implemented prepared statement for postgres and sqlite

backends/postgres/include/postgres_connection.hpp

@@ -14,6 +14,8 @@
 
 #include "matador/sql/connection_impl.hpp"
 
+#include <unordered_map>
+
 #include <libpq-fe.h>
 
 namespace matador::backends::postgres {
@@ -27,7 +29,7 @@ public:
   bool is_open() override;
 
   std::unique_ptr<sql::query_result_impl> fetch(const std::string &stmt) override;
-  void prepare(const std::string &stmt) override;
+  std::unique_ptr<sql::statement_impl> prepare(sql::query_context context) override;
 
   size_t execute(const std::string &stmt) override;
 
@@ -35,8 +37,15 @@ public:
 
   bool exists(const std::string &table_name) override;
 
+private:
+  [[nodiscard]] static std::string generate_statement_name(const sql::query_context &query) ;
+
 private:
   PGconn *conn_{nullptr};
+
+  using string_to_int_map = std::unordered_map<std::string, unsigned long>;
+
+  static string_to_int_map statement_name_map_;
 };
 
 }