1 files changed, 63 insertions, 0 deletions
diff --git a/jstests/core/index_elemmatch2.js b/jstests/core/index_elemmatch2.js
new file mode 100644
index 00000000000..ecd24035284
--- /dev/null
+++ b/jstests/core/index_elemmatch2.js
@@ -0,0 +1,63 @@
+/**
+ * Test that queries containing $elemMatch correctly use an index if each child expression is
+ * compatible with the index.
+ */
+(function() {
+    "use strict";
+
+    load("jstests/libs/analyze_plan.js");
+
+    const coll = db.elemMatch_index;
+    coll.drop();
+
+    assert.writeOK(coll.insert({a: 1}));
+    assert.writeOK(coll.insert({a: [{}]}));
+    assert.writeOK(coll.insert({a: [1, null]}));
+    assert.writeOK(coll.insert({a: [{type: "Point", coordinates: [0, 0]}]}));
+
+    assert.commandWorked(coll.createIndex({a: 1}, {sparse: true}));
+
+    function assertIndexResults(coll, query, useIndex, nReturned) {
+        const explainPlan = coll.find(query).explain("executionStats");
+        assert.eq(isIxscan(explainPlan.queryPlanner.winningPlan), useIndex);
+        assert.eq(explainPlan.executionStats.nReturned, nReturned);
+    }
+
+    assertIndexResults(coll, {a: {$elemMatch: {$exists: false}}}, false, 0);
+
+    // An $elemMatch predicate is treated as nested, and the index should be used for $exists:true.
+    assertIndexResults(coll, {a: {$elemMatch: {$exists: true}}}, true, 3);
+
+    // $not within $elemMatch should not attempt to use a sparse index for $exists:false.
+    assertIndexResults(coll, {a: {$elemMatch: {$not: {$exists: false}}}}, false, 3);
+    assertIndexResults(coll, {a: {$elemMatch: {$gt: 0, $not: {$exists: false}}}}, false, 1);
+
+    // $geo within $elemMatch should not attempt to use a non-geo index.
+    assertIndexResults(
+        coll,
+        {
+          a: {
+              $elemMatch: {
+                  $geoWithin: {
+                      $geometry:
+                          {type: "Polygon", coordinates: [[[0, 0], [0, 1], [1, 0], [0, 0]]]}
+                  }
+              }
+          }
+        },
+        false,
+        1);
+
+    // $in with a null value within $elemMatch should use a sparse index.
+    assertIndexResults(coll, {a: {$elemMatch: {$in: [null]}}}, true, 1);
+
+    // $eq with a null value within $elemMatch should use a sparse index.
+    assertIndexResults(coll, {a: {$elemMatch: {$eq: null}}}, true, 1);
+
+    // A negated regex within $elemMatch should not use an index, sparse or not.
+    assertIndexResults(coll, {a: {$elemMatch: {$not: {$in: [/^a/]}}}}, false, 3);
+
+    coll.dropIndexes();
+    assert.commandWorked(coll.createIndex({a: 1}));
+    assertIndexResults(coll, {a: {$elemMatch: {$not: {$in: [/^a/]}}}}, false, 3);
+})();