Fix bugs with degenerate window ORDER BY clauses in GROUPS/RANGE mode.

nodeWindowAgg.c failed to cope with the possibility that no ordering
columns are defined in the window frame for GROUPS mode or RANGE OFFSET
mode, leading to assertion failures or odd errors, as reported by Masahiko
Sawada and Lukas Eder.  In RANGE OFFSET mode, an ordering column is really
required, so add an Assert about that.  In GROUPS mode, the code would
work, except that the node initialization code wasn't in sync with the
execution code about when to set up tuplestore read pointers and spare
slots.  Fix the latter for consistency's sake (even though I think the
changes described below make the out-of-sync cases unreachable for now).

Per SQL spec, a single ordering column is required for RANGE OFFSET mode,
and at least one ordering column is required for GROUPS mode.  The parser
enforced the former but not the latter; add a check for that.

We were able to reach the no-ordering-column cases even with fully spec
compliant queries, though, because the planner would drop partitioning
and ordering columns from the generated plan if they were redundant with
earlier columns according to the redundant-pathkey logic, for instance
"PARTITION BY x ORDER BY y" in the presence of a "WHERE x=y" qual.
While in principle that's an optimization that could save some pointless
comparisons at runtime, it seems unlikely to be meaningful in the real
world.  I think this behavior was not so much an intentional optimization
as a side-effect of an ancient decision to construct the plan node's
ordering-column info by reverse-engineering the PathKeys of the input
path.  If we give up redundant-column removal then it takes very little
code to generate the plan node info directly from the WindowClause,
ensuring that we have the expected number of ordering columns in all
cases.  (If anyone does complain about this, the planner could perhaps
be taught to remove redundant columns only when it's safe to do so,
ie *not* in RANGE OFFSET mode.  But I doubt anyone ever will.)

With these changes, the WindowAggPath.winpathkeys field is not used for
anything anymore, so remove it.

The test cases added here are not actually very interesting given the
removal of the redundant-column-removal logic, but they would represent
important corner cases if anyone ever tries to put that back.

Tom Lane and Masahiko Sawada.  Back-patch to v11 where RANGE OFFSET
and GROUPS modes were added.

Discussion: https://postgr.es/m/CAD21AoDrWqycq-w_+Bx1cjc+YUhZ11XTj9rfxNiNDojjBx8Fjw@mail.gmail.com
Discussion: https://postgr.es/m/153086788677.17476.8002640580496698831@wrigleys.postgresql.org

src/backend/executor/nodeWindowAgg.c

@@ -1079,6 +1079,7 @@ begin_partition(WindowAggState *winstate)
 {
 	WindowAgg  *node = (WindowAgg *) winstate->ss.ps.plan;
 	PlanState  *outerPlan = outerPlanState(winstate);
+	int			frameOptions = winstate->frameOptions;
 	int			numfuncs = winstate->numfuncs;
 	int			i;
 
@@ -1143,8 +1144,8 @@ begin_partition(WindowAggState *winstate)
 		 * If the frame head is potentially movable, or we have an EXCLUSION
 		 * clause, we might need to restart aggregation ...
 		 */
-		if (!(winstate->frameOptions & FRAMEOPTION_START_UNBOUNDED_PRECEDING) ||
-			(winstate->frameOptions & FRAMEOPTION_EXCLUSION))
+		if (!(frameOptions & FRAMEOPTION_START_UNBOUNDED_PRECEDING) ||
+			(frameOptions & FRAMEOPTION_EXCLUSION))
 		{
 			/* ... so create a mark pointer to track the frame head */
 			agg_winobj->markptr = tuplestore_alloc_read_pointer(winstate->buffer, 0);
@@ -1182,21 +1183,24 @@ begin_partition(WindowAggState *winstate)
 
