SERVER-49766 Indexed and non-indexed collections return different results for null query

4 files changed, 61 insertions, 0 deletions

diff --git a/jstests/core/null_query_semantics.js b/jstests/core/null_query_semantics.js
index e13e0f5d0a8..7c1e3f5ff3b 100644
--- a/jstests/core/null_query_semantics.js
+++ b/jstests/core/null_query_semantics.js
@@ -260,6 +260,27 @@ function testNotEqualsNullSemantics() {
         assert(resultsEq(projectResults, extractAValues(expected)), tojson(projectResults));
     }());
 
+    // Test the semantics of the query {a: {$not: {$gte: null}}}.
+    (function testBasicNotEqualsGTENullQuery() {
+        const noProjectResults = coll.find({a: {$not: {$gte: null}}}).toArray();
+        const expected = [
+            {_id: "a_double_array", a: [[]]},
+            {_id: "a_empty_array", a: []},
+            {_id: "a_empty_subobject", a: {}},
+            {_id: "a_number", a: 4},
+            {_id: "a_object_array_all_b_nulls", a: [{b: null}, {b: undefined}, {b: null}, {}]},
+            {_id: "a_object_array_no_b_nulls", a: [{b: 1}, {b: 3}, {b: "string"}]},
+            {_id: "a_object_array_some_b_nulls", a: [{b: null}, {b: 3}, {b: null}]},
+            {_id: "a_object_array_some_b_undefined", a: [{b: undefined}, {b: 3}]},
+            {_id: "a_object_array_some_b_missing", a: [{b: 3}, {}]},
+            {_id: "a_subobject_b_not_null", a: {b: "hi"}},
+            {_id: "a_subobject_b_null", a: {b: null}},
+            {_id: "a_subobject_b_undefined", a: {b: undefined}},
+            {_id: "a_value_array_no_nulls", a: [1, "string", 4]},
+        ];
+        assert(resultsEq(noProjectResults, expected), tojson(noProjectResults));
+    }());
+
     // Test the semantics of the query {a: {$nin: [null, <number>]}}.
     (function testNotInNullQuery() {
         const query = {a: {$nin: [null, 75]}};
diff --git a/src/mongo/db/query/canonical_query_encoder.cpp b/src/mongo/db/query/canonical_query_encoder.cpp
index 487a8ad2198..ce2f70ec0fb 100644
--- a/src/mongo/db/query/canonical_query_encoder.cpp
+++ b/src/mongo/db/query/canonical_query_encoder.cpp
@@ -42,6 +42,27 @@
 #include "mongo/logv2/log.h"
 
 namespace mongo {
+
+bool isQueryNegatingGTEorLTENull(const mongo::MatchExpression* tree) {
+    // If the query predicate is null, do not reuse the plan since empty arrays ([]) are
+    // encoded as 'null' in the index. Thus we cannot safely invert the index bounds since 'null'
+    // has special comparison semantics.
+    if (tree->matchType() != MatchExpression::NOT) {
+        return false;
+    }
+
+    const MatchExpression* child = tree->getChild(0);
+    switch (child->matchType()) {
+        case MatchExpression::GTE:
+        case MatchExpression::LTE:
+            return static_cast<const ComparisonMatchExpression*>(child)->getData().type() ==
+                BSONType::jstNULL;
+
+        default:
+            return false;
+    }
+}
+
 namespace {
 
 // Delimiters for cache key encoding.
@@ -423,6 +444,12 @@ void encodeKeyForMatch(const MatchExpression* tree, StringBuilder* keyBuilder) {
     else if (tree->isLTMaxKey())
         *keyBuilder << "max";
 
+    // If the query predicate involves comparison to null, do not reuse the plan since empty arrays
+    // ([]) are encoded as null in the index. Thus we cannot safely invert the index bounds.
+    if (isQueryNegatingGTEorLTENull(tree)) {
+        *keyBuilder << "not_gte_lte_null";
+    }
+
     // Traverse child nodes.
     // Enclose children in [].
     if (tree->numChildren() > 0) {
diff --git a/src/mongo/db/query/canonical_query_encoder.h b/src/mongo/db/query/canonical_query_encoder.h
index 73c0eff5fa7..ca9d91bcace 100644
--- a/src/mongo/db/query/canonical_query_encoder.h
+++ b/src/mongo/db/query/canonical_query_encoder.h
@@ -32,6 +32,13 @@
 #include "mongo/db/query/plan_cache.h"
 
 namespace mongo {
+
+/**
+ * Returns true if the query predicate involves a negation of a LTE or GTE comparison to 'null'.
+ */
+bool isQueryNegatingGTEorLTENull(const mongo::MatchExpression* tree);
+
+
 namespace canonical_query_encoder {
 /**
  * Encode the given CanonicalQuery into a string representation which represents the shape of the
diff --git a/src/mongo/db/query/planner_ixselect.cpp b/src/mongo/db/query/planner_ixselect.cpp
index 47e1df683f5..ff0a4753e17 100644
--- a/src/mongo/db/query/planner_ixselect.cpp
+++ b/src/mongo/db/query/planner_ixselect.cpp
@@ -42,6 +42,7 @@
 #include "mongo/db/matcher/expression_geo.h"
 #include "mongo/db/matcher/expression_internal_expr_eq.h"
 #include "mongo/db/matcher/expression_text.h"
+#include "mongo/db/query/canonical_query_encoder.h"
 #include "mongo/db/query/collation/collator_interface.h"
 #include "mongo/db/query/index_tag.h"
 #include "mongo/db/query/indexability.h"
@@ -481,6 +482,11 @@ bool QueryPlannerIXSelect::_compatible(const BSONElement& keyPatternElt,
             }
         }
 
+        if (index.pathHasMultikeyComponent(keyPatternElt.fieldNameStringData()) && !index.sparse &&
+            isQueryNegatingGTEorLTENull(node)) {
+            return false;
+        }
+
         // If this is an $elemMatch value, make sure _all_ of the children can use the index.
         if (node->matchType() == MatchExpression::ELEM_MATCH_VALUE) {
             ElemMatchContext newContext;