CURRENT ROW frame start for ROWS frame

src/Processors/Transforms/WindowTransform.cpp

@@ -285,6 +285,19 @@ void WindowTransform::advanceFrameStartChoose()
             // the new partition starts.
             frame_started = true;
             return;
+        case WindowFrame::BoundaryType::Current:
+            switch (window_description.frame.type)
+            {
+                case WindowFrame::FrameType::Rows:
+                    // CURRENT ROW
+                    frame_start = current_row;
+                    frame_started = true;
+                    return;
+                default:
+                    // Fallthrough to the "not implemented" error.
+                    break;
+            }
+            break;
         case WindowFrame::BoundaryType::Offset:
             switch (window_description.frame.type)
             {
@@ -296,9 +309,6 @@ void WindowTransform::advanceFrameStartChoose()
                     break;
             }
             break;
-        default:
-            // Fallthrough to the "not implemented" error.
-            break;
     }
 
     throw Exception(ErrorCodes::NOT_IMPLEMENTED,

tests/queries/0_stateless/01591_window_functions.reference

@@ -516,7 +516,7 @@ settings max_block_size = 2;
 27	27	29	29
 27	27	29	29
 30	30	30	30
--- ROWS offset frame start
+-- CURRENT ROW and offset for ROWS frame start
 select number, p,
     count(*) over (partition by p order by number
         rows between 1 preceding and unbounded following),

tests/queries/0_stateless/01591_window_functions.sql

@@ -167,6 +167,8 @@ settings max_block_size = 2;
 select number, p,
     count(*) over (partition by p order by number
         rows between 1 preceding and unbounded following),
+    count(*) over (partition by p order by number
+        rows between current row and unbounded following),
     count(*) over (partition by p order by number
         rows between 1 following and unbounded following)
 from (select number, intDiv(number, 5) p from numbers(31))