 	/*
 	 * If we are in RANGE or GROUPS mode, then determining frame boundaries
-	 * requires physical access to the frame endpoint rows, except in
+	 * requires physical access to the frame endpoint rows, except in certain
 	 * degenerate cases.  We create read pointers to point to those rows, to
 	 * simplify access and ensure that the tuplestore doesn't discard the
-	 * endpoint rows prematurely.  (Must match logic in update_frameheadpos
-	 * and update_frametailpos.)
+	 * endpoint rows prematurely.  (Must create pointers in exactly the same
+	 * cases that update_frameheadpos and update_frametailpos need them.)
 	 */
 	winstate->framehead_ptr = winstate->frametail_ptr = -1; /* if not used */
 
-	if ((winstate->frameOptions & (FRAMEOPTION_RANGE | FRAMEOPTION_GROUPS)) &&
-		node->ordNumCols != 0)
+	if (frameOptions & (FRAMEOPTION_RANGE | FRAMEOPTION_GROUPS))
 	{
-		if (!(winstate->frameOptions & FRAMEOPTION_START_UNBOUNDED_PRECEDING))
+		if (((frameOptions & FRAMEOPTION_START_CURRENT_ROW) &&
+			 node->ordNumCols != 0) ||
+			(frameOptions & FRAMEOPTION_START_OFFSET))
 			winstate->framehead_ptr =
 				tuplestore_alloc_read_pointer(winstate->buffer, 0);
-		if (!(winstate->frameOptions & FRAMEOPTION_END_UNBOUNDED_FOLLOWING))
+		if (((frameOptions & FRAMEOPTION_END_CURRENT_ROW) &&
+			 node->ordNumCols != 0) ||
+			(frameOptions & FRAMEOPTION_END_OFFSET))
 			winstate->frametail_ptr =
 				tuplestore_alloc_read_pointer(winstate->buffer, 0);
 	}
@@ -1210,7 +1214,7 @@ begin_partition(WindowAggState *winstate)
 	 */
 	winstate->grouptail_ptr = -1;
 
-	if ((winstate->frameOptions & (FRAMEOPTION_EXCLUDE_GROUP |
+	if ((frameOptions & (FRAMEOPTION_EXCLUDE_GROUP |
 						 FRAMEOPTION_EXCLUDE_TIES)) &&
 		node->ordNumCols != 0)
 	{
@@ -1563,6 +1567,9 @@ update_frameheadpos(WindowAggState *winstate)
 			bool		sub,
 						less;
 
+			/* We must have an ordering column */
+			Assert(node->ordNumCols == 1);
+
 			/* Precompute flags for in_range checks */
 			if (frameOptions & FRAMEOPTION_START_OFFSET_PRECEDING)
 				sub = true;		/* subtract startOffset from current row */
@@ -1814,6 +1821,9 @@ update_frametailpos(WindowAggState *winstate)
 			bool		sub,
 						less;
 
+			/* We must have an ordering column */
+			Assert(node->ordNumCols == 1);
+
 			/* Precompute flags for in_range checks */
 			if (frameOptions & FRAMEOPTION_END_OFFSET_PRECEDING)
 				sub = true;		/* subtract endOffset from current row */
@@ -2318,16 +2328,21 @@ ExecInitWindowAgg(WindowAgg *node, EState *estate, int eflags)
 	winstate->temp_slot_2 = ExecInitExtraTupleSlot(estate, scanDesc);
 
 	/*
-	 * create frame head and tail slots only if needed (must match logic in
-	 * update_frameheadpos and update_frametailpos)
+	 * create frame head and tail slots only if needed (must create slots in
+	 * exactly the same cases that update_frameheadpos and update_frametailpos
+	 * need them)
 	 */
 	winstate->framehead_slot = winstate->frametail_slot = NULL;
 
 	if (frameOptions & (FRAMEOPTION_RANGE | FRAMEOPTION_GROUPS))
 	{
-		if (!(frameOptions & FRAMEOPTION_START_UNBOUNDED_PRECEDING))
+		if (((frameOptions & FRAMEOPTION_START_CURRENT_ROW) &&
+			 node->ordNumCols != 0) ||
+			(frameOptions & FRAMEOPTION_START_OFFSET))
 			winstate->framehead_slot = ExecInitExtraTupleSlot(estate, scanDesc);
-		if (!(frameOptions & FRAMEOPTION_END_UNBOUNDED_FOLLOWING))
+		if (((frameOptions & FRAMEOPTION_END_CURRENT_ROW) &&
+			 node->ordNumCols != 0) ||
+			(frameOptions & FRAMEOPTION_END_OFFSET))
 			winstate->frametail_slot = ExecInitExtraTupleSlot(estate, scanDesc);
 	}
 

src/backend/nodes/outfuncs.c

@@ -2110,7 +2110,6 @@ _outWindowAggPath(StringInfo str, const WindowAggPath *node)
 
