SERVER-44383 Use trivial BSON conversion for sort key generation

3 files changed, 26 insertions, 3 deletions

diff --git a/src/mongo/db/exec/document_value/document.cpp b/src/mongo/db/exec/document_value/document.cpp
index c50648c0e20..dac2e0dde60 100644
--- a/src/mongo/db/exec/document_value/document.cpp
+++ b/src/mongo/db/exec/document_value/document.cpp
@@ -461,6 +461,14 @@ BSONObj Document::toBson() const {
     return bb.obj();
 }
 
+boost::optional<BSONObj> Document::toBsonIfTriviallyConvertible() const {
+    if (!storage().isModified() && !storage().stripMetadata()) {
+        return storage().bsonObj();
+    } else {
+        return boost::none;
+    }
+}
+
 constexpr StringData Document::metaFieldTextScore;
 constexpr StringData Document::metaFieldRandVal;
 constexpr StringData Document::metaFieldSortKey;
diff --git a/src/mongo/db/exec/document_value/document.h b/src/mongo/db/exec/document_value/document.h
index 73a72175db6..2c389fd2056 100644
--- a/src/mongo/db/exec/document_value/document.h
+++ b/src/mongo/db/exec/document_value/document.h
@@ -230,6 +230,15 @@ public:
     BSONObj toBson() const;
 
     /**
+     * Serializes this document iff the conversion is "trivial," meaning that the underlying storage
+     * is already in BSON format and there are no damages. No conversion is necessary; this function
+     * just returns the already existing BSON.
+     *
+     * When the trivial conversion is not possible, this function returns boost::none.
+     */
+    boost::optional<BSONObj> toBsonIfTriviallyConvertible() const;
+
+    /**
      * Like the 'toBson()' method, but includes metadata at the top-level. When
      * 'use42ChangeStreamSortKeys' is true, we assume that any Value in the "sortKey" metadata
      * represents the resume token, which gets assigned directly to the "$sortKey" field. Otherwise,
diff --git a/src/mongo/db/index/sort_key_generator.cpp b/src/mongo/db/index/sort_key_generator.cpp
index a09622bb316..2719682a77b 100644
--- a/src/mongo/db/index/sort_key_generator.cpp
+++ b/src/mongo/db/index/sort_key_generator.cpp
@@ -260,9 +260,15 @@ StatusWith<Value> SortKeyGenerator::extractKeyFast(const Document& doc,
 
 BSONObj SortKeyGenerator::extractKeyWithArray(const Document& doc,
                                               const DocumentMetadataFields& metadata) const {
-    // Convert the Document to a BSONObj, but only do the conversion for the paths we actually need.
-    // Then run the result through the SortKeyGenerator to obtain the final sort key.
-    auto bsonDoc = _sortPattern.documentToBsonWithSortPaths(doc);
+    // Sort key generation requires the Document to be in BSON format. First, we attempt the
+    // "trivial" conversion, which returns the Document's BSON storage.
+    auto optionalBsonDoc = doc.toBsonIfTriviallyConvertible();
+
+    // If the trivial conversion is not possible, we perform a conversion that only converts the
+    // paths we need to generate the sort key.
+    auto bsonDoc = optionalBsonDoc ? std::move(*optionalBsonDoc)
+                                   : _sortPattern.documentToBsonWithSortPaths(doc);
+
     return computeSortKeyFromDocument(bsonDoc, metadata);
 }