SERVER-31072 reorder $limit before $lookup in pipeline if no $unwind is present

1 files changed, 80 insertions, 0 deletions

diff --git a/jstests/noPassthroughWithMongod/lookup_with_limit.js b/jstests/noPassthroughWithMongod/lookup_with_limit.js
new file mode 100644
index 00000000000..3b841422ca3
--- /dev/null
+++ b/jstests/noPassthroughWithMongod/lookup_with_limit.js
@@ -0,0 +1,80 @@
+/**
+ * Tests that the $limit stage is pushed before $lookup stages, except when there is an $unwind.
+ */
+(function() {
+"use strict";
+
+const coll = db.lookup_with_limit;
+const other = db.lookup_with_limit_other;
+coll.drop();
+other.drop();
+
+// Checks that the order of the pipeline stages matches the expected ordering depending on whether
+// the pipeline is optimized or not.
+function checkResults(pipeline, isOptimized, expected) {
+    assert.commandWorked(db.adminCommand({
+        "configureFailPoint": 'disablePipelineOptimization',
+        "mode": isOptimized ? 'off' : 'alwaysOn'
+    }));
+    const explain = coll.explain().aggregate(pipeline);
+    if (expected.length > 0) {
+        assert.eq(explain.stages[0].$cursor.queryPlanner.winningPlan.stage, expected[0], explain);
+    }
+    for (let i = 1; i < expected.length; i++) {
+        assert.eq(Object.keys(explain.stages[i]), expected[i], explain);
+    }
+}
+
+// Insert ten documents into coll: {x: 0}, {x: 1}, ..., {x: 9}.
+const bulk = coll.initializeOrderedBulkOp();
+Array.from({length: 10}, (_, i) => ({x: i})).forEach(doc => bulk.insert(doc));
+assert.commandWorked(bulk.execute());
+
+// Insert twenty documents into other: {x: 0, y: 0}, {x: 0, y: 1}, ..., {x: 9, y: 0}, {x: 9, y: 1}.
+const bulk_other = other.initializeOrderedBulkOp();
+Array.from({length: 10}, (_, i) => ({x: i, y: 0})).forEach(doc => bulk_other.insert(doc));
+Array.from({length: 10}, (_, i) => ({x: i, y: 1})).forEach(doc => bulk_other.insert(doc));
+assert.commandWorked(bulk_other.execute());
+
+// Check that lookup->limit is reordered to limit->lookup, with the limit stage pushed down to query
+// system.
+var pipeline = [
+    {$lookup: {from: other.getName(), localField: "x", foreignField: "x", as: "from_other"}},
+    {$limit: 5}
+];
+checkResults(pipeline, false, ["COLLSCAN", "$lookup", "$limit"]);
+checkResults(pipeline, true, ["LIMIT", "$lookup"]);
+
+// Check that lookup->addFields->lookup->limit is reordered to limit->lookup->addFields->lookup,
+// with the limit stage pushed down to query system.
+pipeline = [
+    {$lookup: {from: other.getName(), localField: "x", foreignField: "x", as: "from_other"}},
+    {$addFields: {z: 0}},
+    {$lookup: {from: other.getName(), localField: "x", foreignField: "x", as: "additional"}},
+    {$limit: 5}
+];
+checkResults(pipeline, false, ["COLLSCAN", "$lookup", "$addFields", "$lookup", "$limit"]);
+checkResults(pipeline, true, ["LIMIT", "$lookup", "$addFields", "$lookup"]);
+
+// Check that lookup->unwind->limit is reordered to lookup->limit, with the unwind stage being
+// absorbed into the lookup stage and preventing the limit from swapping before it.
+pipeline = [
+    {$lookup: {from: other.getName(), localField: "x", foreignField: "x", as: "from_other"}},
+    {$unwind: "$from_other"},
+    {$limit: 5}
+];
+checkResults(pipeline, false, ["COLLSCAN", "$lookup", "$unwind", "$limit"]);
+checkResults(pipeline, true, ["COLLSCAN", "$lookup", "$limit"]);
+
+// Check that lookup->unwind->sort->limit is reordered to lookup->sort, with the unwind stage being
+// absorbed into the lookup stage and preventing the limit from swapping before it, and the limit
+// stage being absorbed into the sort stage.
+pipeline = [
+    {$lookup: {from: other.getName(), localField: "x", foreignField: "x", as: "from_other"}},
+    {$unwind: "$from_other"},
+    {$sort: {x: 1}},
+    {$limit: 5}
+];
+checkResults(pipeline, false, ["COLLSCAN", "$lookup", "$unwind", "$sort", "$limit"]);
+checkResults(pipeline, true, ["COLLSCAN", "$lookup", "$sort"]);
+}());