clflushopt commented on code in PR #23488:
URL: https://github.com/apache/datafusion/pull/23488#discussion_r3588999618
##########
datafusion/substrait/src/logical_plan/producer/substrait_producer.rs:
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@@ -441,6 +450,30 @@ pub trait SubstraitProducer: Send + Sync + Sized {
from_placeholder(self, placeholder)
}
+ // Outer Schema management API.
+ //
+ // These methods manage a stack of outer schemas for correlated subquery
support
+ // such as when entering a subquery, the enclosing query's schema is
pushed onto
+ // the stack.
+ //
+ // Serializing an Expr::OuterReferenceColumn uses these to resolve the
column
+ // against the correct enclosing query and emit an OuterReference field
reference
+ // with the corresponding `steps_out`.
+
+ /// Push an outer schema onto the stack when entering a subquery.
+ fn push_outer_schema(&mut self, _schema: DFSchemaRef) {}
+
+ /// Pop an outer schema from the stack when leaving a subquery.
+ fn pop_outer_schema(&mut self) {}
+
+ /// Get the outer schema at the given nesting depth.
+ /// `steps_out = 1` is the immediately enclosing query, `steps_out = 2`
+ /// is two levels out, etc. Returns `None` if `steps_out` is 0 or
+ /// exceeds the current nesting depth.
+ fn get_outer_schema(&self, _steps_out: usize) -> Option<DFSchemaRef> {
+ None
+ }
Review Comment:
I believe it would break existing custom producers that serialize substraits
because it would be invoked on every substrait boundary. The choice of
defaulting to noops `None` is to match the consumer side as well which has the
same default behaviour.
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