save a few redundant container lookups (#5875)

Summary:
This PR eliminates repeated lookups in associative or ordered containers when a single lookup suffices.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5875

Differential Revision: D17753172

Pulled By: anand1976

fbshipit-source-id: 796b02b760082521d8c42a1cb65a76bf0e6c1b8e

db/version_builder.cc

@@ -303,10 +303,7 @@ class VersionBuilder::Rep {
           levels_[level].added_files.erase(exising);
         }
       } else {
-        auto exising = invalid_levels_[level].find(number);
-        if (exising != invalid_levels_[level].end()) {
-          invalid_levels_[level].erase(exising);
-        } else {
+        if (invalid_levels_[level].erase(number) == 0) {
           // Deleting an non-existing file on invalid level.
           has_invalid_levels_ = true;
         }
@@ -326,8 +323,9 @@ class VersionBuilder::Rep {
         levels_[level].added_files[f->fd.GetNumber()] = f;
       } else {
         uint64_t number = new_file.second.fd.GetNumber();
-        if (invalid_levels_[level].count(number) == 0) {
-          invalid_levels_[level].insert(number);
+        auto& lvls = invalid_levels_[level];
+        if (lvls.count(number) == 0) {
+          lvls.insert(number);
         } else {
           // Creating an already existing file on invalid level.
           has_invalid_levels_ = true;

utilities/transactions/pessimistic_transaction.cc

@@ -473,10 +473,11 @@ Status PessimisticTransaction::LockBatch(WriteBatch* batch,
     void RecordKey(uint32_t column_family_id, const Slice& key) {
       std::string key_str = key.ToString();
 
-      auto iter = (keys_)[column_family_id].find(key_str);
-      if (iter == (keys_)[column_family_id].end()) {
+      auto& cfh_keys = keys_[column_family_id];
+      auto iter = cfh_keys.find(key_str);
+      if (iter == cfh_keys.end()) {
         // key not yet seen, store it.
-        (keys_)[column_family_id].insert({std::move(key_str)});
+        cfh_keys.insert({std::move(key_str)});
       }
     }
 

utilities/transactions/transaction_base.cc

@@ -170,7 +170,7 @@ Status TransactionBaseImpl::RollbackToSavePoint() {
         }
         if (tracked_keys_iter->second.num_reads == 0 &&
             tracked_keys_iter->second.num_writes == 0) {
-          tracked_keys_[column_family_id].erase(tracked_keys_iter);
+          cf_tracked_keys.erase(tracked_keys_iter);
         }
       }
     }