wiedld commented on code in PR #12142:
URL: https://github.com/apache/datafusion/pull/12142#discussion_r1730651101
##########
datafusion/expr/src/logical_plan/builder.rs:
##########
@@ -1331,7 +1331,17 @@ pub fn validate_unique_names<'a>(
})
}
-/// Union two logical plans.
+/// Union two [`LogicalPlan`]s.
+///
+/// Constructs the UNION plan, but does not perform type-coercion. Therefore
the
+/// subtree expressions will not be properly typed until the optimizer pass.
Review Comment:
Also, to clarify. The "incorrect plans" is because the new `union()`
behavior [takes the left node's
schema](https://github.com/apache/datafusion/blob/4fd460d7a2d2b60e6f9dc716148d8596d94d048c/datafusion/expr/src/logical_plan/builder.rs#L1346-L1352)
-- and doesn't type cast the expressions. When we built logical plans with
union + gap filling (adding casted scalar nulls), we started having missing
fields etc when "reading the union" before constructing the next node.
Maybe that part should be a bug?
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