martin-g commented on code in PR #18842:
URL: https://github.com/apache/datafusion/pull/18842#discussion_r2548754381
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datafusion/substrait/src/logical_plan/producer/rel/read_rel.rs:
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@@ -83,26 +84,61 @@ pub fn from_table_scan(
}))
}
+/// Encodes an EmptyRelation as a Substrait VirtualTable.
+///
+/// EmptyRelation represents a relation with no input data. When
`produce_one_row` is true,
+/// it generates a single row with all fields set to their default values
(typically NULL).
+/// This is used for queries without a FROM clause, such as "SELECT 1 AS one"
or
+/// "SELECT current_timestamp()".
+///
+/// When `produce_one_row` is false, it represents a truly empty relation with
no rows,
+/// used in optimizations or as a placeholder.
pub fn from_empty_relation(
producer: &mut impl SubstraitProducer,
e: &EmptyRelation,
) -> datafusion::common::Result<Box<Rel>> {
- if e.produce_one_row {
- return not_impl_err!("Producing a row from empty relation is
unsupported");
- }
- #[allow(deprecated)]
+ let base_schema = to_substrait_named_struct(producer, &e.schema)?;
+
+ let read_type = if e.produce_one_row {
+ // Create one row with default scalar values for each field in the
schema.
+ // For example, an Int32 field gets Int32(NULL), a Utf8 field gets
Utf8(NULL), etc.
+ // This represents the "phantom row" that provides a context for
evaluating
+ // scalar expressions in queries without a FROM clause.
+ let fields = e
+ .schema
+ .fields()
+ .iter()
+ .map(|f| {
+ let scalar = ScalarValue::try_from(f.data_type())?;
+ to_substrait_literal(producer, &scalar)
+ })
+ .collect::<datafusion::common::Result<_>>()?;
+
+ ReadType::VirtualTable(VirtualTable {
+ // Use deprecated 'values' field instead of 'expressions' because
the consumer's
+ // nested expression support (RexType::Nested) is not yet
implemented.
Review Comment:
```suggestion
// nested expression support (RelType::Nested) is not yet
implemented.
```
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