SERVER-61485 Wrong SBE plan reported by explain() for rejected plans

2 files changed, 69 insertions, 3 deletions

diff --git a/jstests/core/sbe/sbe_explain_rejected_plans.js b/jstests/core/sbe/sbe_explain_rejected_plans.js
new file mode 100644
index 00000000000..acf2101a4f2
--- /dev/null
+++ b/jstests/core/sbe/sbe_explain_rejected_plans.js
@@ -0,0 +1,66 @@
+/**
+ * Tests that SBE reports correct rejected plans when calling explain().
+ * @tags: [assumes_unsharded_collection]
+ */
+(function() {
+"use strict";
+
+load("jstests/libs/analyze_plan.js");
+load("jstests/libs/collection_drop_recreate.js");
+load("jstests/libs/sbe_util.js");
+
+const isSBEEnabled = checkSBEEnabled(db);
+if (!isSBEEnabled) {
+    jsTestLog("Skipping test because the SBE feature flag is disabled");
+    return;
+}
+
+const coll = assertDropAndRecreateCollection(db, "sbe_explain_rejected_plans");
+assert.commandWorked(coll.createIndex({a: 1}));
+assert.commandWorked(coll.createIndex({b: 1}));
+assert.commandWorked(coll.createIndex({a: 1, b: 1}));
+
+// Insert data, such that index 'a_1_b_1' is preferred.
+for (let a = 0; a < 10; a++) {
+    for (let b = 0; b < 10; b++) {
+        assert.commandWorked(coll.insert({a: a, b: b}));
+    }
+}
+
+const a1b1IndexName = "a_1_b_1";
+const a1IndexName = "a_1";
+const b1IndexName = "b_1";
+const explain = coll.find({a: 7, b: 9}).explain("executionStats");
+
+// Verify that the winner plan has index scan stage on 'a_1_b_1'.
+let ixscans = getPlanStages(getWinningPlan(explain.queryPlanner), "IXSCAN");
+assert.neq(ixscans.length, 0, explain);
+for (let ixscan of ixscans) {
+    assert.eq(ixscan.indexName, a1b1IndexName, explain);
+}
+
+// Verify that the winning SBE plan has index scan stage on 'a_1_b_1'.
+const executionStages = getExecutionStages(explain);
+assert.neq(executionStages.length, 0, explain);
+for (let executionStage of executionStages) {
+    ixscans = getPlanStages(executionStage, "ixseek");
+    assert.neq(ixscans.length, 0);
+    for (let ixscan of ixscans) {
+        assert.eq(ixscan.indexName, a1b1IndexName, ixscan);
+    }
+}
+
+// Verify that rejected plans should have index scan on 'a_1' or 'b_1'.
+for (let rejectedPlan of getRejectedPlans(explain)) {
+    ixscans = getPlanStages(getRejectedPlan(rejectedPlan), "IXSCAN");
+    assert.neq(ixscans.length, 0, explain);
+    for (let ixscan of ixscans) {
+        assert.contains(ixscan.indexName, [a1IndexName, b1IndexName], ixscan);
+    }
+
+    assert(!rejectedPlan.slotBasedPlan.stages.includes("@\"a_1_b_1\""), explain);
+    assert(rejectedPlan.slotBasedPlan.stages.includes("@\"a_1\"") ||
+               rejectedPlan.slotBasedPlan.stages.includes("@\"b_1\""),
+           explain);
+}
+})();
diff --git a/src/mongo/db/query/plan_explainer_sbe.h b/src/mongo/db/query/plan_explainer_sbe.h
index 7ce73608596..5dc97f90641 100644
--- a/src/mongo/db/query/plan_explainer_sbe.h
+++ b/src/mongo/db/query/plan_explainer_sbe.h
@@ -77,11 +77,11 @@ public:
                                                      ExplainOptions::Verbosity) const final;
 
 private:
-    boost::optional<BSONObj> buildExecPlanDebugInfo(
-        const sbe::PlanStage* root, const stage_builder::PlanStageData* data) const {
+    static boost::optional<BSONObj> buildExecPlanDebugInfo(
+        const sbe::PlanStage* root, const stage_builder::PlanStageData* data) {
         if (root && data) {
             return BSON("slots" << data->debugString() << "stages"
-                                << sbe::DebugPrinter().print(*_root));
+                                << sbe::DebugPrinter().print(*root));
         }
         return boost::none;
     }