1 files changed, 14 insertions, 15 deletions

diff --git a/src/mongo/db/index/expression_index.h b/src/mongo/db/index/expression_index.h
index 3806b1e1c4b..672f78457f1 100644
--- a/src/mongo/db/index/expression_index.h
+++ b/src/mongo/db/index/expression_index.h
@@ -77,19 +77,13 @@ namespace mongo {
             // Look at the cells we cover and all cells that are within our covering and finer.
             // Anything with our cover as a strict prefix is contained within the cover and should
             // be intersection tested.
-            bool considerCoarser = false;
             set<string> intervalSet;
+            set<string> exactSet;
             for (size_t i = 0; i < cover.size(); ++i) {
-                intervalSet.insert(cover[i].toString());
-                // If any of our covers could be covered by something in the index, we have
-                // to look at things coarser.
-                if (cover[i].level() > coarsestIndexedLevel) {
-                    considerCoarser = true;
-                }
-            }
 
-            set<string> exactSet;
-            if (considerCoarser) {
+                S2CellId coveredCell = cover[i];
+                intervalSet.insert(coveredCell.toString());
+
                 // Look at the cells that cover us.  We want to look at every cell that contains the
                 // covering we would index on if we were to insert the query geometry.  We generate
                 // the would-index-with-this-covering and find all the cells strictly containing the
@@ -103,11 +97,16 @@ namespace mongo {
                 // And we've already looked at points with the cell id 211111 from the regex search
                 // created above, so we only want things where the value of the last digit is not
                 // stored (and therefore could be 1).
-                for (size_t i = 0; i < cover.size(); ++i) {
-                    for (S2CellId id = cover[i].parent(); id.level() >= coarsestIndexedLevel;
-                            id = id.parent()) {
-                        exactSet.insert(id.toString());
-                    }
+
+                while (coveredCell.level() > coarsestIndexedLevel) {
+
+                    // Add the parent cell of the currently covered cell since we aren't at the
+                    // coarsest level yet
+                    // NOTE: Be careful not to generate cells strictly less than the
+                    // coarsestIndexedLevel - this can result in S2 failures when level < 0.
+
+                    coveredCell = coveredCell.parent();
+                    exactSet.insert(coveredCell.toString());
                 }
             }