avoid unnecessary memory copy

db/builder.cc

@@ -30,11 +30,14 @@ Status BuildTable(const std::string& dbname, Env* env, const Options& options,
 
     TableBuilder* builder = new TableBuilder(options, file);
     meta->smallest.DecodeFrom(iter->key());
+    Slice key;
     for (; iter->Valid(); iter->Next()) {
-      Slice key = iter->key();
-      meta->largest.DecodeFrom(key);
+      key = iter->key();
       builder->Add(key, iter->value());
     }
+    if(!key.empty()) {
+      meta->largest.DecodeFrom(key);
+    }
 
     // Finish and check for builder errors
     s = builder->Finish();