adriangb commented on code in PR #24125:
URL: https://github.com/apache/datafusion/pull/24125#discussion_r3770064794
##########
datafusion/physical-expr-adapter/src/schema_rewriter.rs:
##########
@@ -282,13 +311,130 @@ impl DefaultPhysicalExprAdapterRewriter {
return Ok(Transformed::yes(transformed));
}
+ if let Some(transformed) = self.try_narrow_struct_cast(&expr)? {
+ return Ok(Transformed::yes(transformed));
+ }
+
if let Some(column) = expr.downcast_ref::<Column>() {
return self.rewrite_column(Arc::clone(&expr), column);
}
Ok(Transformed::no(expr))
}
+ /// Rewrite `get_field(cast(s AS Struct<..>), 'f')` into
+ /// `cast(get_field(s, 'f') AS <type of f>)`.
Review Comment:
Added below:
```rust
/// Narrowing that cast is worthwhile for two reasons:
///
/// 1. Reading one field should not cost a whole struct. The wide form
/// casts every field of the column — including ones the query never
/// reads — to produce a value that is immediately discarded except for
/// one field.
/// 2. It keeps the column visible. Consumers throughout the codebase
/// pattern match on `get_field(column, 'f')` to recognise a struct
/// field access; a cast between the `get_field` and its column defeats
/// that match, and each such consumer then falls back to whatever it
/// does for an unrecognised expression.
```
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