Sends SELECT right after HELLO after a connection.

README.md

@@ -676,6 +676,15 @@ https://lists.boost.org/Archives/boost/2023/01/253944.php.
 
 ### develop (incorporates changes to conform the boost review and more)
 
+* Adds `boost::redis::config::database_index` to make it possible to
+  choose a database before starting running commands e.g. after an
+  automatic reconnection.
+
+* Massive performance improvement. One of my tests went from
+  140k req/s to 390k/s. This was possible after a parser
+  simplification that reduced the number of reschedules and buffer
+  rotations.
+
 * Adds Redis stream example.
 
 * Renames the project to Boost.Redis and moves the code into namespace

include/boost/redis/config.hpp

@@ -9,6 +9,7 @@
 
 #include <string>
 #include <chrono>
+#include <optional>
 
 namespace boost::redis
 {
@@ -48,6 +49,9 @@ struct config {
    /// Client name parameter of the [HELLO](https://redis.io/commands/hello/) command.
    std::string clientname = "Boost.Redis";
 
+   /// Database that will be passed to the [SELECT](https://redis.io/commands/hello/) command.
+   std::optional<int> database_index = 0;
+
    /// Message used by the health-checker in `boost::redis::connection::async_run`.
    std::string health_check_id = "Boost.Redis";
 

include/boost/redis/detail/runner.hpp

@@ -231,6 +231,9 @@ class runner {
          hello_req_.push("HELLO", "3", "AUTH", cfg_.username, cfg_.password);
       else
          hello_req_.push("HELLO", "3", "SETNAME", cfg_.clientname);
+
+      if (cfg_.database_index)
+         hello_req_.push("SELECT", cfg_.database_index.value());
    }
 
    resolver_type resv_;

tests/test_conn_exec.cpp

@@ -20,8 +20,10 @@ namespace net = boost::asio;
 using boost::redis::connection;
 using boost::redis::request;
 using boost::redis::response;
+using boost::redis::generic_response;
 using boost::redis::ignore;
 using boost::redis::operation;
+using boost::redis::config;
 
 // Sends three requests where one of them has a hello with a priority
 // set, which means it should be executed first.
@@ -117,3 +119,38 @@ BOOST_AUTO_TEST_CASE(cancel_request_if_not_connected)
 
    ioc.run();
 }
+
+BOOST_AUTO_TEST_CASE(correct_database)
+{
+   config cfg;
+   cfg.database_index = 2;
+
+   net::io_context ioc;
+
+   auto conn = std::make_shared<connection>(ioc);
+
+   request req;
+   req.push("CLIENT", "LIST");
+
+   generic_response resp;
+
+   conn->async_exec(req, resp, [&](auto ec, auto n){
+         BOOST_TEST(!ec);
+         std::clog << "async_exec has completed: " << n << std::endl;
+         conn->cancel();
+   });
+
+   conn->async_run(cfg, {}, [](auto){
+         std::clog << "async_run has exited." << std::endl;
+   });
+
+   ioc.run();
+
+   assert(!resp.value().empty());
+   auto const& value = resp.value().front().value;
+   auto const pos = value.find("db=");
+   auto const index_str = value.substr(pos + 3, 1);
+   auto const index = std::stoi(index_str);
+   BOOST_CHECK_EQUAL(cfg.database_index.value(), index);
+}
+