From 5125c021566323508211b2e54c2de70b985db1d8 Mon Sep 17 00:00:00 2001
From: Hana Pearlman <hana.pearlman@mongodb.com>
Date: Fri, 16 Apr 2021 14:45:39 +0000
Subject: Don't use set intersection

---
 src/mongo/db/query/get_executor.cpp | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/src/mongo/db/query/get_executor.cpp b/src/mongo/db/query/get_executor.cpp
index 36179df5791..ddca2f4a41a 100644
--- a/src/mongo/db/query/get_executor.cpp
+++ b/src/mongo/db/query/get_executor.cpp
@@ -214,12 +214,13 @@ IndexEntry indexEntryFromIndexCatalogEntry(OperationContext* opCtx,
 
                 // For a compound wildcard index, we also want to get the multikey information
                 // for (non-wildcard) fields in the index which are being queried.
-                auto indexKeys = wam->getKeyPattern().getFieldNames<std::set<std::string>>();
-                std::set_intersection(fields.begin(),
-                                      fields.end(),
-                                      indexKeys.begin(),
-                                      indexKeys.end(),
-                                      std::inserter(projectedFields, projectedFields.begin()));
+                for (auto& indexElem : wam->getKeyPattern()) {
+                    auto indexKey = indexElem.fieldNameStringData();
+                    if (fields.contains(indexKey.toString()) && indexKey != "$**" &&
+                        !indexKey.endsWith(".$**")) {
+                        projectedFields.insert(projectedFields.begin(), indexKey.toString());
+                    }
+                }
 
                 multikeyPathSet =
                     getWildcardMultikeyPathSet(wam, opCtx, projectedFields, &mkAccessStats);
-- 
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