Fix ExprSchema extraction of metadata for Cast expressions. (apache#13305)

* test(12733): reproducers for schema bugs

* fix(12733): properly extract field metadata from Cast expr

* test(12733): update metadata preservation test, for new contract (a.k.a. cast preserves field metadata)

Author: wiedld

datafusion/expr/src/expr_schema.rs

@@ -347,6 +347,7 @@ impl ExprSchemable for Expr {
         match self {
             Expr::Column(c) => Ok(schema.metadata(c)?.clone()),
             Expr::Alias(Alias { expr, .. }) => expr.metadata(schema),
+            Expr::Cast(Cast { expr, .. }) => expr.metadata(schema),
             _ => Ok(HashMap::new()),
         }
     }
@@ -681,13 +682,11 @@ mod tests {
             .with_data_type(DataType::Int32)
             .with_metadata(meta.clone());
 
-        // col and alias should be metadata-preserving
+        // col, alias, and cast should be metadata-preserving
         assert_eq!(meta, expr.metadata(&schema).unwrap());
         assert_eq!(meta, expr.clone().alias("bar").metadata(&schema).unwrap());
-
-        // cast should drop input metadata since the type has changed
         assert_eq!(
-            HashMap::new(),
+            meta,
             expr.clone()
                 .cast_to(&DataType::Int64, &schema)
                 .unwrap()

datafusion/sqllogictest/test_files/metadata.slt

@@ -168,5 +168,74 @@ LIMIT 1;
 2020-09-08T13:42:29.190855123Z
 
 
+
+# distinct (aggregate) alone
+query P
+SELECT
+    DISTINCT ts as dist
+FROM table_with_metadata;
+----
+2020-09-08T13:42:29.190855123
+
+# cast alone
+query D
+SELECT
+    ts::DATE as casted
+FROM table_with_metadata;
+----
+2020-09-08
+2020-09-08
+2020-09-08
+
+# Regression test: distinct with cast
+query D
+SELECT DISTINCT (ts::DATE) AS dist
+    FROM table_with_metadata;
+----
+2020-09-08
+
+
+
+# count distinct with group by
+query II
+SELECT
+    id AS grp,
+    COUNT(DISTINCT nonnull_name) as dist
+FROM table_with_metadata
+GROUP BY grp
+order by 1 asc nulls last;
+----
+1 1
+3 1
+NULL 1
+
+# count (not distinct) & cast, with group by
+query TI
+SELECT
+    CAST(id AS TEXT) AS grp,
+    COUNT(nonnull_name) as dist
+FROM table_with_metadata
+GROUP BY grp
+order by 1 asc nulls last;
+----
+1 1
+3 1
+NULL 1
+
+# Regression test: count distinct & cast, with group by
+query TI
+SELECT
+    CAST(id AS TEXT) AS grp,
+    COUNT(DISTINCT nonnull_name) as dist
+FROM table_with_metadata
+GROUP BY grp
+order by 1 asc nulls last;
+----
+1 1
+3 1
+NULL 1
+
+
+
 statement ok
 drop table table_with_metadata;