 	WRITE_NODE_FIELD(subpath);
 	WRITE_NODE_FIELD(winclause);
-	WRITE_NODE_FIELD(winpathkeys);
 }
 
 static void

src/backend/optimizer/plan/createplan.c

@@ -107,15 +107,6 @@ static WindowAgg *create_windowagg_plan(PlannerInfo *root, WindowAggPath *best_p
 static SetOp *create_setop_plan(PlannerInfo *root, SetOpPath *best_path,
 				  int flags);
 static RecursiveUnion *create_recursiveunion_plan(PlannerInfo *root, RecursiveUnionPath *best_path);
-static void get_column_info_for_window(PlannerInfo *root, WindowClause *wc,
-						   List *tlist,
-						   int numSortCols, AttrNumber *sortColIdx,
-						   int *partNumCols,
-						   AttrNumber **partColIdx,
-						   Oid **partOperators,
-						   int *ordNumCols,
-						   AttrNumber **ordColIdx,
-						   Oid **ordOperators);
 static LockRows *create_lockrows_plan(PlannerInfo *root, LockRowsPath *best_path,
 					 int flags);
 static ModifyTable *create_modifytable_plan(PlannerInfo *root, ModifyTablePath *best_path);
@@ -2160,19 +2151,17 @@ create_windowagg_plan(PlannerInfo *root, WindowAggPath *best_path)
 {
 	WindowAgg  *plan;
 	WindowClause *wc = best_path->winclause;
+	int			numPart = list_length(wc->partitionClause);
+	int			numOrder = list_length(wc->orderClause);
 	Plan	   *subplan;
 	List	   *tlist;
-	int			numsortkeys;
-	AttrNumber *sortColIdx;
-	Oid		   *sortOperators;
-	Oid		   *collations;
-	bool	   *nullsFirst;
 	int			partNumCols;
 	AttrNumber *partColIdx;
 	Oid		   *partOperators;
 	int			ordNumCols;
 	AttrNumber *ordColIdx;
 	Oid		   *ordOperators;
+	ListCell   *lc;
 
 	/*
 	 * WindowAgg can project, so no need to be terribly picky about child
@@ -2183,32 +2172,43 @@ create_windowagg_plan(PlannerInfo *root, WindowAggPath *best_path)
 	tlist = build_path_tlist(root, &best_path->path);
 
 	/*
-	 * We shouldn't need to actually sort, but it's convenient to use
-	 * prepare_sort_from_pathkeys to identify the input's sort columns.
+	 * Convert SortGroupClause lists into arrays of attr indexes and equality
+	 * operators, as wanted by executor.  (Note: in principle, it's possible
+	 * to drop some of the sort columns, if they were proved redundant by
+	 * pathkey logic.  However, it doesn't seem worth going out of our way to
+	 * optimize such cases.  In any case, we must *not* remove the ordering
+	 * column for RANGE OFFSET cases, as the executor needs that for in_range
+	 * tests even if it's known to be equal to some partitioning column.)
 	 */
-	subplan = prepare_sort_from_pathkeys(subplan,
-										 best_path->winpathkeys,
-										 NULL,
-										 NULL,
-										 false,
-										 &numsortkeys,
-										 &sortColIdx,
-										 &sortOperators,
-										 &collations,
-										 &nullsFirst);
+	partColIdx = (AttrNumber *) palloc(sizeof(AttrNumber) * numPart);
+	partOperators = (Oid *) palloc(sizeof(Oid) * numPart);
+
+	partNumCols = 0;
+	foreach(lc, wc->partitionClause)
+	{
+		SortGroupClause *sgc = (SortGroupClause *) lfirst(lc);
+		TargetEntry *tle = get_sortgroupclause_tle(sgc, subplan->targetlist);
+
+		Assert(OidIsValid(sgc->eqop));
+		partColIdx[partNumCols] = tle->resno;
+		partOperators[partNumCols] = sgc->eqop;
+		partNumCols++;
+	}
 
