ircd::db: Improve column find by name from linear to logn; can still be O(1) with more work.

include/ircd/db/database.h

@@ -61,6 +61,7 @@ struct ircd::db::database
 	struct comparator;
 	struct prefix_transform;
 	struct column;
+
 	using description = std::vector<descriptor>;
 
 	// central collection of open databases for iteration (non-owning)
@@ -74,7 +75,9 @@ struct ircd::db::database
 	std::shared_ptr<struct events> events;
 	std::shared_ptr<struct mergeop> mergeop;
 	std::shared_ptr<rocksdb::Cache> cache;
-	std::map<std::string, size_t, std::less<>> column_names;
+	std::vector<descriptor> descriptors;
+	std::vector<string_view> column_names;
+	std::unordered_map<string_view, size_t> column_index;
 	std::vector<std::shared_ptr<column>> columns;
 	custom_ptr<rocksdb::DB> d;
 	unique_const_iterator<decltype(dbs)> dbs_it;

ircd/db.cc

@@ -408,28 +408,42 @@ try
 	const auto lru_cache_size{64_MiB};
 	return rocksdb::NewLRUCache(lru_cache_size);
 }()}
-,column_names{[this, &description]
+,descriptors
 {
-	// Existing columns
-	const auto opts(make_dbopts(std::string(this->optstr)));
+	std::move(description)
+}
+,column_names{[this]
+{
+	// Existing columns at path. If any are left the descriptor set did not
+	// describe all of the columns found in the database at path.
+	const auto opts{make_dbopts(std::string(this->optstr))};
+	const auto required{db::column_names(path, opts)};
+	std::set<string_view> existing{begin(required), end(required)};
 
-	std::set<std::string> existing;
-	for(auto &column_name : db::column_names(path, opts))
-		existing.emplace(std::move(column_name));
-
-	decltype(this->column_names) ret;
-	for(const auto &descriptor : description)
+	// The names of the columns extracted from the descriptor set
+	std::vector<string_view> ret(descriptors.size());
+	std::transform(begin(descriptors), end(descriptors), begin(ret), [&existing]
+	(const auto &descriptor) -> string_view
 	{
 		existing.erase(descriptor.name);
-		ret.emplace(descriptor.name, -1);
-	}
+		return descriptor.name;
+	});
 
 	for(const auto &remain : existing)
-		throw error("Failed to describe existing column '%s'", remain);
+		throw error("Failed to describe existing column '%s' (and %zd others...)",
+		            remain,
+		            existing.size() - 1);
+	return ret;
+}()}
+,column_index{[this]
+{
+	decltype(this->column_index) ret;
+	for(const auto &descriptor : this->descriptors)
+		ret.emplace(descriptor.name, -1);
 
 	return ret;
 }()}
-,d{[this, &description]
+,d{[this]
 {
 	bool fsck{false};
 	bool read_only{false};
@@ -462,7 +476,7 @@ try
 	opts.row_cache = this->cache;
 
 	// Setup column families
-	for(const auto &desc : description)
+	for(const auto &desc : descriptors)
 	{
 		const auto c
 		{
@@ -515,10 +529,12 @@ try
 			rocksdb::DB::Open(opts, path, columns, &handles, &ptr)
 		};
 
+	try // Assign the column index numbers given by db
+	{
 		for(const auto &handle : handles)
 		{
 			this->columns.at(handle->GetID())->handle.reset(handle);
-		this->column_names.at(handle->GetName()) = handle->GetID();
+			this->column_index.at(handle->GetName()) = handle->GetID();
 		}
 
 		for(size_t i(0); i < this->columns.size(); ++i)
@@ -527,6 +543,12 @@ try
 				            i,
 				            db::id(*this->columns[i]),
 				            db::name(*this->columns[i]));
+	}
+	catch(const std::exception &e)
+	{
+		delete ptr;
+		throw;
+	}
 
 	return custom_ptr<rocksdb::DB>
 	{
@@ -645,8 +667,8 @@ ircd::db::database::operator()(const sopts &sopts,
 ircd::db::database::column &
 ircd::db::database::operator[](const string_view &name)
 {
-	const auto it{column_names.find(name)};
-	if(unlikely(it == std::end(column_names)))
+	const auto it{column_index.find(name)};
+	if(unlikely(it == std::end(column_index)))
 		throw schema_error("'%s': column '%s' is not available or specified in schema",
 		                   this->name,
 		                   name);
@@ -671,8 +693,8 @@ const ircd::db::database::column &
 ircd::db::database::operator[](const string_view &name)
 const
 {
-	const auto it{column_names.find(name)};
-	if(unlikely(it == std::end(column_names)))
+	const auto it{column_index.find(name)};
+	if(unlikely(it == std::end(column_index)))
 		throw schema_error("'%s': column '%s' is not available or specified in schema",
 		                   this->name,
 		                   name);