SERVER-47848 Don't assume pipeline needs textScore, unless needsMerge

2 files changed, 26 insertions, 14 deletions

diff --git a/src/mongo/db/pipeline/pipeline.cpp b/src/mongo/db/pipeline/pipeline.cpp
index 156450b961f..f3fc921a6ee 100644
--- a/src/mongo/db/pipeline/pipeline.cpp
+++ b/src/mongo/db/pipeline/pipeline.cpp
@@ -537,13 +537,14 @@ DepsTracker Pipeline::getDependencies(QueryMetadataBitSet unavailableMetadata) c
         deps.needWholeDocument = true;  // don't know all fields we need
 
     if (!unavailableMetadata[DocumentMetadataFields::kTextScore]) {
-        // If there is a text score, assume we need to keep it if we can't prove we don't. If we are
-        // the first half of a pipeline which has been split, future stages might need it.
-        if (!knowAllMeta) {
+        // There is a text score available. If we are the first half of a split pipeline, then we
+        // have to assume future stages might depend on the textScore (unless we've encountered a
+        // stage that doesn't preserve metadata).
+        if (getContext()->needsMerge && !knowAllMeta) {
             deps.setNeedsMetadata(DocumentMetadataFields::kTextScore, true);
         }
     } else {
-        // If there is no text score available, then we don't need to ask for it.
+        // There is no text score available, so we don't need to ask for it.
         deps.setNeedsMetadata(DocumentMetadataFields::kTextScore, false);
     }
 
diff --git a/src/mongo/db/pipeline/pipeline_test.cpp b/src/mongo/db/pipeline/pipeline_test.cpp
index b0dbbfb99ce..d99123a1404 100644
--- a/src/mongo/db/pipeline/pipeline_test.cpp
+++ b/src/mongo/db/pipeline/pipeline_test.cpp
@@ -3032,7 +3032,6 @@ TEST_F(PipelineDependenciesTest, EmptyPipelineShouldRequireWholeDocument) {
     depsTracker =
         pipeline->getDependencies(DepsTracker::kAllMetadata & ~DepsTracker::kOnlyTextScore);
     ASSERT_TRUE(depsTracker.needWholeDocument);
-    ASSERT_TRUE(depsTracker.getNeedsMetadata(DocumentMetadataFields::kTextScore));
 }
 
 //
@@ -3203,19 +3202,31 @@ TEST_F(PipelineDependenciesTest, ShouldThrowIfTextScoreIsNeededButNotPresent) {
     ASSERT_THROWS(pipeline->getDependencies(DepsTracker::kAllMetadata), AssertionException);
 }
 
-TEST_F(PipelineDependenciesTest, ShouldRequireTextScoreIfAvailableAndNoStageReturnsExhaustiveMeta) {
+TEST_F(PipelineDependenciesTest,
+       ShouldRequireTextScoreIfAvailableAndNoStageReturnsExhaustiveMetaAndNeedsMerge) {
     auto ctx = getExpCtx();
+
+    // When needsMerge is true, the consumer might implicitly use textScore, if it's available.
+    ctx->needsMerge = true;
+
     auto pipeline = Pipeline::create({}, ctx);
+    auto deps = pipeline->getDependencies(DepsTracker::kAllMetadata & ~DepsTracker::kOnlyTextScore);
+    ASSERT_TRUE(deps.getNeedsMetadata(DocumentMetadataFields::kTextScore));
 
-    auto depsTracker =
-        pipeline->getDependencies(DepsTracker::kAllMetadata & ~DepsTracker::kOnlyTextScore);
-    ASSERT_TRUE(depsTracker.getNeedsMetadata(DocumentMetadataFields::kTextScore));
+    pipeline = Pipeline::create({DocumentSourceNeedsASeeNext::create(ctx)}, ctx);
+    deps = pipeline->getDependencies(DepsTracker::kAllMetadata & ~DepsTracker::kOnlyTextScore);
+    ASSERT_TRUE(deps.getNeedsMetadata(DocumentMetadataFields::kTextScore));
 
-    auto needsASeeNext = DocumentSourceNeedsASeeNext::create(ctx);
-    pipeline = Pipeline::create({needsASeeNext}, ctx);
-    depsTracker =
-        pipeline->getDependencies(DepsTracker::kAllMetadata & ~DepsTracker::kOnlyTextScore);
-    ASSERT_TRUE(depsTracker.getNeedsMetadata(DocumentMetadataFields::kTextScore));
+    // When needsMerge is false, if no stage explicitly uses textScore then we know it isn't needed.
+    ctx->needsMerge = false;
+
+    pipeline = Pipeline::create({}, ctx);
+    deps = pipeline->getDependencies(DepsTracker::kAllMetadata & ~DepsTracker::kOnlyTextScore);
+    ASSERT_FALSE(deps.getNeedsMetadata(DocumentMetadataFields::kTextScore));
+
+    pipeline = Pipeline::create({DocumentSourceNeedsASeeNext::create(ctx)}, ctx);
+    deps = pipeline->getDependencies(DepsTracker::kAllMetadata & ~DepsTracker::kOnlyTextScore);
+    ASSERT_FALSE(deps.getNeedsMetadata(DocumentMetadataFields::kTextScore));
 }
 
 TEST_F(PipelineDependenciesTest, ShouldNotRequireTextScoreIfAvailableButDefinitelyNotNeeded) {