SERVER-27175 Improve performance of planSummary string generation

3 files changed, 57 insertions, 15 deletions

diff --git a/src/mongo/db/keypattern.cpp b/src/mongo/db/keypattern.cpp
index 2c1bf09497d..ffc2a1f8e71 100644
--- a/src/mongo/db/keypattern.cpp
+++ b/src/mongo/db/keypattern.cpp
@@ -31,7 +31,6 @@
 #include "mongo/db/keypattern.h"
 
 #include "mongo/db/index_names.h"
-#include "mongo/util/mongoutils/str.h"
 
 namespace mongo {
 
@@ -55,6 +54,35 @@ bool KeyPattern::isHashedKeyPattern(const BSONObj& pattern) {
     return IndexNames::HASHED == IndexNames::findPluginName(pattern);
 }
 
+StringBuilder& operator<<(StringBuilder& sb, const KeyPattern& keyPattern) {
+    // Rather than return BSONObj::toString() we construct a keyPattern string manually. This allows
+    // us to avoid the cost of writing numeric direction to the str::stream which will then undergo
+    // expensive number to string conversion.
+    sb << "{ ";
+
+    bool first = true;
+    for (auto&& elem : keyPattern._pattern) {
+        if (first) {
+            first = false;
+        } else {
+            sb << ", ";
+        }
+
+        if (mongo::String == elem.type()) {
+            sb << elem;
+        } else if (elem.number() >= 0) {
+            // The canonical check as to whether a key pattern element is "ascending" or
+            // "descending" is (elem.number() >= 0). This is defined by the Ordering class.
+            sb << elem.fieldNameStringData() << ": 1";
+        } else {
+            sb << elem.fieldNameStringData() << ": -1";
+        }
+    }
+
+    sb << " }";
+    return sb;
+}
+
 BSONObj KeyPattern::extendRangeBound(const BSONObj& bound, bool makeUpperInclusive) const {
     BSONObjBuilder newBound(bound.objsize());
 
diff --git a/src/mongo/db/keypattern.h b/src/mongo/db/keypattern.h
index f9ac83983e3..cfa24da5df2 100644
--- a/src/mongo/db/keypattern.h
+++ b/src/mongo/db/keypattern.h
@@ -29,7 +29,9 @@
 #pragma once
 
 #include "mongo/base/string_data.h"
+#include "mongo/bson/util/builder.h"
 #include "mongo/db/jsobj.h"
+#include "mongo/util/mongoutils/str.h"
 
 namespace mongo {
 
@@ -80,9 +82,13 @@ public:
      * Returns a string representation of this KeyPattern
      */
     std::string toString() const {
-        return toBSON().toString();
+        return str::stream() << *this;
     }
 
+    /**
+     * Writes to 'sb' a string representation of this KeyPattern.
+     */
+    friend StringBuilder& operator<<(StringBuilder& sb, const KeyPattern& keyPattern);
 
     /* Takes a BSONObj whose field names are a prefix of the fields in this keyPattern, and
      * outputs a new bound with MinKey values appended to match the fields in this keyPattern
diff --git a/src/mongo/db/query/explain.cpp b/src/mongo/db/query/explain.cpp
index 929950dc38a..7d83021dbd5 100644
--- a/src/mongo/db/query/explain.cpp
+++ b/src/mongo/db/query/explain.cpp
@@ -31,6 +31,7 @@
 #include "mongo/db/query/explain.h"
 
 #include "mongo/base/owned_pointer_vector.h"
+#include "mongo/bson/util/builder.h"
 #include "mongo/db/exec/cached_plan.h"
 #include "mongo/db/exec/count_scan.h"
 #include "mongo/db/exec/distinct_scan.h"
@@ -41,6 +42,7 @@
 #include "mongo/db/exec/pipeline_proxy.h"
 #include "mongo/db/exec/text.h"
 #include "mongo/db/exec/working_set_common.h"
+#include "mongo/db/keypattern.h"
 #include "mongo/db/query/get_executor.h"
 #include "mongo/db/query/plan_executor.h"
 #include "mongo/db/query/plan_summary_stats.h"
@@ -166,33 +168,39 @@ size_t getDocsExamined(StageType type, const SpecificStats* specific) {
 }
 
 /**
- * Adds to the plan summary string being built by 'ss' for the execution stage 'stage'.
+ * Adds to the plan summary string being built by 'sb' for the execution stage 'stage'.
  */
-void addStageSummaryStr(const PlanStage* stage, mongoutils::str::stream& ss) {
+void addStageSummaryStr(const PlanStage* stage, StringBuilder& sb) {
     // First add the stage type string.
     const CommonStats* common = stage->getCommonStats();
-    ss << common->stageTypeStr;
+    sb << common->stageTypeStr;
 
     // Some leaf nodes also provide info about the index they used.
     const SpecificStats* specific = stage->getSpecificStats();
     if (STAGE_COUNT_SCAN == stage->stageType()) {
         const CountScanStats* spec = static_cast<const CountScanStats*>(specific);
-        ss << " " << spec->keyPattern;
+        const KeyPattern keyPattern{spec->keyPattern};
+        sb << " " << keyPattern;
     } else if (STAGE_DISTINCT_SCAN == stage->stageType()) {
         const DistinctScanStats* spec = static_cast<const DistinctScanStats*>(specific);
-        ss << " " << spec->keyPattern;
+        const KeyPattern keyPattern{spec->keyPattern};
+        sb << " " << keyPattern;
     } else if (STAGE_GEO_NEAR_2D == stage->stageType()) {
         const NearStats* spec = static_cast<const NearStats*>(specific);
-        ss << " " << spec->keyPattern;
+        const KeyPattern keyPattern{spec->keyPattern};
+        sb << " " << keyPattern;
     } else if (STAGE_GEO_NEAR_2DSPHERE == stage->stageType()) {
         const NearStats* spec = static_cast<const NearStats*>(specific);
-        ss << " " << spec->keyPattern;
+        const KeyPattern keyPattern{spec->keyPattern};
+        sb << " " << keyPattern;
     } else if (STAGE_IXSCAN == stage->stageType()) {
         const IndexScanStats* spec = static_cast<const IndexScanStats*>(specific);
-        ss << " " << spec->keyPattern;
+        const KeyPattern keyPattern{spec->keyPattern};
+        sb << " " << keyPattern;
     } else if (STAGE_TEXT == stage->stageType()) {
         const TextStats* spec = static_cast<const TextStats*>(specific);
-        ss << " " << spec->indexPrefix;
+        const KeyPattern keyPattern{spec->indexPrefix};
+        sb << " " << keyPattern;
     }
 }
 
@@ -801,7 +809,7 @@ std::string Explain::getPlanSummary(const PlanStage* root) {
     flattenExecTree(root, &stages);
 
     // Use this stream to build the plan summary string.
-    mongoutils::str::stream ss;
+    StringBuilder sb;
     bool seenLeaf = false;
 
     for (size_t i = 0; i < stages.size(); i++) {
@@ -809,15 +817,15 @@ std::string Explain::getPlanSummary(const PlanStage* root) {
             // This is a leaf node. Add to the plan summary string accordingly. Unless
             // this is the first leaf we've seen, add a delimiting string first.
             if (seenLeaf) {
-                ss << ", ";
+                sb << ", ";
             } else {
                 seenLeaf = true;
             }
-            addStageSummaryStr(stages[i], ss);
+            addStageSummaryStr(stages[i], sb);
         }
     }
 
-    return ss;
+    return sb.str();
 }
 
 // static