feat: support expression in window aggregate bounds

dialects/tests/expressions_test.yaml

@@ -21,3 +21,5 @@ supported_expressions:
       - CURRENT_DATE
       - CURRENT_TIMESTAMP
       - CURRENT_TIMEZONE
+  - expression: WINDOW_FUNCTION
+    supports_offset_expr: false

proto/substrait/algebra.proto

@@ -1402,17 +1402,31 @@ message Expression {
       // Defines that the bound extends this far back from the current record.
       message Preceding {
         // A strictly positive integer specifying the number of records that
-        // the window extends back from the current record. Required. Use
-        // CurrentRow for offset zero and Following for negative offsets.
-        int64 offset = 1;
+        // the window extends back from the current record. Required.
+        // Use CurrentRow for offset zero and Following for negative offsets.
+        // Deprecated: use `offset_expr` instead.
+        int64 offset = 1 [deprecated = true];
+
+        // Expression evaluated into a strictly positive integer specifying
+        // the offset from the current record. Evaluating to null, zero, or
+        // a negative integer should result in an error.
+        // Recommended type for offset is int64.
+        Expression offset_expr = 2;
       }
 
       // Defines that the bound extends this far ahead of the current record.
       message Following {
         // A strictly positive integer specifying the number of records that
-        // the window extends ahead of the current record. Required. Use
-        // CurrentRow for offset zero and Preceding for negative offsets.
-        int64 offset = 1;
+        // the window extends ahead of the current record. Required.
+        // Use CurrentRow for offset zero and Preceding for negative offsets.
+        // Deprecated: use `offset_expr` instead.
+        int64 offset = 1 [deprecated = true];
+
+        // Expression evaluated into a strictly positive integer specifying
+        // the offset from the current record. Evaluating to null, zero, or
+        // a negative integer should result in an error.
+        // Recommended type for offset is int64.
+        Expression offset_expr = 2;
       }
 
       // Defines that the bound extends to or from the current record.

site/docs/expressions/window_functions.md

@@ -18,8 +18,10 @@ When binding a window function, the binding must include the following additiona
 | Property    | Description                                                  | Required                                                     |
 | ----------- | ------------------------------------------------------------ | ------------------------------------------------------------ |
 | Partition   | A list of partitioning expressions.                          | False, defaults to a single partition for the entire dataset |
-| Lower Bound | Bound Following(int64), Bound Trailing(int64) or CurrentRow. | False, defaults to start of partition                        |
-| Upper Bound | Bound Following(int64), Bound Trailing(int64) or CurrentRow. | False, defaults to end of partition                          |
+| Lower Bound | Preceding, Following, CurrentRow, or Unbounded.               | False, defaults to start of partition                        |
+| Upper Bound | Preceding, Following, CurrentRow, or Unbounded.               | False, defaults to end of partition                          |
+
+`Preceding` and `Following` define offsets relative to the current record. `offset_expr` is the recommended way to specify the offset; it should evaluate to a strictly positive integer and should use type `int64`. A literal integer `offset` is still accepted for compatibility but is deprecated. Evaluating `offset_expr` to null, zero, or a negative integer should result in an error. Use `CurrentRow` for offset zero, and use the opposite bound direction for negative offsets.
 
 ## Aggregate Functions as Window Functions
 

text/dialect_schema.yaml

@@ -1100,6 +1100,13 @@ definitions:
       metadata:
         type: object
         description: Arbitrary data created by the dialect author.
+      supports_offset_expr:
+        type: boolean
+        default: false
+        description: >
+          Whether this dialect supports expression-based offsets for window
+          function bounds. If false, only literal offsets are supported for
+          Preceding and Following bounds. Defaults to false.
     required: [expression]
   if_then_expression:
     type: object