SERVER-33607 Use kV2Unique version for new Unique indexes only

3 files changed, 22 insertions, 5 deletions

diff --git a/src/mongo/db/catalog/index_key_validate.cpp b/src/mongo/db/catalog/index_key_validate.cpp
index 4c1695069d5..25f7c43910f 100644
--- a/src/mongo/db/catalog/index_key_validate.cpp
+++ b/src/mongo/db/catalog/index_key_validate.cpp
@@ -220,6 +220,7 @@ StatusWith<BSONObj> validateIndexSpec(
     bool hasNamespaceField = false;
     bool hasVersionField = false;
     bool hasCollationField = false;
+    bool isUniqueIndex = false;
 
     auto fieldNamesValidStatus = validateIndexSpecFieldNames(indexSpec);
     if (!fieldNamesValidStatus.isOK()) {
@@ -364,6 +365,13 @@ StatusWith<BSONObj> validateIndexSpec(
             if (!statusWithMatcher.isOK()) {
                 return statusWithMatcher.getStatus();
             }
+        } else if (IndexDescriptor::kUniqueFieldName == indexSpecElemFieldName) {
+            // Note: Here we only consider whether or not "unique" field is specified and that its
+            // value evaluates to true. "_id" index for instance is unique, but the index spec for
+            // it doesn't carry a "unique" field. "isUniqueIndex" being false for "_id" indexes is
+            // on purpose, and in future if we were to make "_id" index specs include
+            // "unique:true", then we would need to change the logic here to preserve its behavior.
+            isUniqueIndex = indexSpecElem.trueValue();
         } else {
             // We can assume field name is valid at this point. Validation of fieldname is handled
             // prior to this in validateIndexSpecFieldNames().
@@ -372,8 +380,8 @@ StatusWith<BSONObj> validateIndexSpec(
     }
 
     if (!resolvedIndexVersion) {
-        resolvedIndexVersion =
-            IndexDescriptor::getDefaultIndexVersion(featureCompatibility.getVersion());
+        resolvedIndexVersion = IndexDescriptor::getDefaultIndexVersion(
+            featureCompatibility.getVersion(), isUniqueIndex);
     }
 
     if (!hasKeyPatternField) {
diff --git a/src/mongo/db/index/index_descriptor.cpp b/src/mongo/db/index/index_descriptor.cpp
index 9c72069f191..ca9ea36ece0 100644
--- a/src/mongo/db/index/index_descriptor.cpp
+++ b/src/mongo/db/index/index_descriptor.cpp
@@ -130,11 +130,19 @@ Status IndexDescriptor::isIndexVersionAllowedForCreation(
 }
 
 IndexVersion IndexDescriptor::getDefaultIndexVersion(
-    ServerGlobalParams::FeatureCompatibility::Version featureCompatibilityVersion) {
+    ServerGlobalParams::FeatureCompatibility::Version featureCompatibilityVersion,
+    bool isUniqueIndex) {
     // The gating variable would allow creation of V2 format unique index when set to true.
+    // Note: Here "isUniqueIndex" only considers whether or not "unique" field is specified
+    // and that its value evaluates to true. "_id" index for instance is unique, but the
+    // index spec for it doesn't carry a "unique" field. "isUniqueIndex" being false for
+    // "_id" indexes is on purpose, and in future if we were to make "_id" index specs
+    // include "unique:true", then we would need to change the logic here to preserve its
+    // behavior.
     const bool useV2UniqueIndexFormat = false;
-    if (useV2UniqueIndexFormat)
+    if (useV2UniqueIndexFormat && isUniqueIndex) {
         return IndexVersion::kV2Unique;
+    }
 
     return IndexVersion::kV2;
 }
diff --git a/src/mongo/db/index/index_descriptor.h b/src/mongo/db/index/index_descriptor.h
index f5c8a46b76c..f1135ca3f91 100644
--- a/src/mongo/db/index/index_descriptor.h
+++ b/src/mongo/db/index/index_descriptor.h
@@ -133,7 +133,8 @@ public:
      * Returns the index version to use if it isn't specified in the index specification.
      */
     static IndexVersion getDefaultIndexVersion(
-        ServerGlobalParams::FeatureCompatibility::Version featureCompatibilityVersion);
+        ServerGlobalParams::FeatureCompatibility::Version featureCompatibilityVersion,
+        bool isUniqueIndex = false);
 
     //
     // Information about the key pattern.