Remove unused variable `single_column_family_mode_` (#10078)

Summary:
This variable is actually not being used for anything meaningful, thus remove it.

This can make https://github.com/facebook/rocksdb/issues/7516 slightly simpler by reducing the amount of state that must be made lock-free.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10078

Test Plan: make check

Reviewed By: ajkr

Differential Revision: D36779817

Pulled By: riversand963

fbshipit-source-id: ffb0d9ad6149616917ae5e02bb28102cb90fc406

db/db_impl/db_impl.cc

@@ -2772,7 +2772,6 @@ Status DBImpl::CreateColumnFamilyImpl(const ColumnFamilyOptions& cf_options,
       s = cfd->AddDirectories(&dummy_created_dirs);
     }
     if (s.ok()) {
-      single_column_family_mode_ = false;
       auto* cfd =
           versions_->GetColumnFamilySet()->GetColumnFamily(column_family_name);
       assert(cfd != nullptr);

db/db_impl/db_impl.h

@@ -1207,9 +1207,6 @@ class DBImpl : public DB {
   // only used for dynamically adjusting max_total_wal_size. it is a sum of
   // [write_buffer_size * max_write_buffer_number] over all column families
   uint64_t max_total_in_memory_state_;
-  // If true, we have only one (default) column family. We use this to optimize
-  // some code-paths
-  bool single_column_family_mode_;
 
   // The options to access storage files
   const FileOptions file_options_;

db/db_impl/db_impl_open.cc

@@ -561,10 +561,6 @@ Status DBImpl::Recover(
     default_cf_handle_ = new ColumnFamilyHandleImpl(
         versions_->GetColumnFamilySet()->GetDefault(), this, &mutex_);
     default_cf_internal_stats_ = default_cf_handle_->cfd()->internal_stats();
-    // TODO(Zhongyi): handle single_column_family_mode_ when
-    // persistent_stats is enabled
-    single_column_family_mode_ =
-        versions_->GetColumnFamilySet()->NumberOfColumnFamilies() == 1;
 
     // Recover from all newer log files than the ones named in the
     // descriptor (new log files may have been added by the previous

db/db_impl/db_impl_secondary.cc

@@ -61,8 +61,6 @@ Status DBImplSecondary::Recover(
     default_cf_handle_ = new ColumnFamilyHandleImpl(
         versions_->GetColumnFamilySet()->GetDefault(), this, &mutex_);
     default_cf_internal_stats_ = default_cf_handle_->cfd()->internal_stats();
-    single_column_family_mode_ =
-        versions_->GetColumnFamilySet()->NumberOfColumnFamilies() == 1;
 
     std::unordered_set<ColumnFamilyData*> cfds_changed;
     s = FindAndRecoverLogFiles(&cfds_changed, &job_context);

db/db_impl/db_impl_write.cc

@@ -1041,13 +1041,19 @@ Status DBImpl::PreprocessWrite(const WriteOptions& write_options,
 
   PERF_TIMER_GUARD(write_scheduling_flushes_compactions_time);
 
-  assert(!single_column_family_mode_ ||
-         versions_->GetColumnFamilySet()->NumberOfColumnFamilies() == 1);
-  if (UNLIKELY(status.ok() && !single_column_family_mode_ &&
-               total_log_size_ > GetMaxTotalWalSize())) {
+  if (UNLIKELY(status.ok() && total_log_size_ > GetMaxTotalWalSize())) {
+    assert(versions_);
+    const ColumnFamilySet* const column_families =
+        versions_->GetColumnFamilySet();
+    assert(column_families);
+    size_t num_cfs = column_families->NumberOfColumnFamilies();
+
+    assert(num_cfs >= 1);
+    if (num_cfs > 1) {
       WaitForPendingWrites();
       status = SwitchWAL(write_context);
     }
+  }
 
   if (UNLIKELY(status.ok() && write_buffer_manager_->ShouldFlush())) {
     // Before a new memtable is added in SwitchMemtable(),