SERVER-24401 allow DISTINCT_SCAN plans over collator comparison keys

1 files changed, 21 insertions, 0 deletions

diff --git a/jstests/core/collation.js b/jstests/core/collation.js
index c682a47e7bc..20df0f73489 100644
--- a/jstests/core/collation.js
+++ b/jstests/core/collation.js
@@ -470,6 +470,27 @@
     assert.commandWorked(coll.ensureIndex({a: 1}, {collation: {locale: "en_US"}}));
     var explain = coll.explain("queryPlanner").distinct("a");
     assert(planHasStage(explain.queryPlanner.winningPlan, "DISTINCT_SCAN"));
+    assert(planHasStage(explain.queryPlanner.winningPlan, "FETCH"));
+
+    // Distinct scan on strings can be used over an index with a collation when the predicate has
+    // exact bounds.
+    explain = coll.explain("queryPlanner").distinct("a", {a: {$gt: "foo"}});
+    assert(planHasStage(explain.queryPlanner.winningPlan, "DISTINCT_SCAN"));
+    assert(planHasStage(explain.queryPlanner.winningPlan, "FETCH"));
+    assert(!planHasStage(explain.queryPlanner.winningPlan, "PROJECTION"));
+
+    // Distinct scan cannot be used over an index with a collation when the predicate has inexact
+    // bounds.
+    explain = coll.explain("queryPlanner").distinct("a", {a: {$exists: true}});
+    assert(planHasStage(explain.queryPlanner.winningPlan, "IXSCAN"));
+    assert(planHasStage(explain.queryPlanner.winningPlan, "FETCH"));
+    assert(!planHasStage(explain.queryPlanner.winningPlan, "DISTINCT_SCAN"));
+
+    // Distinct scan can be used without a fetch when predicate has exact non-string bounds.
+    explain = coll.explain("queryPlanner").distinct("a", {a: {$gt: 3}});
+    assert(planHasStage(explain.queryPlanner.winningPlan, "DISTINCT_SCAN"));
+    assert(planHasStage(explain.queryPlanner.winningPlan, "PROJECTION"));
+    assert(!planHasStage(explain.queryPlanner.winningPlan, "FETCH"));
 
     // Distinct should not use index when no collation specified and collection default collation is
     // incompatible with index collation.