SERVER-14608 Optimize out empty match stage in aggregation pipeline

3 files changed, 32 insertions, 4 deletions

diff --git a/src/mongo/db/pipeline/document_source.h b/src/mongo/db/pipeline/document_source.h
index bbbe429e1bd..a2b6c2225da 100644
--- a/src/mongo/db/pipeline/document_source.h
+++ b/src/mongo/db/pipeline/document_source.h
@@ -567,6 +567,7 @@ namespace mongo {
         virtual const char *getSourceName() const;
         virtual bool coalesce(const boost::intrusive_ptr<DocumentSource>& nextSource);
         virtual Value serialize(bool explain = false) const;
+        virtual boost::intrusive_ptr<DocumentSource> optimize();
         virtual void setSource(DocumentSource* Source);
 
         /**
diff --git a/src/mongo/db/pipeline/document_source_match.cpp b/src/mongo/db/pipeline/document_source_match.cpp
index ab2784acb97..b6c6e005446 100644
--- a/src/mongo/db/pipeline/document_source_match.cpp
+++ b/src/mongo/db/pipeline/document_source_match.cpp
@@ -53,6 +53,10 @@ namespace mongo {
         return Value(DOC(getSourceName() << Document(getQuery())));
     }
 
+    intrusive_ptr<DocumentSource> DocumentSourceMatch::optimize() {
+        return getQuery().isEmpty() ? nullptr : this;
+    }
+
     boost::optional<Document> DocumentSourceMatch::getNext() {
         pExpCtx->checkForInterrupt();
 
diff --git a/src/mongo/dbtests/pipelinetests.cpp b/src/mongo/dbtests/pipelinetests.cpp
index ef3d24dd546..11f9230e5b7 100644
--- a/src/mongo/dbtests/pipelinetests.cpp
+++ b/src/mongo/dbtests/pipelinetests.cpp
@@ -97,13 +97,34 @@ namespace PipelineTests {
                 }
             };
 
-            class RemoveSkipZeroKeepProject : public Base {
+            class RemoveEmptyMatch : public Base {
                 string inputPipeJson() override {
-                    return "[{$skip: 0}, {$project: {i: true}}]";
+                    return "[{$match: {}}]";
                 }
 
                 string outputPipeJson() override {
-                    return "[{$project: {i: true}}]";
+                    return "[]";
+                }
+            };
+
+            class RemoveMultipleEmptyMatches : public Base {
+                string inputPipeJson() override {
+                    return "[{$match: {}}, {$match: {}}]";
+                }
+
+                string outputPipeJson() override {
+                    // TODO: The desired behavior here is to end up with an empty array.
+                    return "[{$match: {$and: [{}, {}]}}]";
+                }
+            };
+
+            class DoNotRemoveNonEmptyMatch : public Base {
+                string inputPipeJson() override {
+                    return "[{$match: {_id: 1}}]";
+                }
+
+                string outputPipeJson() override {
+                    return "[{$match: {_id: 1}}]";
                 }
             };
 
@@ -278,7 +299,9 @@ namespace PipelineTests {
         void setupTests() {
             add<Optimizations::Local::RemoveSkipZero>();
             add<Optimizations::Local::DoNotRemoveSkipOne>();
-            add<Optimizations::Local::RemoveSkipZeroKeepProject>();
+            add<Optimizations::Local::RemoveEmptyMatch>();
+            add<Optimizations::Local::RemoveMultipleEmptyMatches>();
+            add<Optimizations::Local::DoNotRemoveNonEmptyMatch>();
             add<Optimizations::Sharded::Empty>();
             add<Optimizations::Sharded::moveFinalUnwindFromShardsToMerger::OneUnwind>();
             add<Optimizations::Sharded::moveFinalUnwindFromShardsToMerger::TwoUnwind>();