From 8bc18f2367d616ef1e68b3be4acfaa1d2e8daa6e Mon Sep 17 00:00:00 2001
From: David Storch <david.storch@10gen.com>
Date: Wed, 21 Jun 2017 15:07:52 -0400
Subject: SERVER-29647 Fix $match swapping to avoid moving before $sort-$limit.

(cherry picked from commit 8b8845703f0c92bafca58ce5d9f36fd2327301d1)

Conflicts:
	jstests/core/views/views_aggregation.js
---
 src/mongo/db/pipeline/document_source_sort.h |  4 +++-
 src/mongo/db/pipeline/pipeline_test.cpp      | 23 +++++++++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

(limited to 'src')

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::
-- 
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