)`: Represents statistical information (e.g. distribution,
mean, variance) of a single (possibly unknown) value or an estimate. This is
the focus of this PR, which provides the baseline mechanism to evaluate this
for arbitrary expressions.
2. `ColumnStatistics`: It will collect a bunch of `Statistics(V2)` objects
that represent estimations about the population of values in a column; e.g. its
maximum value, average etc.
3. `TableStatistics`: Similar to 2, but for relations.
Revamp of the current implementations of 2 and 3, based on 1, will be the
focus of subsequent PRs.
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