SERVER-33967 Include executionTimeMillis for each shard in the explain output

3 files changed, 139 insertions, 16 deletions

diff --git a/buildscripts/resmokeconfig/suites/sharding_last_stable_mongos_and_mixed_shards.yml b/buildscripts/resmokeconfig/suites/sharding_last_stable_mongos_and_mixed_shards.yml
index 039bd4a42ff..dec06467969 100644
--- a/buildscripts/resmokeconfig/suites/sharding_last_stable_mongos_and_mixed_shards.yml
+++ b/buildscripts/resmokeconfig/suites/sharding_last_stable_mongos_and_mixed_shards.yml
@@ -134,6 +134,7 @@ selector:
   # Enable if SERVER-36966 is backported or 4.2 becomes last-stable
   - jstests/sharding/mr_output_sharded_validation.js
   # Enable when 4.4 becomes last stable
+  - jstests/sharding/explain_exec_stats_on_shards.js
   - jstests/sharding/refine_collection_shard_key_basic.js
   # Enable when SERVER-41237 is backported
   - jstests/sharding/graph_lookup.js
diff --git a/jstests/sharding/explain_exec_stats_on_shards.js b/jstests/sharding/explain_exec_stats_on_shards.js
new file mode 100644
index 00000000000..d9c9067f3ef
--- /dev/null
+++ b/jstests/sharding/explain_exec_stats_on_shards.js
@@ -0,0 +1,137 @@
+// Tests for the mongos explain command to ensure that the 'executionStats' section of the explain
+// output is populated correctly for each shard.
+(function() {
+    'use strict';
+
+    // Verifies that the explain output for the given shard contains all expected fields and that
+    // field values match the values specified in the 'expected*' arguments. This function also
+    // updates the 'totals' object which holds accumulated values for certain fields from each
+    // shard.
+    function verifyExecStatsOnShard({explain,
+                                     expectedShardName,
+                                     expectedNReturned,
+                                     expectedKeysExamined,
+                                     expectedDocsExamined,
+                                     totals}) {
+        assert(explain.executionSuccess, tojson(explain));
+        assert.eq(explain.shardName, expectedShardName, tojson(explain));
+        assert.eq(explain.nReturned, expectedNReturned, tojson(explain));
+        assert.gte(explain.executionTimeMillis, 0, tojson(explain));
+        assert.eq(explain.totalKeysExamined, expectedKeysExamined, tojson(explain));
+        assert.eq(explain.totalDocsExamined, expectedDocsExamined, tojson(explain));
+        assert(explain.hasOwnProperty("executionStages"), tojson(explain));
+
+        totals.nReturned += explain.nReturned;
+        totals.executionTimeMillis += explain.executionTimeMillis;
+        totals.keysExamined += explain.totalKeysExamined;
+        totals.docsExamined += explain.totalDocsExamined;
+    }
+
+    // Create a cluster with 2 shards.
+    const numShards = 2;
+    const st = new ShardingTest({shards: numShards});
+    const db = st.s.getDB(`${jsTest.name()}_db`);
+
+    // Enable sharding on the database and use shard0 as the primary shard.
+    assert.commandWorked(db.adminCommand({enableSharding: db.getName()}));
+    st.ensurePrimaryShard(db.getName(), st.shard0.shardName);
+
+    // Test that the explain's 'executionStats' section includes all relevant fields for each shard
+    // when the 'explain' command is executed against a sharded collection.
+    (function testExplainExecutionStatsWithShardedCollection() {
+        // Set up a collection, shard on {a:1}, split at {a:4}, and move the {a:4} chunk to shard1.
+        const shardedColl = db.getCollection(`${jsTest.name()}_sharded`);
+        shardedColl.drop();
+        st.shardColl(shardedColl, {a: 1}, {a: 4}, {a: 4});
+
+        // Put documents on each shard.
+        const numDocs = 10;
+        for (let i = 0; i < numDocs; i++) {
+            assert.commandWorked(shardedColl.insert({_id: i, a: i}));
+        }
+        assert.eq(shardedColl.find().itcount(), numDocs);
+
+        // Explain the find command and check that all expected explain fields are present in the
+        // output.
+        const explain = shardedColl.find({}).explain("executionStats");
+        assert(explain.hasOwnProperty("executionStats"), tojson(explain));
+        assert(explain.executionStats.hasOwnProperty("executionStages"), tojson(explain));
+        assert(explain.executionStats.executionStages.hasOwnProperty("shards"), tojson(explain));
+        assert.eq(explain.executionStats.executionStages.shards.length, numShards);
+
+        // Verify execution stats on each shards and accumulate totals.
+        const totals = {nReturned: 0, executionTimeMillis: 0, keysExamined: 0, docsExamined: 0};
+        const executionStages = explain.executionStats.executionStages;
+        // Sort the shards array by the 'shardName' to guarantee consistent results, as explain
+        // outputs from the shards may arrive in an arbitrary order.
+        executionStages.shards.sort((a, b) => a.shardName.localeCompare(b.shardName));
