statement cache progress

include/matador/sql/connection_pool.hpp

@@ -6,6 +6,7 @@
 #include <chrono>
 #include <mutex>
 #include <string>
+#include <optional>
 #include <thread>
 #include <unordered_map>
 
@@ -48,8 +49,14 @@ public:
   Connection* operator->() { return &connection_->second; }
   Connection& operator*() { return connection_->second; }
 
-  [[nodiscard]] size_t id() const { return connection_->first; }
-
+  [[nodiscard]] std::optional<size_t> id() const
+  {
+    if (connection_) {
+      return connection_->first;
+    } else {
+      return std::nullopt;
+    }
+  }
   [[nodiscard]] bool valid() const { return connection_ != nullptr; }
 
 private: