fast-path for ShardKey::extractFields to create less garbage if already in key form

2 files changed, 18 insertions, 2 deletions

diff --git a/s/chunk.cpp b/s/chunk.cpp
index b1984179864..6a195d8d25d 100644
--- a/s/chunk.cpp
+++ b/s/chunk.cpp
@@ -666,8 +666,10 @@ namespace mongo {
             }
 
             if ( c ) {
-                if ( c->contains( obj ) )
+                if ( c->contains( key ) ){
+                    dassert(c->contains(key)); // doesn't use fast-path in extractKey
                     return c;
+                }
 
                 PRINT(foo);
                 PRINT(*c);
diff --git a/s/shardkey.h b/s/shardkey.h
index 96301ffe093..976cff09591 100644
--- a/s/shardkey.h
+++ b/s/shardkey.h
@@ -102,7 +102,21 @@ namespace mongo {
     };
 
     inline BSONObj ShardKeyPattern::extractKey(const BSONObj& from) const {
-        BSONObj k = from.extractFields(pattern);
+        BSONObj k = from;
+        bool needExtraction = false;
+
+        BSONObjIterator a(from);
+        BSONObjIterator b(pattern);
+        while (a.more() && b.more()){
+            if (strcmp(a.next().fieldName(), b.next().fieldName()) != 0){
+                needExtraction = true;
+                break;
+            }
+        }
+
+        if (needExtraction || a.more() != b.more())
+            k = from.extractFields(pattern);
+
         uassert(13334, "Shard Key must be less than 512 bytes", k.objsize() < 512);
         return k;
     }