CURRENT ROW frame start for RANGE frame

src/Processors/Transforms/WindowTransform.cpp

@@ -295,18 +295,14 @@ void WindowTransform::advanceFrameStartChoose()
             frame_started = true;
             return;
         case WindowFrame::BoundaryType::Current:
-            switch (window_description.frame.type)
-            {
-                case WindowFrame::FrameType::Rows:
-                    // CURRENT ROW
-                    frame_start = current_row;
+            // CURRENT ROW differs between frame types only in how the peer
+            // groups are accounted.
+            assert(partition_start <= peer_group_start);
+            assert(peer_group_start < partition_end);
+            assert(peer_group_start <= current_row);
+            frame_start = peer_group_start;
             frame_started = true;
             return;
-                default:
-                    // Fallthrough to the "not implemented" error.
-                    break;
-            }
-            break;
         case WindowFrame::BoundaryType::Offset:
             switch (window_description.frame.type)
             {
@@ -651,6 +647,13 @@ void WindowTransform::appendChunk(Chunk & chunk)
 //                current_row, frame_start, frame_end,
 //                frame_started, frame_ended);
 
+            // We now know that the current row is valid, so we can update the
+            // peer group start.
+            if (!arePeers(peer_group_start, current_row))
+            {
+                peer_group_start = current_row;
+            }
+
             // Advance the frame start.
             advanceFrameStart();
 
@@ -703,6 +706,8 @@ void WindowTransform::appendChunk(Chunk & chunk)
             writeOutCurrentRow();
 
             // Move to the next row. The frame will have to be recalculated.
+            // The peer group start is updated at the beginning of the loop,
+            // because current_row might now be past-the-end.
             advanceRowNumber(current_row);
             first_not_ready_row = current_row;
             frame_ended = false;
@@ -738,6 +743,7 @@ void WindowTransform::appendChunk(Chunk & chunk)
         prev_frame_start = partition_start;
         prev_frame_end = partition_start;
         assert(current_row == partition_start);
+        peer_group_start = partition_start;
 
 //        fmt::print(stderr, "reinitialize agg data at start of {}\n",
 //            new_partition_start);
@@ -925,6 +931,7 @@ void WindowTransform::work()
         assert(next_output_block_number >= first_block_number);
         assert(frame_start.block >= first_block_number);
         assert(current_row.block >= first_block_number);
+        assert(peer_group_start.block >= first_block_number);
     }
 }
 

src/Processors/Transforms/WindowTransform.h

@@ -257,6 +257,10 @@ public:
 
     // The row for which we are now computing the window functions.
     RowNumber current_row;
+    // The start of current peer group, needed for CURRENT ROW frame start.
+    // For ROWS frame, always equal to the current row, and for RANGE and GROUP
+    // frames may be earlier.
+    RowNumber peer_group_start;
 
     // The frame is [frame_start, frame_end) if frame_ended && frame_started,
     // and unknown otherwise. Note that when we move to the next row, both the

tests/queries/0_stateless/01591_window_functions.reference

@@ -568,3 +568,42 @@ SELECT number, max(number) OVER (PARTITION BY intDiv(number, 7) ORDER BY number
 -- a corner case
 select count() over ();
 1
+-- RANGE CURRENT ROW frame start
+select number, p, o,
+    count(*) over (partition by p order by o
+        range between current row and unbounded following)
+from (select number, intDiv(number, 5) p, mod(number, 3) o
+    from numbers(31))
+order by p, o, number
+settings max_block_size = 2;
+0	0	0	5
+3	0	0	5
+1	0	1	3
+4	0	1	3
+2	0	2	1
+6	1	0	5
+9	1	0	5
+7	1	1	3
+5	1	2	2
+8	1	2	2
+12	2	0	5
+10	2	1	4
+13	2	1	4
+11	2	2	2
+14	2	2	2
+15	3	0	5
+18	3	0	5
+16	3	1	3
+19	3	1	3
+17	3	2	1
+21	4	0	5
+24	4	0	5
+22	4	1	3
+20	4	2	2
+23	4	2	2
+27	5	0	5
+25	5	1	4
+28	5	1	4
+26	5	2	2
+29	5	2	2
+30	6	0	1

tests/queries/0_stateless/01591_window_functions.sql

@@ -182,3 +182,12 @@ SELECT number, max(number) OVER (PARTITION BY intDiv(number, 7) ORDER BY number
 
 -- a corner case
 select count() over ();
+
+-- RANGE CURRENT ROW frame start
+select number, p, o,
+    count(*) over (partition by p order by o
+        range between current row and unbounded following)
+from (select number, intDiv(number, 5) p, mod(number, 3) o
+    from numbers(31))
+order by p, o, number
+settings max_block_size = 2;