SERVER-29647 Fix $match swapping to avoid moving before $sort-$limit.

(cherry picked from commit 8b8845703f0c92bafca58ce5d9f36fd2327301d1)

Conflicts:
	jstests/core/views/views_aggregation.js

3 files changed, 46 insertions, 1 deletions

diff --git a/jstests/aggregation/bugs/match_swap_limit.js b/jstests/aggregation/bugs/match_swap_limit.js
new file mode 100644
index 00000000000..3de26d6f4b5
--- /dev/null
+++ b/jstests/aggregation/bugs/match_swap_limit.js
@@ -0,0 +1,20 @@
+/**
+ * Ensure that $match is always applied after $limit.
+ */
+(function() {
+    "use strict";
+
+    let coll = db.jstests_match_swap_limit;
+    coll.drop();
+
+    assert.writeOK(coll.insert({_id: 0, x: 1, y: 3}));
+    assert.writeOK(coll.insert({_id: 1, x: 2, y: 2}));
+    assert.writeOK(coll.insert({_id: 2, x: 3, y: 1}));
+
+    assert.eq([{_id: 1, x: 2, y: 2}],
+              coll.aggregate([{$sort: {x: -1}}, {$limit: 2}, {$match: {y: {$gte: 2}}}]).toArray());
+
+    assert.writeOK(coll.createIndex({x: 1}));
+    assert.eq([{_id: 1, x: 2, y: 2}],
+              coll.aggregate([{$sort: {x: -1}}, {$limit: 2}, {$match: {y: {$gte: 2}}}]).toArray());
+}());
diff --git a/src/mongo/db/pipeline/document_source_sort.h b/src/mongo/db/pipeline/document_source_sort.h
index d9266011a55..0e4a427e77f 100644
--- a/src/mongo/db/pipeline/document_source_sort.h
+++ b/src/mongo/db/pipeline/document_source_sort.h
@@ -51,7 +51,9 @@ public:
     }
 
     bool canSwapWithMatch() const final {
-        return true;
+        // Can't swap with a $match if a limit has been absorbed, since in general match can't swap
+        // with limit.
+        return !limitSrc;
     }
 
     BSONObjSet getOutputSorts() final {
diff --git a/src/mongo/db/pipeline/pipeline_test.cpp b/src/mongo/db/pipeline/pipeline_test.cpp
index 0df5816a905..ed4ea7aefee 100644
--- a/src/mongo/db/pipeline/pipeline_test.cpp
+++ b/src/mongo/db/pipeline/pipeline_test.cpp
@@ -870,6 +870,27 @@ class MatchShouldNotSwapBeforeSkip : public Base {
     }
 };
 
+class MatchCannotSwapWithLimit : public Base {
+    string inputPipeJson() final {
+        return "[{$limit: 3}, {$match: {x: {$gt: 0}}}]";
+    }
+    string outputPipeJson() final {
+        return inputPipeJson();
+    }
+};
+
+class MatchCannotSwapWithSortLimit : public Base {
+    string inputPipeJson() final {
+        return "[{$sort: {x: -1}}, {$limit: 3}, {$match: {x: {$gt: 0}}}]";
+    }
+    string outputPipeJson() final {
+        return "[{$sort: {sortKey: {x: -1}, limit: 3}}, {$match: {x: {$gt: 0}}}]";
+    }
+    string serializedPipeJson() override {
+        return inputPipeJson();
+    }
+};
+
 }  // namespace Local
 
 namespace Sharded {
@@ -1492,6 +1513,8 @@ public:
         add<Optimizations::Local::NeighboringMatchesShouldCoalesce>();
         add<Optimizations::Local::MatchShouldNotSwapBeforeLimit>();
         add<Optimizations::Local::MatchShouldNotSwapBeforeSkip>();
+        add<Optimizations::Local::MatchCannotSwapWithLimit>();
+        add<Optimizations::Local::MatchCannotSwapWithSortLimit>();
         add<Optimizations::Sharded::Empty>();
         add<Optimizations::Sharded::coalesceLookUpAndUnwind::ShouldCoalesceUnwindOnAs>();
         add<Optimizations::Sharded::coalesceLookUpAndUnwind::