SERVER-56449 Allow $derivative and $integral to use descending sortBy (document windows)

4 files changed, 41 insertions, 9 deletions

diff --git a/jstests/aggregation/sources/setWindowFields/derivative.js b/jstests/aggregation/sources/setWindowFields/derivative.js
index 173f7e5e493..f125bf96872 100644
--- a/jstests/aggregation/sources/setWindowFields/derivative.js
+++ b/jstests/aggregation/sources/setWindowFields/derivative.js
@@ -117,6 +117,23 @@ assert.docEq(result, [
     // time: 4 and time: 7.
     {time: 7, y: 118, dy: +3 / 3},
 ]);
+// Because the derivative is the same irrespective of sort order (as long as we reexpress the
+// bounds) we can compare this result with the result of the previous aggregation.
+const resultDesc =
+    coll.aggregate([
+            {
+                $setWindowFields: {
+                    sortBy: {time: -1},
+                    output: {
+                        dy: {$derivative: {input: "$y"}, window: {documents: [-1, +3]}},
+                    }
+                }
+            },
+            {$unset: "_id"},
+            {$sort: {time: 1}},
+        ])
+        .toArray();
+assert.docEq(result, resultDesc);
 
 // Example with range-based bounds.
 coll.drop();
diff --git a/jstests/aggregation/sources/setWindowFields/integral.js b/jstests/aggregation/sources/setWindowFields/integral.js
index 097ea70536a..196c3d463a8 100644
--- a/jstests/aggregation/sources/setWindowFields/integral.js
+++ b/jstests/aggregation/sources/setWindowFields/integral.js
@@ -70,6 +70,26 @@ assert.sameMembers(result, [
     {partitionID: 2, x: 4, y: 107, integral: 212},  // (105 + 107) * 2 / 2 = 212
     {partitionID: 2, x: 6, y: -100, integral: 7},   // (107 - 100) * 2 / 2 = 7
 ]);
+// Because the integral from a to b is the same as the inverse of the integral from b to a, we can
+// invert the input, sort order, and bounds so that the results are the same as the previous
+// integral.
+const resultDesc = coll.aggregate([
+                           {
+                               $setWindowFields: {
+                                   partitionBy: "$partitionID",
+                                   sortBy: {x: -1},
+                                   output: {
+                                       integral: {
+                                           $integral: {input: {$subtract: [0, "$y"]}},
+                                           window: {documents: [0, +1]}
+                                       },
+                                   }
+                               }
+                           },
+                           {$unset: "_id"},
+                       ])
+                       .toArray();
+assert.sameMembers(result, resultDesc);
 
 // 'outputUnit' only supports 'week' and smaller.
 coll.drop();
diff --git a/src/mongo/db/pipeline/window_function/window_function_exec.cpp b/src/mongo/db/pipeline/window_function/window_function_exec.cpp
index 00ca373d6a3..4619ef46bdd 100644
--- a/src/mongo/db/pipeline/window_function/window_function_exec.cpp
+++ b/src/mongo/db/pipeline/window_function/window_function_exec.cpp
@@ -56,9 +56,8 @@ boost::intrusive_ptr<Expression> translateInputExpression(
     if (auto integral = dynamic_cast<window_function::ExpressionIntegral*>(expr.get())) {
         auto expCtx = integral->expCtx();
         tassert(5558802,
-                "$integral requires a 1-field ascending sortBy",
-                sortBy && sortBy->size() == 1 && !sortBy->begin()->expression &&
-                    sortBy->begin()->isAscending);
+                "$integral requires a 1-field sortBy",
+                sortBy && sortBy->size() == 1 && !sortBy->begin()->expression);
         auto sortByExpr = ExpressionFieldPath::createPathFromString(
             expCtx, sortBy->begin()->fieldPath->fullPath(), expCtx->variablesParseState);
         return ExpressionArray::create(
@@ -97,9 +96,8 @@ std::unique_ptr<mongo::WindowFunctionExec> translateDerivative(
     const boost::optional<SortPattern>& sortBy,
     MemoryUsageTracker::PerFunctionMemoryTracker* memTracker) {
     tassert(5490703,
-            "$derivative requires a 1-field ascending sortBy",
-            sortBy && sortBy->size() == 1 && !sortBy->begin()->expression &&
-                sortBy->begin()->isAscending);
+            "$derivative requires a 1-field sortBy",
+            sortBy && sortBy->size() == 1 && !sortBy->begin()->expression);
     auto sortExpr =
         ExpressionFieldPath::createPathFromString(expr->expCtx(),
                                                   sortBy->begin()->fieldPath->fullPath(),
diff --git a/src/mongo/db/pipeline/window_function/window_function_expression.h b/src/mongo/db/pipeline/window_function/window_function_expression.h
index 526c519ede4..52fe79e5b48 100644
--- a/src/mongo/db/pipeline/window_function/window_function_expression.h
+++ b/src/mongo/db/pipeline/window_function/window_function_expression.h
@@ -484,9 +484,6 @@ protected:
         uassert(ErrorCodes::FailedToParse,
                 str::stream() << accumulatorName << " requires a non-expression sortBy",
                 !sortBy->begin()->expression);
-        uassert(ErrorCodes::FailedToParse,
-                str::stream() << accumulatorName << " requires an ascending sortBy",
-                sortBy->begin()->isAscending);
     }
 
     boost::optional<long long> convertTimeUnitToMillis(boost::optional<TimeUnit> outputUnit) const {