gene-bordegaray commented on code in PR #24670:
URL: https://github.com/apache/datafusion/pull/24670#discussion_r3865359658
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datafusion/physical-plan/src/projection.rs:
##########
@@ -1326,17 +1340,44 @@ pub fn update_join_filter(
})
}
+/// Returns whether a projection defines metadata that its expressions and
input
+/// schema cannot reproduce.
+///
+/// Such a projection is an execution boundary: a parent expression such as
+/// `arrow_metadata` can observe its output field metadata.
+fn projection_overrides_metadata(projection: &ProjectionExec) -> Result<bool> {
+ let derived_schema = projection
+ .projector
+ .projection()
+ .project_schema(projection.input().schema().as_ref())?;
+ let output_schema = projection.schema();
+ Ok(derived_schema.metadata() != output_schema.metadata()
+ || derived_schema
+ .fields()
+ .iter()
+ .zip(output_schema.fields())
+ .any(|(derived, output)| derived.metadata() != output.metadata()))
+}
+
/// Collapse a chain of consecutive [`ProjectionExec`]s into one. Returns
/// `None` if nothing could be merged.
fn try_collapse_projection_chain(
outer: &ProjectionExec,
) -> Result<Option<Arc<dyn ExecutionPlan>>> {
+ if projection_overrides_metadata(outer)? {
+ return Ok(None);
+ }
+
let mut current_exprs: Vec<ProjectionExpr> = outer.expr().to_vec();
let mut current_input: Arc<dyn ExecutionPlan> = Arc::clone(outer.input());
let mut column_ref_map: HashMap<Column, usize> = HashMap::new();
let mut collapsed_any = false;
'outer: while let Some(inner_proj) =
current_input.downcast_ref::<ProjectionExec>() {
+ if projection_overrides_metadata(inner_proj)? {
+ break;
+ }
+
// Collect the column references usage in the outer projection.
Review Comment:
This is sometimes collapsable because take the example I put in the PR
description. You might have this:
```text
ProjectionExec: arrow_metadata(i, 'event_field') AS metadata
ProjectionExec: i@0 AS i
output metadata = {"event_field": "true"}
DataSourceExec: i
metadata = {}
```
The outer query does not have override metadata but the inner does. The
correct result is `true`.
The previous projection logic would collapse these, but the outer projection
expr reads the metadata. So it would make:
```text
ProjectionExec: arrow_metadata(i, 'event_field') AS metadata
DataSourceExec: i
metadata = {}
```
Giving use result as `NULL` now.
This is kinda ocnservative as we could probably just chekc if everything in
the outer projection is just referencing columns but I am just trying to get to
correctness first. Then could do the optimixation. What you think?
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