SERVER-20347 $in containing a regex element cannot use a hashed index

2 files changed, 24 insertions, 1 deletions

diff --git a/src/mongo/db/query/planner_ixselect.cpp b/src/mongo/db/query/planner_ixselect.cpp
index 0b274c69ebf..6cb0062befc 100644
--- a/src/mongo/db/query/planner_ixselect.cpp
+++ b/src/mongo/db/query/planner_ixselect.cpp
@@ -236,7 +236,14 @@ bool QueryPlannerIXSelect::compatible(const BSONElement& elt,
         invariant(0);
         return true;
     } else if (IndexNames::HASHED == indexedFieldType) {
-        return exprtype == MatchExpression::MATCH_IN || exprtype == MatchExpression::EQ;
+        if (exprtype == MatchExpression::EQ) {
+            return true;
+        }
+        if (exprtype == MatchExpression::MATCH_IN) {
+            const InMatchExpression* expr = static_cast<const InMatchExpression*>(node);
+            return expr->getData().numRegexes() == 0;
+        }
+        return false;
     } else if (IndexNames::GEO_2DSPHERE == indexedFieldType) {
         if (exprtype == MatchExpression::GEO) {
             // within or intersect.
diff --git a/src/mongo/db/query/query_planner_test.cpp b/src/mongo/db/query/query_planner_test.cpp
index 3d3a07e9e45..fbb7be3069f 100644
--- a/src/mongo/db/query/query_planner_test.cpp
+++ b/src/mongo/db/query/query_planner_test.cpp
@@ -65,6 +65,15 @@ TEST_F(QueryPlannerTest, EqualityIndexScanWithTrailingFields) {
     assertSolutionExists("{fetch: {filter: null, node: {ixscan: {pattern: {x: 1, y: 1}}}}}");
 }
 
+// $eq can use a hashed index because it looks for values of type regex;
+// it doesn't evaluate the regex itself.
+TEST_F(QueryPlannerTest, EqCanUseHashedIndexWithRegex) {
+    addIndex(BSON("a"
+                  << "hashed"));
+    runQuery(fromjson("{a: {$eq: /abc/}}"));
+    ASSERT_EQUALS(getNumSolutions(), 2U);
+}
+
 //
 // indexFilterApplied
 // Check that index filter flag is passed from planner params
@@ -1983,6 +1992,13 @@ TEST_F(QueryPlannerTest, InWithSortAndLimitTrailingField) {
         " {ixscan: {pattern: {a:1,b:-1,c:1}}}]}}}}}}");
 }
 
+TEST_F(QueryPlannerTest, InCantUseHashedIndexWithRegex) {
+    addIndex(BSON("a"
+                  << "hashed"));
+    runQuery(fromjson("{a: {$in: [/abc/]}}"));
+    ASSERT_EQUALS(getNumSolutions(), 1U);
+}
+
 //
 // Multiple solutions
 //