+        verifyExecStatsOnShard({
+            explain: executionStages.shards[0],
+            expectedShardName: st.shard0.shardName,
+            expectedNReturned: 4,
+            expectedKeysExamined: 0,
+            expectedDocsExamined: 4,
+            totals: totals
+        });
+        verifyExecStatsOnShard({
+            explain: executionStages.shards[1],
+            expectedShardName: st.shard1.shardName,
+            expectedNReturned: 6,
+            expectedKeysExamined: 0,
+            expectedDocsExamined: 6,
+            totals: totals
+        });
+
+        // Ensure that execution stats accumulated across all shards matches the values in the
+        // top-level 'executionStages' section of the explain output.
+        assert.eq(executionStages.nReturned, totals.nReturned, tojson(explain));
+        assert.eq(executionStages.totalChildMillis, totals.executionTimeMillis, tojson(explain));
+        assert.eq(executionStages.totalKeysExamined, totals.keysExamined, tojson(explain));
+        assert.eq(executionStages.totalDocsExamined, totals.docsExamined, tojson(explain));
+    })();
+
+    // Test that the explain's 'executionStats' section includes all relevant fields when the
+    // 'explain' command is executed against an unsharded collection.
+    (function testExplainExecutionStatsWithUnshardedCollection() {
+        // Setup an unsharded collection.
+        const unshardedColl = db.getCollection(`${jsTest.name()}_unsharded`);
+        unshardedColl.drop();
+        assert.commandWorked(unshardedColl.ensureIndex({a: 1}));
+
+        // Add documents to the collection.
+        const numDocs = 10;
+        for (let i = 0; i < numDocs; i++) {
+            assert.commandWorked(unshardedColl.insert({_id: i, a: i}));
+        }
+        assert.eq(unshardedColl.count(), numDocs);
+
+        // Explain the find command and check that all expected explain fields are present in the
+        // output.
+        const explain = unshardedColl.find({}).explain("executionStats");
+        assert(explain.hasOwnProperty("executionStats"), tojson(explain));
+        assert(explain.executionStats.hasOwnProperty("executionStages"), tojson(explain));
+        assert(explain.executionStats.executionStages.hasOwnProperty("shards"), tojson(explain));
+        assert.eq(explain.executionStats.executionStages.shards.length, 1);
+
+        // Verify execution stats on the primary shard and accumulate totals.
+        const totals = {nReturned: 0, executionTimeMillis: 0, keysExamined: 0, docsExamined: 0};
+        const executionStages = explain.executionStats.executionStages;
+        verifyExecStatsOnShard({
+            explain: executionStages.shards[0],
+            expectedShardName: st.shard0.shardName,
+            expectedNReturned: 10,
+            expectedKeysExamined: 0,
+            expectedDocsExamined: 10,
+            totals: totals
+        });
+
+        // Ensure that execution stats on the primary shard matches the values in the top-level
+        // 'executionStages' section of the explain output.
+        assert.eq(executionStages.nReturned, totals.nReturned, tojson(explain));
+        assert.eq(executionStages.totalChildMillis, totals.executionTimeMillis, tojson(explain));
+        assert.eq(executionStages.totalKeysExamined, totals.keysExamined, tojson(explain));
+        assert.eq(executionStages.totalDocsExamined, totals.docsExamined, tojson(explain));
+    })();
+
+    st.stop();
+})();
diff --git a/src/mongo/s/commands/cluster_explain.cpp b/src/mongo/s/commands/cluster_explain.cpp
index d814f6080d9..fb3b748f4c5 100644
--- a/src/mongo/s/commands/cluster_explain.cpp
+++ b/src/mongo/s/commands/cluster_explain.cpp
@@ -304,25 +304,10 @@ void ClusterExplain::buildExecStats(const vector<Strategy::CommandResult>& shard
     BSONArrayBuilder execShardsBuilder(executionStagesBob.subarrayStart("shards"));
     for (size_t i = 0; i < shardResults.size(); i++) {
         BSONObjBuilder singleShardBob(execShardsBuilder.subobjStart());
-
         BSONObj execStats = shardResults[i].result["executionStats"].Obj();
-        BSONObj execStages = execStats["executionStages"].Obj();
 
         singleShardBob.append("shardName", shardResults[i].shardTargetId.toString());
-
-        // Append error-related fields, if present.
-        if (!execStats["executionSuccess"].eoo()) {
-            singleShardBob.append(execStats["executionSuccess"]);
-        }
-        if (!execStats["errorMessage"].eoo()) {
-            singleShardBob.append(execStats["errorMessage"]);
-        }
-        if (!execStats["errorCode"].eoo()) {
-            singleShardBob.append(execStats["errorCode"]);
-        }
-
-        appendIfRoom(&singleShardBob, execStages, "executionStages");
-
+        appendElementsIfRoom(&singleShardBob, execStats);
         singleShardBob.doneFast();
     }
     execShardsBuilder.doneFast();