Fixed potential performance issue in "Context::getTable" method

dbms/src/Client/Connection.cpp

@@ -774,9 +774,7 @@ std::unique_ptr<Exception> Connection::receiveException()
 {
     //LOG_TRACE(log_wrapper.get(), "Receiving exception");
 
-    Exception e;
-    readException(e, *in, "Received from " + getDescription());
-    return std::unique_ptr<Exception>{ e.clone() };
+    return std::make_unique<Exception>{ readException(*in, "Received from " + getDescription()) };
 }
 
 

dbms/src/IO/ReadHelpers.cpp

@@ -959,7 +959,7 @@ void skipJSONField(ReadBuffer & buf, const StringRef & name_of_field)
 }
 
 
-void readException(Exception & e, ReadBuffer & buf, const String & additional_message)
+Exception readException(ReadBuffer & buf, const String & additional_message)
 {
     int code = 0;
     String name;
@@ -986,14 +986,12 @@ void readException(Exception & e, ReadBuffer & buf, const String & additional_me
     if (!stack_trace.empty())
         out << " Stack trace:\n\n" << stack_trace;
 
-    e = Exception(out.str(), code);
+    return Exception(out.str(), code);
 }
 
 void readAndThrowException(ReadBuffer & buf, const String & additional_message)
 {
-    Exception e;
-    readException(e, buf, additional_message);
-    e.rethrow();
+    readException(buf, additional_message).rethrow();
 }
 
 

dbms/src/IO/ReadHelpers.h

@@ -939,7 +939,7 @@ void skipJSONField(ReadBuffer & buf, const StringRef & name_of_field);
   * (type is cut to base class, 'message' replaced by 'displayText', and stack trace is appended to 'message')
   * Some additional message could be appended to exception (example: you could add information about from where it was received).
   */
-void readException(Exception & e, ReadBuffer & buf, const String & additional_message = "");
+Exception readException(ReadBuffer & buf, const String & additional_message = "");
 void readAndThrowException(ReadBuffer & buf, const String & additional_message = "");
 
 

dbms/src/Interpreters/Context.cpp

@@ -922,7 +922,7 @@ StoragePtr Context::tryGetExternalTable(const String & table_name) const
 
 StoragePtr Context::getTable(const String & database_name, const String & table_name) const
 {
-    Exception exc;
+    std::optional<Exception> exc;
     auto res = getTableImpl(database_name, table_name, &exc);
     if (!res)
         throw exc;
@@ -932,11 +932,11 @@ StoragePtr Context::getTable(const String & database_name, const String & table_
 
 StoragePtr Context::tryGetTable(const String & database_name, const String & table_name) const
 {
-    return getTableImpl(database_name, table_name, nullptr);
+    return getTableImpl(database_name, table_name, {});
 }
 
 
-StoragePtr Context::getTableImpl(const String & database_name, const String & table_name, Exception * exception) const
+StoragePtr Context::getTableImpl(const String & database_name, const String & table_name, std::optional<Exception> * exception) const
 {
     String db;
     DatabasePtr database;
@@ -958,7 +958,7 @@ StoragePtr Context::getTableImpl(const String & database_name, const String & ta
         if (shared->databases.end() == it)
         {
             if (exception)
-                *exception = Exception("Database " + backQuoteIfNeed(db) + " doesn't exist", ErrorCodes::UNKNOWN_DATABASE);
+                exception->emplace("Database " + backQuoteIfNeed(db) + " doesn't exist", ErrorCodes::UNKNOWN_DATABASE);
             return {};
         }
 
@@ -969,7 +969,7 @@ StoragePtr Context::getTableImpl(const String & database_name, const String & ta
     if (!table)
     {
         if (exception)
-            *exception = Exception("Table " + backQuoteIfNeed(db) + "." + backQuoteIfNeed(table_name) + " doesn't exist.", ErrorCodes::UNKNOWN_TABLE);
+            exception->emplace("Table " + backQuoteIfNeed(db) + "." + backQuoteIfNeed(table_name) + " doesn't exist.", ErrorCodes::UNKNOWN_TABLE);
         return {};
     }
 

dbms/src/Interpreters/Context.h

@@ -589,7 +589,7 @@ private:
 
     EmbeddedDictionaries & getEmbeddedDictionariesImpl(bool throw_on_error) const;
 
-    StoragePtr getTableImpl(const String & database_name, const String & table_name, Exception * exception) const;
+    StoragePtr getTableImpl(const String & database_name, const String & table_name, std::optional<Exception> * exception) const;
 
     SessionKey getSessionKey(const String & session_id) const;