-	/* Now deconstruct that into partition and ordering portions */
-	get_column_info_for_window(root,
-							   wc,
-							   subplan->targetlist,
-							   numsortkeys,
-							   sortColIdx,
-							   &partNumCols,
-							   &partColIdx,
-							   &partOperators,
-							   &ordNumCols,
-							   &ordColIdx,
-							   &ordOperators);
+	ordColIdx = (AttrNumber *) palloc(sizeof(AttrNumber) * numOrder);
+	ordOperators = (Oid *) palloc(sizeof(Oid) * numOrder);
+
+	ordNumCols = 0;
+	foreach(lc, wc->orderClause)
+	{
+		SortGroupClause *sgc = (SortGroupClause *) lfirst(lc);
+		TargetEntry *tle = get_sortgroupclause_tle(sgc, subplan->targetlist);
+
+		Assert(OidIsValid(sgc->eqop));
+		ordColIdx[ordNumCols] = tle->resno;
+		ordOperators[ordNumCols] = sgc->eqop;
+		ordNumCols++;
+	}
 
 	/* And finally we can make the WindowAgg node */
 	plan = make_windowagg(tlist,
@@ -2234,112 +2234,6 @@ create_windowagg_plan(PlannerInfo *root, WindowAggPath *best_path)
 	return plan;
 }
 
