1, if the virtual machine does not support the dynamic expansion of the stack, it will throw an exception StackOverFlow. If you support the dynamic expansion, then the stack will request an additional extension of space. Of course, memory is not infinite, if frequent extended memory, that it can not continue to expand, and this time will throw OutOfMemory exception. In addition, the heap space is limited. Since the object is to be created in the heap allocation of memory, so if the heap space is insufficient, there is not enough memory space to allocate memory for the new object, this time can also throw OutOfMemory exception.
2, for the select statement, before parsing the query, the server will check the query cache (Query Cache), if we can find the corresponding query in which the server will not have to execute the query parsing, optimization and execution of the whole process, but directly returns query result set cache. (Source: "High Performance MySQL" P3)