1 files changed, 75 insertions, 0 deletions
diff --git a/jstests/core/query/explain/explain_delete.js b/jstests/core/query/explain/explain_delete.js
new file mode 100644
index 00000000000..2b2d7933531
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+++ b/jstests/core/query/explain/explain_delete.js
@@ -0,0 +1,75 @@
+/**
+ * Tests for explaining the delete command.
+ *
+ * @tags: [
+ *  no_selinux,
+ *  requires_fastcount,
+ *  requires_fcv_61,
+ *  requires_non_retryable_writes,
+ *  ]
+ */
+
+(function() {
+"use strict";
+
+var collName = "jstests_explain_delete";
+var t = db[collName];
+t.drop();
+
+var explain;
+
+/**
+ * Verify that the explain command output 'explain' shows a BATCHED_DELETE stage with an
+ * nWouldDelete value equal to 'nWouldDelete'.
+ */
+function checkNWouldDelete(explain, nWouldDelete) {
+    assert.commandWorked(explain);
+    assert("executionStats" in explain);
+    var executionStats = explain.executionStats;
+    assert("executionStages" in executionStats);
+
+    // If passed through mongos, then BATCHED_DELETE stage(s) should be below the SHARD_WRITE
+    // mongos stage.  Otherwise the BATCHED_DELETE stage is the root stage.
+    var execStages = executionStats.executionStages;
+    if ("SHARD_WRITE" === execStages.stage) {
+        let totalToBeDeletedAcrossAllShards = 0;
+        execStages.shards.forEach(function(shardExplain) {
+            const rootStageName = shardExplain.executionStages.stage;
+            assert(rootStageName === "BATCHED_DELETE", tojson(execStages));
+            totalToBeDeletedAcrossAllShards += shardExplain.executionStages.nWouldDelete;
+        });
+        assert.eq(totalToBeDeletedAcrossAllShards, nWouldDelete, explain);
+    } else {
+        assert(execStages.stage === "BATCHED_DELETE", explain);
+        assert.eq(execStages.nWouldDelete, nWouldDelete, explain);
+    }
+}
+
+// Explain delete against an empty collection.
+assert.commandWorked(db.createCollection(t.getName()));
+explain = db.runCommand({explain: {delete: collName, deletes: [{q: {a: 1}, limit: 0}]}});
+checkNWouldDelete(explain, 0);
+
+// Add an index but no data, and check that the explain still works.
+t.createIndex({a: 1});
+explain = db.runCommand({explain: {delete: collName, deletes: [{q: {a: 1}, limit: 0}]}});
+checkNWouldDelete(explain, 0);
+
+// Add some copies of the same document.
+for (var i = 0; i < 10; i++) {
+    t.insert({a: 1});
+}
+assert.eq(10, t.count());
+
+// Run an explain which shows that all 10 documents *would* be deleted.
+explain = db.runCommand({explain: {delete: collName, deletes: [{q: {a: 1}, limit: 0}]}});
+checkNWouldDelete(explain, 10);
+
+// Make sure all 10 documents are still there.
+assert.eq(10, t.count());
+
+// If we run the same thing without the explain, then all 10 docs should be deleted.
+var deleteResult = db.runCommand({delete: collName, deletes: [{q: {a: 1}, limit: 0}]});
+assert.commandWorked(deleteResult);
+assert.eq(0, t.count());
+}());