-/*
- * get_column_info_for_window
- *		Get the partitioning/ordering column numbers and equality operators
- *		for a WindowAgg node.
- *
- * This depends on the behavior of planner.c's make_pathkeys_for_window!
- *
- * We are given the target WindowClause and an array of the input column
- * numbers associated with the resulting pathkeys.  In the easy case, there
- * are the same number of pathkey columns as partitioning + ordering columns
- * and we just have to copy some data around.  However, it's possible that
- * some of the original partitioning + ordering columns were eliminated as
- * redundant during the transformation to pathkeys.  (This can happen even
- * though the parser gets rid of obvious duplicates.  A typical scenario is a
- * window specification "PARTITION BY x ORDER BY y" coupled with a clause
- * "WHERE x = y" that causes the two sort columns to be recognized as
- * redundant.)	In that unusual case, we have to work a lot harder to
- * determine which keys are significant.
- *
- * The method used here is a bit brute-force: add the sort columns to a list
- * one at a time and note when the resulting pathkey list gets longer.  But
- * it's a sufficiently uncommon case that a faster way doesn't seem worth
- * the amount of code refactoring that'd be needed.
- */
-static void
-get_column_info_for_window(PlannerInfo *root, WindowClause *wc, List *tlist,
-						   int numSortCols, AttrNumber *sortColIdx,
-						   int *partNumCols,
-						   AttrNumber **partColIdx,
-						   Oid **partOperators,
-						   int *ordNumCols,
-						   AttrNumber **ordColIdx,
-						   Oid **ordOperators)
-{
-	int			numPart = list_length(wc->partitionClause);
-	int			numOrder = list_length(wc->orderClause);
-
-	if (numSortCols == numPart + numOrder)
-	{
-		/* easy case */
-		*partNumCols = numPart;
-		*partColIdx = sortColIdx;
-		*partOperators = extract_grouping_ops(wc->partitionClause);
-		*ordNumCols = numOrder;
-		*ordColIdx = sortColIdx + numPart;
-		*ordOperators = extract_grouping_ops(wc->orderClause);
-	}
-	else
-	{
-		List	   *sortclauses;
-		List	   *pathkeys;
-		int			scidx;
-		ListCell   *lc;
-
-		/* first, allocate what's certainly enough space for the arrays */
-		*partNumCols = 0;
-		*partColIdx = (AttrNumber *) palloc(numPart * sizeof(AttrNumber));
-		*partOperators = (Oid *) palloc(numPart * sizeof(Oid));
-		*ordNumCols = 0;
-		*ordColIdx = (AttrNumber *) palloc(numOrder * sizeof(AttrNumber));
-		*ordOperators = (Oid *) palloc(numOrder * sizeof(Oid));
-		sortclauses = NIL;
-		pathkeys = NIL;
-		scidx = 0;
-		foreach(lc, wc->partitionClause)
-		{
-			SortGroupClause *sgc = (SortGroupClause *) lfirst(lc);
-			List	   *new_pathkeys;
-
-			sortclauses = lappend(sortclauses, sgc);
-			new_pathkeys = make_pathkeys_for_sortclauses(root,
-														 sortclauses,
-														 tlist);
-			if (list_length(new_pathkeys) > list_length(pathkeys))
-			{
-				/* this sort clause is actually significant */
-				(*partColIdx)[*partNumCols] = sortColIdx[scidx++];
-				(*partOperators)[*partNumCols] = sgc->eqop;
-				(*partNumCols)++;
-				pathkeys = new_pathkeys;
-			}
-		}
-		foreach(lc, wc->orderClause)
-		{
-			SortGroupClause *sgc = (SortGroupClause *) lfirst(lc);
-			List	   *new_pathkeys;
-
-			sortclauses = lappend(sortclauses, sgc);
-			new_pathkeys = make_pathkeys_for_sortclauses(root,
-														 sortclauses,
-														 tlist);
-			if (list_length(new_pathkeys) > list_length(pathkeys))
-			{
-				/* this sort clause is actually significant */
-				(*ordColIdx)[*ordNumCols] = sortColIdx[scidx++];
-				(*ordOperators)[*ordNumCols] = sgc->eqop;
-				(*ordNumCols)++;
-				pathkeys = new_pathkeys;
-			}
-		}
-		/* complain if we didn't eat exactly the right number of sort cols */
-		if (scidx != numSortCols)
-			elog(ERROR, "failed to deconstruct sort operators into partitioning/ordering operators");
-	}
-}
-
 /*
  * create_setop_plan
  *

src/backend/optimizer/plan/planner.c

@@ -4603,8 +4603,7 @@ create_one_window_path(PlannerInfo *root,
 		path = (Path *)
 			create_windowagg_path(root, window_rel, path, window_target,
 								  wflists->windowFuncs[wc->winref],
-								  wc,
-								  window_pathkeys);
+								  wc);
 	}
 
 	add_path(window_rel, path);
@@ -5465,8 +5464,6 @@ make_window_input_target(PlannerInfo *root,
  * The required ordering is first the PARTITION keys, then the ORDER keys.
  * In the future we might try to implement windowing using hashing, in which
  * case the ordering could be relaxed, but for now we always sort.
- *
- * Caution: if you change this, see createplan.c's get_column_info_for_window!
  */
 static List *
 make_pathkeys_for_window(PlannerInfo *root, WindowClause *wc,

src/backend/optimizer/util/pathnode.c

@@ -3072,10 +3072,9 @@ create_minmaxagg_path(PlannerInfo *root,
  * 'target' is the PathTarget to be computed
  * 'windowFuncs' is a list of WindowFunc structs
  * 'winclause' is a WindowClause that is common to all the WindowFuncs
- * 'winpathkeys' is the pathkeys for the PARTITION keys + ORDER keys
  *
- * The actual sort order of the input must match winpathkeys, but might
- * have additional keys after those.
+ * The input must be sorted according to the WindowClause's PARTITION keys
+ * plus ORDER BY keys.
  */
 WindowAggPath *
 create_windowagg_path(PlannerInfo *root,
@@ -3083,8 +3082,7 @@ create_windowagg_path(PlannerInfo *root,
 					  Path *subpath,
 					  PathTarget *target,
 					  List *windowFuncs,
-					  WindowClause *winclause,
-					  List *winpathkeys)
+					  WindowClause *winclause)
 {
 	WindowAggPath *pathnode = makeNode(WindowAggPath);
 
