james-willis commented on code in PR #27143:
URL: https://github.com/apache/datafusion/pull/27143#discussion_r4529678206
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datafusion/physical-expr/src/expressions/case.rs:
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@@ -1307,6 +1325,24 @@ impl PhysicalExpr for CaseExpr {
}
}
+ fn return_field(&self, input_schema: &Schema) -> Result<FieldRef> {
Review Comment:
I wish this just wouldn't plan but I didn't make the change in this or my
prior PR (#27143) because of some unpleasant implications.
In my prior planning PR we made mismatched "ARROW:extension:name" an error
but the pair of a present and a null one goes to the present value.
IMO this should be a planning error because ARROW:extension:name really
defines the type and types should match between branches but it leads to some
unpleasant situations especially around literals, for example
`COALESCE(json_col, '{}')` would fail.
This kind of pain is just kind of a side effect of `ARROW:extension:name`
being an insufficient type system. Here its limitation is that it cant describe
what kinds of up/down casts are allowed
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