alamb commented on code in PR #20999:
URL: https://github.com/apache/datafusion/pull/20999#discussion_r2949099648
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datafusion/common/src/join_type.rs:
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@@ -72,11 +72,26 @@ pub enum JoinType {
/// Same logic as the LeftMark Join above, however it returns a record for
each record from the
/// right input.
RightMark,
+ /// Left Single Join
+ ///
+ /// Like a left outer join, but returns an error if more than one row from
the right
+ /// side matches any given row from the left side. This is used for
decorrelating scalar
+ /// subqueries: a scalar subquery must produce at most one row per outer
row, so a single
+ /// join enforces this constraint at runtime.
+ ///
+ /// Semantics:
+ /// - If a left row has exactly one match on the right: returns the
matched pair
+ /// - If a left row has no match on the right: returns the left row with
NULLs for right columns
+ /// - If a left row has more than one match on the right: returns a
runtime error
+ LeftSingle,
Review Comment:
At vertica we called this type of join `PK` join or a `Unique` join as it
assuemes any particular join key is unique
Maybe we can use a similar term here (rather than Single)
`LeftUnique` 🤔
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