@@ -3102,7 +3100,6 @@ create_windowagg_path(PlannerInfo *root,
 
 	pathnode->subpath = subpath;
 	pathnode->winclause = winclause;
-	pathnode->winpathkeys = winpathkeys;
 
 	/*
 	 * For costing purposes, assume that there are no redundant partitioning

src/backend/parser/parse_clause.c

@@ -2795,6 +2795,16 @@ transformWindowDefinitions(ParseState *pstate,
 			wc->inRangeNullsFirst = sortcl->nulls_first;
 		}
 
+		/* Per spec, GROUPS mode requires an ORDER BY clause */
+		if (wc->frameOptions & FRAMEOPTION_GROUPS)
+		{
+			if (wc->orderClause == NIL)
+				ereport(ERROR,
+						(errcode(ERRCODE_WINDOWING_ERROR),
+						 errmsg("GROUPS mode requires an ORDER BY clause"),
+						 parser_errposition(pstate, windef->location)));
+		}
+
 		/* Process frame offset expressions */
 		wc->startOffset = transformFrameOffset(pstate, wc->frameOptions,
 											   rangeopfamily, rangeopcintype,

src/include/nodes/relation.h

@@ -1653,17 +1653,12 @@ typedef struct MinMaxAggPath
 
 /*
  * WindowAggPath represents generic computation of window functions
- *
- * Note: winpathkeys is separate from path.pathkeys because the actual sort
- * order might be an extension of winpathkeys; but createplan.c needs to
- * know exactly how many pathkeys match the window clause.
  */
 typedef struct WindowAggPath
 {
 	Path		path;
 	Path	   *subpath;		/* path representing input source */
 	WindowClause *winclause;	/* WindowClause we'll be using */
-	List	   *winpathkeys;	/* PathKeys for PARTITION keys + ORDER keys */
 } WindowAggPath;
 
 /*

src/include/optimizer/pathnode.h

@@ -214,8 +214,7 @@ extern WindowAggPath *create_windowagg_path(PlannerInfo *root,
 					  Path *subpath,
 					  PathTarget *target,
 					  List *windowFuncs,
-					  WindowClause *winclause,
-					  List *winpathkeys);
+					  WindowClause *winclause);
 extern SetOpPath *create_setop_path(PlannerInfo *root,
 				  RelOptInfo *rel,
 				  Path *subpath,

src/test/regress/expected/window.out

@@ -2834,6 +2834,101 @@ SELECT count(*) OVER (PARTITION BY four) FROM (SELECT * FROM tenk1 UNION ALL SEL
 -------
 (0 rows)
 
