jonathanc-n commented on code in PR #17153:
URL: https://github.com/apache/datafusion/pull/17153#discussion_r2275259627
##########
datafusion/common/src/join_type.rs:
##########
@@ -111,6 +111,51 @@ impl JoinType {
| JoinType::RightAnti
)
}
+
+ /// Determines whether each input of the join is preserved for WHERE
clause filters.
+ ///
+ /// A join input is "preserved" if every row from that input appears in at
least one
+ /// output row. This property determines whether filters referencing only
columns
+ /// from that input can be safely pushed below the join.
+ ///
+ /// For example:
+ /// - In an inner join, both sides are preserved, because each row of
the output
+ /// maps directly to a row from each side.
+ /// - In a left join, the left side is preserved (we can push
predicates) but
+ /// the right is not, because there may be rows in the output that
don't
+ /// directly map to a row in the right input (due to nulls filling
where there
+ /// is no match on the right).
+ /// - In semi joins, only the preserved side's columns appear in the
output,
+ /// so filters can only reference and be pushed to that side.
+ ///
+ /// # Returns
+ /// A tuple of `(left_preserved, right_preserved)` booleans.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use datafusion_common::JoinType;
+ ///
+ /// assert_eq!(JoinType::Inner.lr_is_preserved(), (true, true));
+ /// assert_eq!(JoinType::Left.lr_is_preserved(), (true, false));
+ /// assert_eq!(JoinType::LeftSemi.lr_is_preserved(), (true, false));
+ /// ```
+ pub fn lr_is_preserved(self) -> (bool, bool) {
Review Comment:
i think we should use a slightly more clear name, maybe like
join_side_is_preserved() or something along those lines
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