ozankabak commented on code in PR #14699:
URL: https://github.com/apache/datafusion/pull/14699#discussion_r1966457481
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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:
The `i`th element of the `children` slice here denotes the statistics of the
`i`th child expression. It follows the same pattern with how `evaluate_bounds`
works.
Once this PR merges and we have the machinery to calculate statistics of
scalar values defined by an expression tree, we will indeed move on to things
like `TableStatistics`, `ColumnStatistics` and others which will be built on
top of this machinery. So stay tuned 🚀
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