[GitHub] [arrow-datafusion] alamb commented on a diff in pull request #2354: Add SQL query planner support for Scalar Subqueries

[GitBox <gi...@apache.org>]

alamb commented on code in PR #2354:
URL: https://github.com/apache/arrow-datafusion/pull/2354#discussion_r860185643


##########
datafusion/core/src/sql/planner.rs:
##########
@@ -1913,6 +1911,16 @@ impl<'a, S: ContextProvider> SqlToRel<'a, S> {
         })
     }
 
+    fn parse_scalar_subquery(
+        &self,
+        subquery: &Query,
+        input_schema: &DFSchema,
+    ) -> Result<Expr> {
+        Ok(Expr::ScalarSubquery(Subquery {
+            subquery: Arc::new(self.subquery_to_plan(subquery.clone(), input_schema)?),

Review Comment:
   So the scalar subquery must produce a single row typically otherwise it is a runtime error
   
   Do you have thoughts about in what stage a query like the following would throw an error?
   
   ```
   "SELECT p.id, (SELECT id FROM person) FROM person p"
   ```



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