Mysql Join_buffer_size的使用原理

class JOIN_CACHE(sql_join_buffer.h) 中的 uchar *buff;

void JOIN_CACHE::set_constants() 设置buff的大小
bool JOIN_CACHE::alloc_buffer() 申请buff大小的内存
void JOIN_CACHE::free() join操作完成以后，立马释放内存
void JOIN::destroy() 销毁join类

mysql中每次join操作都会调用my_malloc、my_free函数申请/释放joib_buffer_size的大小的内存。
join_buffer_size是按照每次操作join表的操作的次数申请和释放joib_buffer_size.


(gdb) JOIN_CACHE::set_constants
Undefined command: "JOIN_CACHE". Try "help".
(gdb) br JOIN_CACHE::set_constants
Breakpoint 1 at 0x9a8ac9: file /home/os1/Open/mysql5616/sql/sql_join_buffer.cc, line 432.
(gdb) br JOIN_CACHE::alloc_buffer
Breakpoint 2 at 0x9a8cb0: file /home/os1/Open/mysql5616/sql/sql_join_buffer.cc, line 468.
(gdb) r
(gdb) br JOIN_CACHE::free 
(gdb) br JOIN::destroy