A database system based on just-in-time (JIT) compilation performs a branch optimization and a loop optimization on the stored procedure. An interpreted query processor generating a structured query language (SQL) execution plan regarding an input online transaction processing (OLTP) query, an online analytical processing (OLAP) analysis, and a stored procedure. A JIT-based query processor receives the SQL execution plan, optimizes a branch and a loop in the received SQL execution plan, and then generates an optimized JIT execution plan. The JIT-based query processor generates a query processing executor performing query processing based on the JIT execution plan and provides a result of the query processing performed by the query processing executor.