+-- check some degenerate cases
+create temp table t1 (f1 int, f2 int8);
+insert into t1 values (1,1),(1,2),(2,2);
+select f1, sum(f1) over (partition by f1
+                         range between 1 preceding and 1 following)
+from t1 where f1 = f2;  -- error, must have order by
+ERROR:  RANGE with offset PRECEDING/FOLLOWING requires exactly one ORDER BY column
+LINE 1: select f1, sum(f1) over (partition by f1
+                                ^
+explain (costs off)
+select f1, sum(f1) over (partition by f1 order by f2
+                         range between 1 preceding and 1 following)
+from t1 where f1 = f2;
+           QUERY PLAN            
+---------------------------------
+ WindowAgg
+   ->  Sort
+         Sort Key: f1
+         ->  Seq Scan on t1
+               Filter: (f1 = f2)
+(5 rows)
+
+select f1, sum(f1) over (partition by f1 order by f2
+                         range between 1 preceding and 1 following)
+from t1 where f1 = f2;
+ f1 | sum 
+----+-----
+  1 |   1
+  2 |   2
+(2 rows)
+
+select f1, sum(f1) over (partition by f1, f1 order by f2
+                         range between 2 preceding and 1 preceding)
+from t1 where f1 = f2;
+ f1 | sum 
+----+-----
+  1 |    
+  2 |    
+(2 rows)
+
+select f1, sum(f1) over (partition by f1, f2 order by f2
+                         range between 1 following and 2 following)
+from t1 where f1 = f2;
+ f1 | sum 
+----+-----
+  1 |    
+  2 |    
+(2 rows)
+
+select f1, sum(f1) over (partition by f1
+                         groups between 1 preceding and 1 following)
+from t1 where f1 = f2;  -- error, must have order by
+ERROR:  GROUPS mode requires an ORDER BY clause
+LINE 1: select f1, sum(f1) over (partition by f1
+                                ^
+explain (costs off)
+select f1, sum(f1) over (partition by f1 order by f2
+                         groups between 1 preceding and 1 following)
+from t1 where f1 = f2;
+           QUERY PLAN            
+---------------------------------
+ WindowAgg
+   ->  Sort
+         Sort Key: f1
+         ->  Seq Scan on t1
+               Filter: (f1 = f2)
+(5 rows)
+
+select f1, sum(f1) over (partition by f1 order by f2
+                         groups between 1 preceding and 1 following)
+from t1 where f1 = f2;
+ f1 | sum 
+----+-----
+  1 |   1
+  2 |   2
+(2 rows)
+
+select f1, sum(f1) over (partition by f1, f1 order by f2
+                         groups between 2 preceding and 1 preceding)
+from t1 where f1 = f2;
+ f1 | sum 
+----+-----
+  1 |    
+  2 |    
+(2 rows)
+
+select f1, sum(f1) over (partition by f1, f2 order by f2
+                         groups between 1 following and 2 following)
+from t1 where f1 = f2;
+ f1 | sum 
+----+-----
+  1 |    
+  2 |    
+(2 rows)
+
 -- ordering by a non-integer constant is allowed
 SELECT rank() OVER (ORDER BY length('abc'));
  rank 

src/test/regress/sql/window.sql

@@ -795,6 +795,44 @@ WINDOW w AS (ORDER BY x groups between 1 preceding and 1 following);
 -- with UNION
 SELECT count(*) OVER (PARTITION BY four) FROM (SELECT * FROM tenk1 UNION ALL SELECT * FROM tenk2)s LIMIT 0;
 
+-- check some degenerate cases
+create temp table t1 (f1 int, f2 int8);
+insert into t1 values (1,1),(1,2),(2,2);
+
+select f1, sum(f1) over (partition by f1
+                         range between 1 preceding and 1 following)
+from t1 where f1 = f2;  -- error, must have order by
+explain (costs off)
+select f1, sum(f1) over (partition by f1 order by f2
+                         range between 1 preceding and 1 following)
+from t1 where f1 = f2;
+select f1, sum(f1) over (partition by f1 order by f2
+                         range between 1 preceding and 1 following)
+from t1 where f1 = f2;
+select f1, sum(f1) over (partition by f1, f1 order by f2
+                         range between 2 preceding and 1 preceding)
+from t1 where f1 = f2;
+select f1, sum(f1) over (partition by f1, f2 order by f2
+                         range between 1 following and 2 following)
+from t1 where f1 = f2;
+
+select f1, sum(f1) over (partition by f1
+                         groups between 1 preceding and 1 following)
+from t1 where f1 = f2;  -- error, must have order by
+explain (costs off)
+select f1, sum(f1) over (partition by f1 order by f2
+                         groups between 1 preceding and 1 following)
+from t1 where f1 = f2;
+select f1, sum(f1) over (partition by f1 order by f2
+                         groups between 1 preceding and 1 following)
+from t1 where f1 = f2;
+select f1, sum(f1) over (partition by f1, f1 order by f2
+                         groups between 2 preceding and 1 preceding)
+from t1 where f1 = f2;
+select f1, sum(f1) over (partition by f1, f2 order by f2
+                         groups between 1 following and 2 following)
+from t1 where f1 = f2;
+
 -- ordering by a non-integer constant is allowed
 SELECT rank() OVER (ORDER BY length('abc'));