alamb commented on code in PR #5820:
URL: https://github.com/apache/arrow-datafusion/pull/5820#discussion_r1156127545
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datafusion/optimizer/src/type_coercion.rs:
##########
@@ -638,19 +606,130 @@ fn coerce_agg_exprs_for_signature(
.collect::<Result<Vec<_>>>()
}
+fn coerce_case_expression(case: Case, schema: &DFSchemaRef) -> Result<Case> {
+ // Given expressions like:
+ //
+ // CASE a1
+ // WHEN a2 THEN b1
+ // WHEN a3 THEN b2
+ // ELSE b3
+ // END
+ //
+ // or:
+ //
+ // CASE
+ // WHEN x1 THEN b1
+ // WHEN x2 THEN b2
+ // ELSE b3
+ // END
+ //
+ // Then all aN (a1, a2, a3) must be converted to a common data type in the
first example
+ // (case-when expression coercion)
+ //
+ // All xN (x1, x2) must be converted to a boolean data type in the second
example
+ // (when-boolean expression coercion)
+ //
+ // And all bN (b1, b2, b3) must be converted to a common data type in both
examples
+ // (then-else expression coercion)
+ //
+ // If any fail to find and cast to a common/specific data type, will
return error
+ //
+ // Note that case-when and when-boolean expression coercions are mutually
exclusive
+ // Only one or the other can occur for a case expression, whilst then-else
expression coercion will always occur
+
+ // prepare types
+ let case_type = case
+ .expr
+ .as_ref()
+ .map(|expr| expr.get_type(&schema))
+ .transpose()?;
+ let then_types = case
+ .when_then_expr
+ .iter()
+ .map(|(_when, then)| then.get_type(&schema))
+ .collect::<Result<Vec<_>>>()?;
+ let else_type = case
+ .else_expr
+ .as_ref()
+ .map(|expr| expr.get_type(&schema))
+ .transpose()?;
+
+ // find common coercible types
Review Comment:
I found this code really easy to read. Thank you @Jefffrey
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