[GitHub] [arrow-datafusion] Dandandan commented on a change in pull request #334: Add window expression part 1 - logical and physical planning, structure, to/from proto, and explain, for empty over clause only

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Dandandan commented on a change in pull request #334:
URL: https://github.com/apache/arrow-datafusion/pull/334#discussion_r635518750



##########
File path: ballista/rust/core/proto/ballista.proto
##########
@@ -151,6 +153,25 @@ message AggregateExprNode {
   LogicalExprNode expr = 2;
 }
 
+enum BuiltInWindowFunction {
+  ROW_NUMBER = 0;
+  RANK = 1;
+  DENSE_RANK = 2;
+  LAG = 3;
+  LEAD = 4;
+  FIRST_VALUE = 5;
+  LAST_VALUE = 6;
+}
+
+message WindowExprNode {
+  oneof window_function {
+    AggregateFunction aggr_function = 1;

Review comment:
       I checked whether this makes sense to reuse aggregate functions for window expressions - I think it does! E.g. PostgreSQL also says:
   https://www.postgresql.org/docs/9.3/functions-window.html
   > In addition to these functions, any built-in or user-defined aggregate function can be used as a window function (see Section 9.20 for a list of the built-in aggregates). Aggregate functions act as window functions only when an OVER clause follows the call; otherwise they act as regular aggregates.




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